Implementation of Lead-Free Soldering in Highly
Reliable Applications
Ove Berglund
Master Thesis
Department of Management and Engineering
LIU-IEI-TEK-A--07/0083--SE
Implementation of Lead-Free Soldering in Highly
Reliable Applications
Ove Berglund
Master Thesis
Department of Management and Engineering
LIU-IEI-TEK-A--07/0083--SE
Abstract
The directive of the European parliament and of the council on the Restriction of the use of certain Hazardous Substances (RoHS) in Electrical and Electronic Equipment (EEE) took eect in the European Union on July 1, 2006. Japan, California, China and Korea are all closed markets for exporters of components containing lead from July 1, 2007. Taiwan and Australia are working with similar directives. The RoHS directive is the reason why this thesis about the implementation of lead-free soldering in highly reliable applications is necessary. The European Lead Free soldering NETwork (ELFNET) status survey from 2005 shows that the majority of the companies are well informed, but 20% are still not active in lead-free soldering. The Swedish industry is for the most part prepared and 95% of the components are lead-free. The transition to lead-free soldering will have a major aect on logistics and administration, because the RoHS directive is 90% about administration and logistics problems. Only 10% is technical problems.
The higher melting point in lead-free soldering aects every stage of the lead-free manufacturing, from assembly to testing and repair.
The major concern for highly reliable applications are that there are not enough data to understand to what grade lead-free solders will perform dierently from lead based solders. Five dierent types of reliability testing were studied in this thesis; vibration, mechanical shock, thermal shock, thermal cycling and combined environments. Whiskers, voids, brittle fractures and mixed assembly problems were also studied. Individual tests alone should not be used to make de nite decisions on free soldering reliability. The lower reliability for lead-free solders in some tests does not necessarily mean that lead-lead-free solders not can be used in highly reliable applications like defence electronics.
The most important conclusions from this thesis are:
• Update or change the logistic system and mark/label according to avail-able standards.
• Secure a good board layout. • Secure a good process control.
• Alternative surface board should be used. Tin-silver-copper (SAC) is the most reliable solder and Electroless Nickel/Immersion Gold (ENIG) is the most reliable surface nish.
• Remember that the higher temperature aects every stage of the manu-facturing.
• No increased problems with whiskers or risk of high voiding levels. • Mixed assembly is a risk. Compatibility and contamination risks must be
taken seriously.
• Which environment will the applications be in? If it is not a highly vibrat-ing and thermal cyclvibrat-ing environment, lead-free soldervibrat-ing should be safe to use.
Sammanfattning
Europaparlamentets och rådets direktiv om begränsning av användningen av vissa farliga ämnen i elektriska och elektroniska produkter började gälla i Eu-ropeiska unionen 1 juli, 2006. Japan, Kalifornien, Kina och Korea är alla stängda marknader för exportörer av komponenter som innehåller bly från och med 1 juli, 2007. Taiwan och Australien arbetar med liknande direktiv. RoHS-direktivet är anledningen till varför detta examensarbete om implementeringen av blyfri lödning i högtillförlitliga applikationer är nödvändigt.
En undersökning från 2005 av ELFNET visar att majoriteten av företagen är väl informerade, men 20% är fortfarande i aktiva med blyfri lödning. Den svenska industrin är till största delen väl förberedd och 95% av komponenterna är blyfria. Övergången till blyfri lödning kommer att ha stor eekt på logistik och administration, därför att 90% är administrations- och logistikproblem i RoHS-direktivet. Bara 10% är tekniska problem.
Den högre smälttemperaturen i blyfri lödning påverkar varje steg av den blyfria tillverkningen, från montering till testning och reparation.
Den stora oron för högtillförlitliga applikationer är att det inte nns tillräckligt med data för att förstå i vilken grad som blyfria lod kommer att bete sig annor-lunda jämfört med blybaserade lod. Fem olika typer av tillförlitlighetstester har undersökts i detta examensarbete; vibration, mekanisk chock, termisk chock, termisk cykling och kombinerade tester. Whiskers, voids, sprödbrott och blan-dad montering studerades också. Individuella tester ska inte användas för att ta några de nitiva beslut om blyfri lödnings tillförlitlighet. Den lägre tillför-litligheten för blyfria lod i en del tester behöver nödvändigtvis inte betyda att blyfria lod inte kan användas i högtillförlitliga applikationer som försvars-elektronik.
De viktigaste slutsatserna från detta examensarbete är:
• Uppdatera eller byt logistiskt system och märk enligt tillgängliga stan-darder.
• Säkerställ en bra kretskortsdesign. • Säkerställ en bra processkontroll.
• Alternativa mönsterkort bör användas. SAC är det tillförlitligaste lodet och ENIG är den tillförlitligaste ytbehandlingen.
• Kom ihåg att den ökade temperaturen påverkar varje steg i tillverkningen. • Inga ökade problem med whiskers eller stort antal voids.
• Blandmontage är riskfyllt. Kompatibilitet och risker med kontaminering måste tas på allvar.
• Vilken miljö kommer applikationen att be nna sig i? Är det inte en starkt vibrerande eller temperaturcyklisk miljö bör blyfri lödning vara säkert att använda.
Acknowledgements
This thesis was carried out at the Department of Management and Engineering, Linköping Institute of Technology and at Saab Systems, Naval Systems Division Sweden. The work was carried out between September 2006 and March 2007 at Saab Systems main o ce at Järfälla, near Stockholm. The thesis is a compulsory part of the education to receive a degree in Master of Science in Electronics Design from Linköping Institute of Technology. The examiner at Linköping Institute of Technology was Mattias Lindahl. The supervisor at Saab Systems was Christer Melander.
I would like to thank my supervisor Christer Melander and Saab personnel at Saab Systems who have been very helpful during this thesis. Christer helped me getting in contact with relevant people and collecting information. I was given the opportunity to participate in meetings and a seminar arrange by KIMAB and IVF.
I would like to thank my examiner Mattias Lindahl who supported me with his analysing and questioning attitude. He also helped me with questions regarding the Asian market.
I would like to thank Per-Erik Tegehall, Lars-Gunnar Klang, Conny Svensson, Jan-Eric Spjuth, Magnus Porsmark, Thomas Cadring and Peter Back.
I would also like to thank Håkan Hådeby, Anders Ekelöf, Benny Gustafson, Ove Isaksson, Kent Stenberg and Axel Tchimanga at Ericsson in Kista, near Stockholm. I would like to thank ABB and their connections with ENICS, which gave me the opportunity to meat Peter Back at ENICS in Malmö. Finally I would like to thank everyone who supported and helped me during my thesis. I am grateful to Saab Systems for giving me this opportunity.
Stockholm, March 2007 Ove Berglund
Nomenclature & De nitions
AOI Automatic Optical Inspection
BGA Ball Grid Array. The pins are replaced by balls of solder stuck to the bottom of the package
Bodycote In house analyse company to Saab. Supplier of specialist testing and thermal processing services. A vital provider of heat treat-ments, hot isostatic pressing, metallurgical coatings and testing services to industry.
CET Combined Environment Tests
CSP Chip Scale Package is a type of integrated circuit chip carrier that has no pins or wires but uses contact pads instead.
Delta T Delta T is the dierence between the highest and lowest temper-ature observed across the board.
DoD U.S. Department of Defense
EEE Electrical and Electronic Equipment
ELFNET European Lead Free soldering NETwork. ELFNET is a European research network of the national organisations, technical experts and industry bodies in micro-electronics. ELFNET provides a platform to coordinate, integrate and optimise research, enabling electronics producers in the EU to meet an EU directive to intro-duce lead-free soldering by 1 July 2006.
ENICS Subcontractor (not to Saab Systems) ENIG Electroless Nickel/Immersion Gold EoL End of Life
EPA Environmental Protection Agency
EPHC Environment Protection and Heritage Council
FR4 Flame Resistant 4. Designation for a berglass and epoxy sub-strate material. A type of material used for making a printed circuit board.
GPL Green Purchasing Law
HARL Home Appliances Recycling Law HASL Hot Air Solder Leveling
I-Ag Immersion silver I-Sn Immersion tin
iNEMI The International Electronics Manufacturing Initiative (iNEMI) is an industry-led consortium whose mission is to assure leadership of the global electronics manufacturing supply chain.
IPC IPC is the formal name of a United States-based trade associ-ation dedicated to furthering the competitive excellence and -nancial success of its members worldwide, who are participants in the electronic interconnect industry. In pursuit of these objec-tives, IPC will devote resources to management improvement and technology enhancement programs, the creation of relevant stan-dards, protection of the environment, and pertinent government relations.
IVF Industrial Research and Development Corporation is the Swedish engineering industry's research institute.
JCAA/JG Joint Council on Aging Aircraft/Joint Group. A DoD sponsored consortium was founded in May of 2001 to evaluate lead-free sol-ders and nishes and to determine whether they are suitable for use in high reliability electronics. This consortium is jointly man-aged by the JCAA and the Joint Group on Pollution Prevention (JG-PP). The consortium's project is called the JCAA/JG-PP Lead-Free Solder Project and it has members from all branches of the Armed Services, NASA, Boeing, Rockwell-Collins, Raytheon, BAE Systems, ACI, Lockheed Martin, Texas Instruments, NCMS, JPL, Sandia National Labs and Marshall Space Flight Center among others.
JEDEC The JEDEC Solid State Technology Association (Once known as the Joint Electron Device Engineering Council). Is the semicon-ductor engineering standardization body of the Electronic Indus-tries Alliance (EIA), a trade association that represents all areas of the electronics industry.
JEITA Japan Electronics and Information Technology Industries Associ-ation. Their objective is to promote the healthy manufacturing, international trade and consumption of electronics products and components in order to contribute to the overall development of the electronics and information technology (IT) industries, and thereby further Japan's economic development and cultural pros-perity.
LEAB Leab Group is a contract manufacturer Legotronic PCB subcontractor to Saab Systems Lindebergh PCB subcontractor to Saab Systems
LPEUR Law for the Promotion of Eective Utilisation of Resources NASA National Aeronautics and Space Administration
NOTE NOTE is a contract supplier of exible electronics production and customised logistics.
NPL National Physical Laboratory. NPL is the United Kingdom's na-tional standards laboratory, an internana-tionally respected and inde-pendent centre of excellence in research, development and knowl-edge transfer in measurement and materials science.
NSD Naval Systems Division
MII Ministry of Information Industry OSP Organic Solderability Preservative PBB Polybrominated Biphenyls PBDE Polybrominated Diphenyl Ethers PBGA Plastic Ball Grid Array
PCB Printed Circuit Board PWB Printed Wiring Board
QFP Quad Flat Pack. High lead count package. Fine-pitch devices, lead pitch is often 0.66 mm down to 0.3 mm.
RoHS Restriction of the use of certain Hazardous Substances. SAC SnAgCu. Tin-Silver-Copper
SAC305 The rst two numbers are the percentage of silver (3.0%) and the last number the percentage of copper (0.5%).
SACB SnAgCuBi. Tin-Silver-Copper-Bismuth SAEEC South African Electrotechnical Export Council SMT Surface Mount Technology
SMTA Surface Mount Technology Association. Established in 1984, is a non-pro t international association of companies and individuals involved in all aspects of the electronics industry. The Associa-tion is dedicated to the advancement of the electronics industry through member education and interaction.
SnAg Tin-Silver SnBi Tin-Bismuth SnCu Tin-Copper SnCuNi Tin-Copper-Nickel SnPb Tin-Lead SnPbAg Tin-Lead-Silver SnZn Tin-Zinc
TAC Technical Adaptation Committee TAL Time Above Liquid
Td Decomposition temperature Tg Glass-transition temperature
TTG Technical Transfer Group. A working group in Saab which are working with issues regarding the RoHS directive.
VOC Volatile Organic Compound
Contents
1 Introduction 1 1.1 Company description . . . 1 1.2 Background . . . 1 1.3 Purpose . . . 2 1.4 Research questions . . . 2 1.5 Delimitations . . . 2 2 Method 5 2.1 Research methodology . . . 52.1.1 Qualitative - Quantitative research . . . 6
2.2 Interview methodology . . . 6
2.3 Theoretical studies . . . 7
2.4 Data sources . . . 7
2.4.1 Saab . . . 7
2.4.2 External sources . . . 7
2.5 Structure of the thesis . . . 8
3 International legislations 11 3.1 EU, RoHS directive . . . 12
3.1.1 Exceptions . . . 13 3.2 USA . . . 13 3.3 Japan . . . 13 3.4 China . . . 14 3.5 Australia . . . 14 3.6 South Africa . . . 15
4 Current status on lead-free soldering 17 4.1 Current status at Saab . . . 18
5 Logistics 21 5.1 Storage . . . 21
5.2 Marking . . . 21
5.2.1 IPC1066/IPC-JEDEC STD NR 97 . . . 22
6 Lead-free issues and alternatives 25 6.1 Education . . . 25
6.2 Temperature . . . 26
6.3 Soldering process . . . 28 6.4 Solder . . . 29 6.4.1 Tin-silver-copper . . . 29 6.4.2 Tin-silver-copper-bismuth/Tin-silver-bismuth . . . 30 6.4.3 Tin-zinc . . . 30 6.4.4 Tin-copper . . . 30 6.4.5 Tin-silver . . . 30 6.5 Components . . . 31 6.5.1 Supply . . . 31
6.6 Printed surface boards . . . 31
6.7 Flux . . . 32
6.8 Surface nishes . . . 32
6.8.1 Electroless Nickel/Immersion Gold . . . 32
6.8.2 Immersion silver . . . 33
6.8.3 Hot Air Solder Levelling . . . 34
6.8.4 Organic Solderability Preservative . . . 34
6.8.5 Immersion tin . . . 34
6.9 Inspection . . . 35
6.10 Rework and repair . . . 35
7 Reliability of lead-free soldering 37 7.1 Reliability testing . . . 38 7.1.1 Vibration . . . 38 7.1.2 Mechanical shock . . . 38 7.1.3 Thermal shock . . . 38 7.1.4 Thermal cycling . . . 39 7.1.5 Combined environments . . . 41 7.1.6 Salt fog . . . 43 7.1.7 Humidity . . . 43 7.1.8 Test summary . . . 43 7.2 Whiskers . . . 44 7.3 Voids . . . 45 7.4 Brittle fractures . . . 46 7.5 Mixed assembly . . . 47 7.5.1 Metallurgical imbalance . . . 49 7.5.2 Contamination . . . 49 7.5.3 Compatibility . . . 49 8 Discussion 51 8.1 What legislations are there . . . 51
8.2 Status on lead-free soldering . . . 51
8.3 Impact on logistics and administration . . . 52
8.4 Aect on the solder process . . . 52
8.5 Aects of the increased solder temperature . . . 57
8.6 Is it safe to use lead-free soldering for highly reliable applications? 57 8.7 Criticism of chosen methodology . . . 59
8.7.1 Sources of error . . . 60
8.7.2 Validity . . . 61
9 Conclusions 63 9.1 General . . . 63 9.2 Saab Systems . . . 64 9.3 Future work . . . 65
A Glossary 73
List of Figures
5.1 Humidity labels [48] . . . 22 5.2 Lead-free labels [46, 47] . . . 23 6.1 Temperature pro les [31] . . . 26 6.2 Storage eect [20] . . . 34 7.1 Cross-over point [33] . . . 407.2 Failure after thermal cycling [63] . . . 41
7.3 Failure regions [21] . . . 42
7.4 Image of whiskers [25] . . . 44
7.5 Optical and electron microscope images of voids [64] . . . 46
7.6 Image of a crack in a solder joint [30] . . . 47
List of Tables
3.1 Legislations . . . 11
3.2 Categories covered by the RoHS and WEEE directives . . . 12
3.3 China implementation standards . . . 14
3.4 Categories covered by the China RoHS directive . . . 14
5.1 Solder denotation . . . 23
6.1 Visual inspection results . . . 27
6.2 Lead and lead-free temperature characteristics [31] . . . 28
6.3 Surface nishes environmental aect . . . 32
7.1 Solder performance . . . 44 8.1 SAC . . . 53 8.2 SACB/SnAgBi . . . 53 8.3 SnZn . . . 53 8.4 SnCu . . . 54 8.5 SnAg . . . 54 8.6 Solder comparison . . . 54 8.7 ENIG . . . 55 8.8 I-Ag . . . 55 8.9 HASL . . . 56 8.10 OSP . . . 56 8.11 I-Sn . . . 56
8.12 Surface nishes comparison . . . 56
Chapter 1
Introduction
1.1 Company description
Saab Systems, Naval Systems Division (NSD) encompasses naval related busi-ness activities in Sweden and Australia. Naval Systems Division Sweden shall in cooperation with Saab Systems other divisions and sta units sell, develop, and deliver naval Combat Management Systems. Reliable through-life support of deliveries is an essential part of Naval Systems Division customer image. The customers are shipyards and governmental organizations in Sweden and inter-nationally.
1.2 Background
The background to this thesis is the RoHS directive which took aect in July 1, 2006. The purpose of the directive is to approximate the laws of the EU member states on RoHS in EEE. RoHS should also contribute to the protection of human health and the environment.
There are six restricted substances in the RoHS directive and lead is one of them. No new EEE released after July 1, 2006 are allowed to contain the restricted substances. There are two categories and several exceptions in the EU directives. Military applications (like Saab Systems) are excepted from the RoHS directive. Exceptions will be carried out only on technical criteria. The exceptions are valid for a maximum period of four years and then subject to a review. The exceptions are granted for speci c applications and not to whole products.
However, Saab Systems is aected in several ways, through customer demands and component manufacturers. Because of the indirect consequences for Saab Systems and the uncertainty regarding how long the exceptions are valid, a study about the implementation of lead-free soldering in highly reliable applications were needed.
Introduction
1.3 Purpose
The purpose of this thesis is to investigate the consequences of the transition to lead-free soldering in highly reliable applications. This shall be made by litera-ture studies, interviews, meetings with Saab and their subcontractors and other companies. The purpose is also to present available solutions to the problems with lead-free soldering.
1.4 Research questions
A number of questions were arisen before and during the thesis process. These were the questions that Saab Systems in Järfälla were interested in making an investigation on:
1. What legislations are there
Lead-free soldering has to be used because of the RoHS directive. This question had to be answered to understand the RoHS directive and the legislations in other relevant coutries.
2. Status on lead-free soldering
This question had to be answered to know how far the transition to lead-free soldering has come.
3. Impact on logistics and administration
This question had to be answered to understand the RoHS directive impact on logistics and administration.
4. Aect on the solder process
This question had to be answered to understand the impact on the whole solder process.
5. Aects of the increased solder temperature
This question had to be answered to understand the problems with the elevated temperature in lead-free soldering.
6. Is it safe to use lead-free soldering for highly reliable applications? Probably one of the most important question within lead-free soldering today.
1.5 Delimitations
This thesis work is limited to 20 working weeks and some delimitation need to be carried out to reach the objective within the timeframe. Therefore it was not possible to investigate the consequences of the transition to lead-free soldering for every relevant department in Naval Systems Division Sweden. Consequences for the development, logistics and production departments were analyzed. No investigation has been made for the purchase, test and delivery, after sales and ILS (Intergrated Logistic Support) departments. Areas that are mentioned
1.5 Delimitations in this thesis, but need more attention are rework and repair, compatibility and
ux.
There are six restricted substances in the RoHS directive; lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyls and polybrominated diphenyl ethers. Lead is the only substance investigated in this thesis.
Chapter 2
Method
This chapter describes the choice of methodology. Some theory about research methodology and interview methodology is presented. Criticism about the cho-sen method is discussed in chapter eight, Discussion. The following books have been used to write this chapter [54, 55, 56, 57].
A research methodology was used to understand the issues with implementing lead-free soldering. A qualitative study had to be used because not enough work has been done to make a quantitative study. Interviews and literature studies was a natural way to investigate the lead-free phenomenon. Therefore people in the ELFNET and the Industrial Research and Development Corporation (IVF) were contacted. To understand other companies problems and status on lead-free soldering interviews with subcontractors and other companies were made. Documents from laboratories and research institutes were used to make the literature study. Relevant articles, journals and other written sources were also used. The National Physical Laboratory (NPL) in the United Kingdom, Surface Mount Technology Association (SMTA), Joint Council on Aging Aircraft/Joint Group (JCAA/JG) and the ELFNET were found to be reliable sources and had some interesting documents. New information was collected to the end of the thesis.
2.1 Research methodology
This thesis is the result of a case study at Saab Systems main o ce in Järfälla, near Stockholm about the implementation of lead-free soldering in highly reli-able applications. A case study can be made in several approaches, Patel and Davidsson describes the research process in sex stages. [57]
Method
1. Collect knowledge 2. Specify the problem 3. Decide
- research group - research planning
- technique to collect information 4. Carrying through
5. Process/Analyse 6. Account/Report
It would be best to follow the stages step by step, but that is not possible in re-ality. The literature study, data collection and analyse were made parallel. The writing of this thesis has been made continuously as new information arrived. The sources to this thesis are interviews, internal documents and other written sources.
2.1.1 Qualitative - Quantitative research
Quantitative case studies testing a theory and qualitative case studies create a theory. Qualitative data consists of detailed descriptions of situations, oc-currences, people, interaction and observed behaviours. The information can consist of quotations, protocols, letters, and case records.
A qualitative study is presented with words, however qualitative data is pre-sented with numbers. Quantitative information can tell us how many, how much and what the distribution looks like. Qualitative case studies are based on qualitative information which is collected from interviews, observations and documents. Quantitative information from e.g. survey studies can be used to support the results from the qualitative data. This thesis is made with the assumption of a qualitative method, because the purpose is to understand the meaning of a certain phenomenon.
2.2 Interview methodology
The interviews were semi structured, which means that they were formal and controlled by a number of questions or problems which will be explored. The results are a combination of open and rm answers. This kind of interviews makes it possible to adapt to the situation as it develops.
In some cases the interviews have been unstructured because insu cient infor-mation has been available for relevant questions. This was the case especially in the beginning of the study. The unstructured interview is supposed to give enough information to ask related questions later in the case study.
2.3 Theoretical studies
2.3 Theoretical studies
The base of this thesis work is a literature study of books, articles, science reports, internal documents and interviews.
2.4 Data sources
The following data sources were used in this thesis work, with a short description of why the source was used.
2.4.1 Saab
• Internal documents
The internal documents were important for the currant status analyses. • Meetings
Meetings with subcontractors etc. were important for the currant status analyses and lead-free issues.
• Interviews with:
- Christer Melander, Senior Production Engineer, Saab Systems Supervisor
- Jan-Eric Spjuth, Production Engineering Manager, Saab Avitronics Chairman in a working group in Saab, which is working with problems regarding lead-free soldering. Spjuth is also a member in the European Lead Free NETwork.
- Conny Svensson, Design Support, Saab Avitronics/Bodycote
Good knowedge in lead-free soldering, will have a road-show at Saab Sys-tems about lead-free soldering.
- Magnus Porsmark, Saab Avitronics - Thomas Cadring, Saab Avitronics
2.4.2 External sources
• Research documents
Research documents and jounals were necessary to answer the question if lead-free soldering is reliable etc.
• Seminar
A seminar about reliability risks and the need of Swedish research. • Interviews with:
- Håkan Hådeby, Manager, Strategic Technology Development, Ericsson - Anders Ekelöf, Production Engineer, Ericsson
Method
- Benny Gustafson, Specialist-SMA Process, Engineering PBA Technology, Ericsson
- Ove Isaksson, Production Engineer, Ericsson
- Kent Stenberg, Senior Production Engineer, Ericsson - Axel Tchimanga, PBA Technology, Ericsson
Ericsson is also producing highly reliable applications, therefore they could provide much useful information. Their information was useful for an-swering research questions number 2-6.
- Peter Back, Director Manufacturing Engineering, Enics
Back has worked with lead-free issues for a long time and could therefore provide much useful information. He had information regarding research questions number 2-6.
- Per-Erik Tegehall, Ph.D. IVF
IVF is the Swedish national network in the European Lead Free NET-work. Tegehall was interviewed because he had information about lead-free reliability, research question number six.
- Lars-Gunnar Klang, Cross Technology Solutions
Klang has a long experience in lead-free soldering issues. He had infor-mation about lead-free reliability, research question number six.
2.5 Structure of the thesis
The thesis is divided into nine chapters and two appendixes. Chapter 1: Introduction to the thesis.
Chapter 2: Describes the methodology in the thesis.
Chapter 3: Describes the international legislations on hazardous substances in the EU, USA, Japan, China, Australia and South Africa. The Euro-pean countries were the rst to restrict the use of hazardous substances. The RoHS directive is described in the beginning of this chapter. The legislations in the other regions are also described.
Chapter 4: Presents the current status on lead-free soldering with focus on Europe and Saab.
Chapter 5: Describes issues regarding storage and marking.
Chapter 6: Describes issues with lead-free soldering such as education, inspec-tion, temperature, rework and repair. Lead-free alternatives regarding solder, surface nishes, etc. are presented.
Chapter 7: Presents the results of some reliability testing. Problems with whiskers, voids, mixed assembly, etc. are also presented.
Chapter 8: The questions of interest are discussed and some criticism of the chosen methodology.
2.5 Structure of the thesis Chapter 9: Compiles the general conclusions.
Appendix A: Glossary
Chapter 3
International legislations
This chapter describes the EU, RoHS directive and the legislations in the USA, Japan, China, Australia and South Africa.
The EU, Japan, California (the USA has no domestic regulation) and China are closed markets for exporters from the dates as set out. South Africa has no pending directive. Table 3.1 shows the dierence between four regions. [60]
EU July 1, 2006 Japan July 1, 2006 California Jan 1, 2007 China Mar 1, 2007 Korea July 1, 2007 Taiwan Pending Australia Pending
EU China Japan California
Restricted
Materials 6 materials Same as EU SameEU, butas info. only
4 havy met-als
Scope 10
cate-gories Long(incl. cap-list ital equip-ment) 7 home app. & computer products Video dis-play device Exemption De ned No petition Not
apllica-ble Same as EU
Marking
Req. None Yes Yes None
Packaging
Materials No impact Non-toxic /recyclable None None Tetsing/
Certi ca-tion
No
prerequi-site China Com-pulsory Cer-ti cation
No
prerequi-site No prerequi-site Table 3.1: Legislations
International legislations
3.1 EU, RoHS directive
The directive 2002/95/EC of the European parliament and of the council on the Restriction of the use of certain Hazardous Substances in electrical and elec-tronic equipment took eect in the European Union on July 1, 2006. [41] The purpose of the directive is to approximate the laws of the EU member states on restriction of the use of hazardous substances in Electrical and Electronic Equipment. RoHS should also contribute to the protection of human health and the environmentally sound recovery and disposal of WEEE (Waste Electri-cal and Electronic Equipment). [42] The RoHS directive is complementary to the WEEE directive. They were created for the same reasons, but with dierent purposes. There are six restricted substances in the RoHS directive; lead, cad-mium, mercury, hexavalent chrocad-mium, PBB (Polybrominated Biphenyls) and PBDE (Polybrominated Diphenyl Ethers). The directive increases the possibil-ity to make an economic pro tabilpossibil-ity of recycling of WEEE and decrease the negative health impact on workers in recycling plants. The regulations con-cerning the producer responsibility of the use of certain hazardous substances in EEE covers eight of ten categories in the WEEE directive. See table 3.2 for the categories covered by the RoHS and WEEE directives. [41] RoHS covers all products in the WEEE directive except two categories; medical equipment (category eight) and monitoring and control equipment (category nine). The maximum concentration level, by weight in homogenous materials is 0.1% for all the substances except cadmium, which has a maximum of 0.01%. [24] Homogenous material is a concept in the RoHS directive which is a material that can not be mechanically disjointed into other materials. Homogenous means "of uniform composition throughout". Mechanical disjointed means that materials can be separated by mechanical actions such as unscrewing, cutting, crushing, grinding and abrasive processes. [24]
1. Large household appliances 2. Small household appliances
3. IT and telecommunications equipment 4. Consumer equipment
5. Lighting equipment
6. Electrical and electronic tools (with the exception of large-scale sta-tionary industrial tools)
7. Toys, leisure and sports equipment 10. Automatic dispensers
Table 3.2: Categories covered by the RoHS and WEEE directives Another concept in the RoHS directive is "put on the market", which is the initial action of making a product available for the rst time on the community market. This happens when the product is transferred from the producer or a distributor or nal consumer or user on the community market. Even if the product model was on the market before July 1, 2006, it has to be converted to RoHS if it is transferred on the community market after July 1, 2006. [24, 26, 42]
3.2 USA
3.1.1 Exceptions
The RoHS directive will expand as soon as scienti c evidence is available and will take into account the precautionary principle. The prohibition of other hazardous substances and their substitution by more environmentally friendly alternatives which ensure at least the same level of protection of consumers should be examined. [41] A Technical Adaptation Committee (TAC) advice the EU on the exceptions. Exceptions will be carried out only on technical criteria. The exceptions are valid for a maximum period of four years and subject to a review. For every new exception a stakeholder consultation will be organized. The exceptions are granted for speci c applications and not to whole products. Exception is possible for materials and components if elimination or substitution via design changes is technically or scienti cally impracticable. If negative environmental health and/or consumer safety impacts caused by substitution are likely to outweigh the environmental health and/or consumer safety bene ts thereof, is an exception possible. [24, 42] Two categories (category eight and nine) and military applications have exceptions today. In general it is expected that all exceptions (category eight and nine) will be in the scope of RoHS, but not before 2010. [24]
3.2 USA
The United States Environmental Protection Agency (EPA) has rated lead as one of the top 17 chemicals which implies the greatest threat to human health. [31] However there is no domestic regulation that directs US electronics manu-facturers to introduce lead-free solder. [39]
3.3 Japan
Japanese electronics manufacturers acted early on the EU directives and have come farthest with lead-free products. Japanese Electronics and Information Technology Industries Association (JEITA) is an important organization in Japan which actively promotes environmental protection measures. JEITA pro-poses industry policies, supports technological development and promotes the diusion of products in new elds. [31]
Japan has the following legislations: [62]
Fundamental Law for Establishing a Sound Material-Cycle Society (2001) LPEUR, Law for the Promotion of Eective Utilisation of Resources (2001) HARL, below LPEUR comes the Home Appliances Recycling Law (2001) GPL, Green Purchasing Law (2001)
International legislations
Japanese RoHS (2006). The Japanese RoHS is a marking rather than re-striction law. Many Japanese companies put in place voluntary RoHS agreements in the late 90s.
3.4 China
A RoHS like law has been introduced by the Ministry of Information Industry (MII), "Management Methods for Pollution Prevention and Control in the Pro-duction of Electronic Information Products". [31] China RoHS will take eect in March 1, 2007 and control the same six substances as EU. China has published three implementation standards to support China RoHS. See table 3.3 and 3.4 for the three standards and the categories in the China RoHS. [43, 52]
1. The Limitation of Hazardous Substances in Electronic Products 2. Testing Methods for Hazardous Substances in Electronic Information Products
3. Marking for control of pollution caused by electronic information products
Table 3.3: China implementation standards
1. Radar 2. Telecom
3. Broadcast and TV 4. IT equipment
5. Household Electronic Appliance 6. Electronic Measuring Instrument
7. Electronic Industry Production and Manufactring Equipment 8. Electronic Component and device (including battery)
9. Medical equipment 10. Electric special material
Table 3.4: Categories covered by the China RoHS directive
3.5 Australia
The RoHS directive has not aected Australia so much, however it has implica-tions for local manufacturers. Australia's Environment Protection and Heritage Council (EPHC) are exploring the possibility of introducing RoHS measures. EPHC consists of state, territory and federal environment ministers. Issues as-sociated with implementing a RoHS-like scheme in Australia was discussed in
3.6 South Africa October 2006 on the behalf of the EPHC. A preliminary economic and
envi-ronmental assessment of the implications to implement RoHS in Australia is in progress. [58, 59] Australia is one of Saab Systems "domestic markets" [1].
3.6 South Africa
South Africa runs the risk of being left behind, according to Eileen Leopold CEO of South African Electrotechnical Export Council (SAEEC). Many South African companies are unaware of the full implications of the directive and are not taking the necessary steps to deal with them. Some initiatives have however been taken at company level and by industry associations. The SAEEC will set up a workshop to look at the implications across aected industry sectors. Industry needs to lobby for appropriate legislations locally and work with the export councils to develop support programs, according to Leopold. [60, 61] South Africa is one of Saab Systems "domestic markets" [1].
Chapter 4
Current status on lead-free
soldering
ELFNET published a lead-free soldering implementation status survey in 2006. The results of that survey were that the majority of the companies felt well informed. 60% of the companies are producing lead-free products, but about 20% are still not active in lead-free soldering. Main problems are temperature issues and component supply. SAC (tin-silver-copper) is clearly favoured for all types of soldering, SAC305 (3.0% silver and 0.5% copper) is the dominating SAC solder. Alternatives to SAC solders are considered. [24] SnCu is the second favourite for wave and manual soldering. Solder ball and nish materials were varied widely. More than 80% reported that they changed technical equipment due to lead-free introduction, 72% are making large or moderate changes. A big majority change or will change their assembly/component design. 60% of the companies would prefer a "RoHS compliant" label, only 20% opting for "Lead-Free". However less than 30% felt that labelling is necessary at all, less than 20% were already labelling products. About half do not intend to label at component level. 60% were aware of industry standard labelling systems such as IPC1066/JESD 97, but less than 10% were using them in practice. Around half of the respondents had been requested to supply RoHS compliant products. A majority have changed logistic system due to lead-free introduction. [23] The industry in Sweden is for the most part prepared, 95% of the components are lead-free. Awareness is generally high, but some are still in a wait and see mode. RoHS and lead-free education is an issue according to the Swedish national network IVF. Logistics concerns are the major challenge in all European countries. Availability of RoHS compliant components is one important issue, another problem is that fake lead-free components are sold in some countries. The main topic for further research is reliability aspects of lead-free solders. In many European countries it is expected that small assembly companies will be out of business after July 1, 2006. They can not aord the investments and cost associated with the transition to lead-free soldering. [24]
Japanese electronics manufacturers have worked with lead-free production for a long time and have a big lead e.g. to European countries. More than 90%
Current status on lead-free soldering
of the domestic electronics in Japan was expected to be lead-free by the end of 2003. [31] 73% of the Japanese companies prefer to use SAC in lead-free soldering. [63] Japanese manufacturers should have noticed reliability problems with lead-free soldering, because they have used it for several years now. In some tests in China at the end of November 2006 lead-free solder joints showed very little dierence to lead joints. Good wetting properties but a little more greyish solder joints were the results.
Siemens, Ericsson and Nokia were leading the development towards lead-free sol-dering around year 2000, according to Peter Back at ENICS. However it ceased between the years 2002-2004. [43] Ericsson had to slow down the changing pro-cess because of the lack of lead-free components and the high cost of lead-free components. When the supply increased a few years later they continued the transition process. Some of Ericssons applications manufactured in Kista are excepted from the RoHS directive, but some of them are produced lead-free anyway. They do that to simplify the logistics and because of the last time buy of lead components. Ericsson has a mixed production today, but up to 90% of the components are lead-free. [16] When HP, Motorola, Microsoft and Dell started their work at year 2004, the pressure at the component manufacturers increased again. During the end of year 2005 it came a new solder paste every third week. ENICS have produced lead-free products the recent six months, they believe they have an advantage to many others because they made the transition to lead-free soldering slowly. If the process is well controlled ENICS recommend using lead-free soldering. It may be a little trial and error in the beginning. The lead-free process have changed and improved a lot the last two years.
Peter Back at ENICS has a feeling that the reliability has decreased for consumer electronics the recent years, which could be a possible concern for lead-free soldering. However consumer products are not high reliability applications. He is also concerned that most tests are not made during full capacity in the factory. The test conditions are not the same as in mass production. [43]
4.1 Current status at Saab
A working group called TTG-RoHS (Technical Transfer Group) has been started in Saab. They are working with issues regarding the RoHS directive. Problems regarding lead-free soldering are their main task. [12] Saab Systems is a company with very special products in very small batches. The most of Saab Systems applications are excepted from the EU directives. All business units are using electronic equipment in their products so every department in Saab Systems will be aected. Some of Saab Systems customers are demanding lead-free applications. It is Saab Systems opinion that it is important to discuss the technical consequences with using lead-free soldering, because of the uncertainty in reliability. Saabs working team visited "Elektronikproduktionsmässan" in Älvsjö and everyone agreed that the RoHS directive will cause a lot of work and problems. One positive eect of the directive is that the communication between manufacturers and designers increases according to Saab. Manufacturers believe that the problems will be solved, but investments are necessary. [5, 6, 7, 9]
4.1 Current status at Saab Saab Systems is planning to use lead-free processes as far as possible in new
projects. Component selection must be made so it is possible to produce in lead processes for 2-5 years from now, according to Saab. Because of the concern of critical components such as BGA components, Saab believes it is imprtant to secure the supply of these components with lead design. Old applications are the major concern. [8, 10, 11]
Chapter 5
Logistics
The RoHS directive is 90% about administration and logistics problems and 10% about technical problems. [25]
5.1 Storage
The higher solder temperature in a lead-free process makes it even more impor-tant to secure that the components have low moisture content. Moisture can cause delamination and pop corning. The risk of cracks also increase, therefore it is crucial to follow the storage recommendations. There are no new recom-mendations, but it is even more important than before to follow the existing recommendations (IPC-JEDEC J-STD 033A). It could be necessary to have controlled storage environments. Storage in a nitrogen environment could be an alternative for lead-free components, which is recommended by some component and board manufacturers. [4, 5, 6] Lead-free components and boards are consid-ered to have a shorter shelf life. Routine checks are needed so the components are used before the performance, etc. decreases. Today, help systems to prevent this problem exist. It is some kind of indicator that shows the time left before the components needs to be assembled. The recommendation for storage time, etc. from the suppliers should therefore be followed. [1, 10, 13] See gure 5.1 for humidity labels.
Saab Avitronics in Kista has a relative humidity of 15-20% in their storage facility. The storage limit should be one year for lead-free components, but they have a storage time of up to ve years. [15] It is necessary for Saab Systems to do an inventory check and separate lead and lead-free components, according to Saab Systems. [8, 10]
5.2 Marking
It is important to mark components and boards, this according to Conny Svens-son at Bodycote, a company making analyses for Saab. [13] One concern is
Logistics
Figure 5.1: Humidity labels [48]
that dierent component manufacturers are using dierent labelling systems. [9] Lead-free components are not always marked, which makes it di cult to separate lead and lead-free components. Some components may be marked to contain lead, but actually found to be lead-free and vice versa. Therefore it is important to test and inspect incoming items. [6, 39] Saab Systems opinion is that all lead-free components, boards, etc. should be marked according to avail-able standards. [12] Lindebergh one of Saab Systems subcontractors is marking components, but they do not mark the printed boards. [3] Ericsson in Kista is marking their printed boards according to the IPC standard IPC-JEDEC Std nr 97, where SAC solder has the denotation e1. Ericsson is also labelling their products after repair, but no marking is used on component level. [16]
5.2.1 IPC1066/IPC-JEDEC STD NR 97
To be able to detect the kind of alloy that has been used on the product that comes in from eld for fault nding and repair, it is important to mark according to the available standards. The categories in table 5.1 are meant to describe the Pb-free 2nd level interconnect terminal nish/material of components and/or the solder paste/solder used in board assembly.
Lead-free identi cation label is a label that indicates that the enclosed com-ponent/devices and/or assemblies do not contain any lead. See gure 5.2 for dierent lead-free labels. [46, 47]
5.2 Marking
e1 - SnAgCu (SAC)
e2 - Other Sn alloys (e.g. SnCu, SnAg, SnAgCuX, etc.) (no Bi or Zn) e3 - Sn
e4 - Precious metals (e.g. Ag, Au, NiPd, NiPdAu) (no Sn) e5 - SnZn, ZnX (no Bi)
e6 - Contains Bi
e7 - Low temperature solder (< 150◦C) containing In but no Bi Table 5.1: Solder denotation
Chapter 6
Lead-free issues and
alternatives
The personnel will need special training because of the transition to lead-free soldering and the inspection methods will have to be revised and updated. [27] When the transition to a lead-free process is made, it is important to modify the product to be material and process compatible. It is crucial to nd alternative solutions and components, when direct replacements are not available. [50] A product previously manufactured as non RoHS compliant, converted to RoHS compliant is changed in form, t or function. Therefore ENICS requires that the part number is changed, either in version, revision or part number. [51]
6.1 Education
The personnel need to be trained so they know why they have to do things dierently and the consequences of not changing. A lot of training and se-cure routines are needed. [1, 13] A common training and a special training for assemble and repair personnel are necessary. [8, 10] The employees at Saab Systems subcontractor Lindebergh will be trained to understand the dierence with lead-free soldering. Everyone at Ericsson in Kista has participated in a 30 minutes lead-free course. Repair personnel had a special external training. Ericsson wants everyone to know the meaning of the RoHS directive and that it is not only lead that is banned in the RoHS directive. [16] Saab on the other hand is considering introducing a solder certi cate for those who are soldering RoHS applications. A green area could also be introduced in the production, to prevent mixing lead and lead-free soldering. [1, 2] Ericsson is marking lead-free equipment and benches, but they have not separated lead and lead-free pro-duction. However in the repair area lead and lead-free work is separated. [16] Lindebergs do not have a green area, they only mark their lead-free equipment. [3]
Lead-free issues and alternatives
6.2 Temperature
Elevated temperatures may cause damage to circuit, components, insulations, plastic parts and delamination of circuit boards. [39] The higher melting point aects every stage of the lead-free manufacturing, from assembly to the testing processes. [31]
The solder temperature increases 30-40◦Cwith lead-free soldering, see gure 6.1 for the temperature pro les. Small heat sensitive components are a big problem together with BGA (Ball Grid Array) components which need high temperature to re ow. It is important to think about the layout, copper balance, low delta T and consider using high temperature surface boards. [9, 10] Ericsson has design guidelines for the board layout, but no demands. [16] Nitrogen can be used to manage the tighter process window. Vapour phase is another alternative. New types of uxes needs to be developed to meet the dierent process demands. The possibility to repair lead-free products will be signi cantly reduced. [5, 12, 13]
Figure 6.1: Temperature pro les [31]
SAC solder has a melting point of 217◦Ccompared to SnPb (tin-lead) solder which has a melting point of 183◦C. Old components can withstand a maxi-mum temperature of 245-250◦C, which mean that the process window for re ow soldering has narrowed. A small deviation outside of the process window could have a dramatic eect on the reliability. If the temperature is too low, the solder will not form a proper joint. If more heat than required is applied it will impact reliability. The peak temperature to be achieved during the re ow process needs to be approximately 15-20◦Cabove the melting temperature of the solder paste. Delta T is the dierence between the highest and lowest temperature observed across the board. It is very di cult to balance the temperatures across the board in a mixed technology PCB (Printed Circuit Board) assembly. Tests with various peak temperatures (Tpeak) and time above liquid (TAL) were carried
6.2 Temperature out with QFP (Quad Flat Pack) and BGA components. The BGA components
in this test had lead-free solder bumps, the other components had pure tin nish on the leads. In some combinations, the solder paste did not even melt. Even if time above liquid increased, lower peak temperature produced incomplete re ow. Higher peak temperatures and increased time above liquids produced very dull solder joints. Higher temperatures (240◦C) and lower time above liquids (25 sec.) produced shiny joints. See table 6.1 for the results of the visual inspection of the joints at dierent TAL and Tpeak.
Run Set Tpeak Set TAL Visual inspection of joints
1 225 25 Incomplete re ow
2 225 50 Incomplete re ow
3 225 95 Incomplete re ow
4 225 125 Incomplete re ow
5 240 25 Shiny joints
6 240 50 Less shiny joints
7 240 95 Dull joints
8 240 125 Dull joints
9 255 25 Shiny joints
10 255 50 Less shiny joints
11 255 95 Dull joints
12 255 125 Dull joints
Control 246 64 Less shiny joints
Table 6.1: Visual inspection results
This phenomenon could be attributed to the ux and the transformation it undergoes due to the higher temperature. Passive components of all sizes showed good solder joints. Solder wicking was observed during visual inspection of the solder joint for some combinations. If too much wicking occurs, there will not be enough solder left in the pad to form a good solder joint. It could also damage the component. The peak temperature has a major role in the amount of solder wicking. When time above liquids and peak temperature are speci ed outside the solder paste manufacturer speci cations, it in uences the strength of the solder joint. A lead-free process is less forgiving, pro ling revealed higher delta T across the PCB. However, no physical damage was noticed to the passive or active components by moving the peak temperature and time above liquids outside the process window. The eect of deviation from the process window on the fatigue life of the solder joints should be investigated. [19]
The peak temperature during re ow has to reach at least 240◦C, compared to 220◦Cfor SnPb solder. The process window in re ow decreases from 30-40◦C for SnPb to 20-25◦C for SAC solder. Total wetting on the PCB could be at risk. Tighter process window is a result of the need to minimize the upper temperature to avoid damage on sensitive components. The recommendation is to extend the time above liquid instead. The higher process temperature means that IR ovens will probably not be able to manufacture good quality solder joints on complex PCBs. IR ovens have larger temperature dierence between dierent components on the board. It is strongly recommended when soldering with SAC solder to build the temperature pro le in the oven according to the
Lead-free issues and alternatives
solder manufacturers recommendations. The pro le has to be optimized for the paste used, component types, board type, etc. See table 6.2 for temperature characteristics. [31]
Table 6.2: Lead and lead-free temperature characteristics [31]
6.2.1 Nitrogen
Nitrogen gives shorter wetting time, better wetting and fewer voids in the solder joints. The number of voids is much less and the size of the voids is smaller when nitrogen has been used in re ow soldering. NPL measurements with SAC solder showed that nitrogen decreases the wetting by half the time needed for wetting in air. The time above liquid can therefore be shortened by the use of nitrogen. The stability of the process increases dramatically when using nitrogen. If simple standard PCBs are manufactured, nitrogen is probably not necessary to get good solder joints. [31]
ENICS does not believe that nitrogen is necessary today, but if the layout is very bad it could be wise to use nitrogen. Six years ago it was necessary to use nitrogen, but today's solder pastes have solved the problem. Nitrogen only makes the process window a little bigger and creates shinier solder joints. [43]
6.3 Soldering process
Vapour phase is an alternative to use nitrogen, the main bene t is the low delta T across the board. Low delta T is important to get good soldering across the board. Vapour phase is a slow process but with fast heating of the board, which can be both good and bad. [9] It is easier to control a vapour phase process than a conventional process and mixed assembly could be possible. [13] This is a reason why Saab Avitronics in Kista is using vapour phase. [15] Ericsson is using a vapour phase oven for small batches, but conventional ovens in the mass production. [16] Saab Systems subcontractor Lindebergh is using a conventional oven with dierent heat temperature pro les for lead and lead-free soldering. It is important to be extra careful with complex boards. [10] Due to the transition
6.4 Solder to lead-free soldering Ericsson replaced old equipment that could not reach the
maximum temperature of 235◦Cin ten seconds. [16]
The recommendations are to use a solder bath temperature between 250-260◦C, with 250◦C as an absolute minimum. A Norwegian lead-free project shows acceptable or good results at 250, 255 and 260◦Cfor SnCu and SAC alloys. [31]
6.4 Solder
The industry has studied a wide range of alloys to replace the SnPb alloy dur-ing the last decade. The selection of a new alloy is based on a number of considerations; toxicity, melting temperature, surface tension, wetting ability, mechanical properties, electrochemical properties, cost, etc. The candidates for wave soldering are SAC, SnAg (tin-silver) and SnCu (tin-copper). Currently SAC alloy for re ow and SAC or Sn0,7Cu for wave soldering are the favourites. The general belief is that SAC alloys with a silver content of 3.0-4.0% are all acceptable compositions. Studies by IPC, solder suppliers and electronic man-ufacturing companies show that there is no signi cant dierence in the process performance and thermo mechanical reliability for these alloy compositions. [27, 31] Ericsson follows these recommendations and allows SAC alloys in these in-tervals. The SAC alloy is used for the entire board, they allow SAC, SnAg, SnCu and SnCuNi (tin-copper-nickel) in wave soldering and selective soldering. [16]
SAC alloys outperform the SnCu alloy in terms of wetting ability and reliability. SnCu has a much lower cost than SAC and that makes it an attractive alterna-tive alloy for wave soldering. Most manufacturers prefer to use the same alloy for the entire board. Some volume production use SAC for re ow soldering and SnCu for wave soldering on the same board. In those cases, methods for inspec-tion and rework must be compatible with both alloys. SAC and SnCu are the lead-free solder alloy choice for the most of the worldwide electronic industry. [27] Manufacturers in Asia are using SAC for high reliability applications and some other alloys for consumer goods. [13]
6.4.1 Tin-silver-copper
There are two recommendations, one by iNEMI (International Electronics Man-ufacturing Initiative) SAC396 (for re ow) and one by JEITA SAC305 (for re ow, wave solder and hand solder). Japanese electronics companies will start to use SAC305 and is therefore more interesting. [63] NEMI has proposed and tested SAC396 for re ow soldering and SnCu for wave soldering. [22] SAC is the most popular replacement to SnPb solder. SAC tested solder alloys with Ag between 3-4% and Cu between 0.5-1.5% are recommended. The concerns with this alloy family are higher processing temperatures, leaching of silver from the solder into land lls and metals cost 2.5 times that of SnPb eutectic. [31]
Assembly of 160 PCBs at IVF printed with SAC solder paste did not show any signi cant dierence compared to assembling on SnPb paste. One hour between printing and mounting gave no visible decrease in assembly quality. Lead-free
Lead-free issues and alternatives
solders have higher surface tension than SnPb solder, however the solderability is not as good for SAC and the time above liquid has to be longer to give a good re ow. Components self centre almost as well in SAC solder according to tests performed by NPL. [31]
Tests made by NOTE shows good thermal cycling and mechanical stress with SAC305. It is less sensitive for lead contamination than the other alternatives and is recommended by several manufacturers. Asian manufacturers are in-creasing their use of SAC305. The melting point is 30◦C above SnPb solder, the wetting ability is not as good as for SnPb. [13] NOTE recommend a SAC solder process. [29] Toshiba and Hitachi are using SAC396. [22] Mitsubishi is using some kind of SAC solder. [53] Lindebergh is also using SAC (SAC387). [3]
6.4.2 Tin-silver-copper-bismuth/Tin-silver-bismuth
The advantages of SACB (tin-silver-copper-bismuth) compared to SAC are a lower melting point and better wetting ability. The disadvantages are that too much bismuth decreases the mechanical properties and problems with lift-o in printed hole assembly. It is also very sensitive to lead contamination. [29] The SnAgBi (tin-silver-bismuth) alloy is a candidate for SMT (Surface Mount Technology) applications. One big concern with bismuth are llet lifting that occurs in SnPb through hole applications, toxicity and low melting phase. Other concerns are lack of compatibility with lead bearing nishes, leaching of silver into land lls, metals cost 2.5 times that of SnPb eutectic. [31]
6.4.3 Tin-zinc
SnZn (tin-zinc) has a lower melting temperature (200◦C), a thin melting area and lower metals cost. The concerns are oxidation of zinc, long term corrosion of the nished soldering joint, which requires special ux and wetting that is not as good as with SAC. It is used for the toy industry. [29, 31]
6.4.4 Tin-copper
The SnCu alloy is a low cost alternative for wave soldering, it is compatible with most lead bearing nishes. Process considerations must be taken because of the high melting temperature (227◦C). The mechanical properties are not as good as for SnPb. Metals cost 1.5 times that of SnPb eutectic. [29, 31] It is used by Matsushita. [22] SnCu is recommended by iNEMI as wave solder and also by JEITA (their second choice for wave solder).
6.4.5 Tin-silver
The SnAg alloy has a little higher melting temperature (221◦C) than SAC and similar cost to SAC. SnAg has been used for years in special applications. [31] SnAg is recommended by JEITA as re ow solder (their second choice). [63]
6.5 Components
6.5 Components
The peak temperature required for components for lead-free soldering is 260◦C. The actual component body temperature may be dierent from the tempera-ture measured on the board, because of component thermal characteristics and locations on the board. The IPC/JEDEC standard J-STD-020-C speci es that a lead-free component shall be capable of being reworked at 260◦Cwithin eight hours of removal from dry storage or bake. Soldering temperatures and tol-erances are captured in the standard. [27] Ericsson make demands on their component suppliers that they should follow the IPC/JEDEC standard J-STD-020-C, to secure that the components can withstand the higher temperature. [16]
6.5.1 Supply
Saab believes that the supply of lead components will stop. They are concerned that big electronic manufacturers will make end of life (EoL) purchases, which will make it hard to get lead components. Most component manufacturers mean that the supply of lead-free components will not be a problem, but the supply of old components will be a problem. [5, 6] It is important to secure the supply on critical components such as BGA. [10, 11]
6.6 Printed surface boards
The higher process temperature has to be considered before the printed board selection. Common sti boards as FR4 (Flame Resistant 4) will not be more seriously aected in wave soldering compared to SnPb, because of the short exposure time to the molten solder on the wave side. For re ow soldering there is an increased risk of bending the board which can cause fall o of components in the second re ow cycle in double sided mounting. [31] The surface boards only manage 4-5 re ow cycles in lead-free soldering, after the rst repair three or four of the available cycles are used. [43] Alternative board selection must be considered if several re ow soldering processes are applied. One solution is to use a support in the middle of the board, but this requires component free areas where the support contacts the board. Higher Tg laminates is another alternative. [31]
According to Saabs working team, common FR4 is not appropriate in lead-free soldering, because of the high temperature. [6] For complex multilayer boards, common FR4 will probably not be a good alternative. For each re ow process the board is aected negatively. An alternative to common FR4 is HTg FR4 which has a 190◦CTg compared to 130◦Cfor common FR4. 370HR is an HTg board type which should manage six re ow soldering processes. [9] Ericsson is still using common FR4 boards. They made demands on their suppliers that the printed boards should withstand six re ow cycles. Their suppliers could only guarantee four re ow cycles. Ericsson is therefore investigating if they need to start using high temperature printed boards. They believe it is important to
Lead-free issues and alternatives
make demands on the suppliers regarding which temperature and how many re ow cycles the board should withstand. [16]
6.7 Flux
Attempts to mix no-clean ux (developed for SnPb) with lead-free solder alloys gave catastrophic results. The higher temperature for lead-free solders requires greater stability of the ux at higher temperatures. Water-soluble uxes for lead-free solder paste and wave soldering applications will also be needed. [27] To improve the wetting properties, development of the ux chemistry is a key issue. [31]
6.8 Surface nishes
The purpose of the surface nish is to provide a solderable surface for com-ponent assembly. It protects the copper surface from oxidation and provides an appropriate surface. [40] Board manufacturers must select a surface nish based on cost, reliability and shelf life. [31] HASL and OSP have a low cost in comparison with other nishes. The best wetting results on fresh boards for lead-free soldering have ENIG and immersion tin, followed by immersion sil-ver and OSP. Immersion tin degrades fastest, followed by immersion silsil-ver and OSP. If it is a fresh board, the immersion silver nish can withstand up to four re ow cycles before the nal re ow soldering process. Two re ow cycles before the wave soldering process is possible. [27] Table 6.3 shows the environmental aect of the dierent surface nishes, low numbers are preferred. [45]
HASL ENIG OSP I-Sn I-Ag
Hazardous substances 5 5 2 5 2 Energy consumtion 3 5 1 3 3 Water consumption 3 5 1 4 1 Material in uence 4 5 1 5 2 Process control 3 5 3 3 3 Recycling 5 5 1 2 2 Total cost 3 5 1 2 3 Environmental index 26 35 10 19 16
Table 6.3: Surface nishes environmental aect
6.8.1 Electroless Nickel/Immersion Gold
ENIG is a metallic surface nish plated onto the copper base with a chemical deposition process. The advantages with ENIG are e.g. excellent corrosion resistance, excellent solderability, excellent atness for ne-pitch technology and excellent shelf life (12 months). [40] Other advantages are good surface contrast, contact resistance and resistance to damage during handling/processing. [31,
6.8 Surface nishes 40] The disadvantages are a more narrow process window, black pads, high
cost process (1.5-2 times higher than HASL) and fatigue failures on large BGA packages. [28, 40]
ENIG provides good solderability and contact interfaces for most applications. It provides a more reliable surface nish, but is in general more brittle than joints between tin and copper. After storage and heat exposure ENIG nishes still have excellent wetting. Tight plating process control is necessary to avoid "black pad" failures. Black pad is due to oxidation of the nickel layer during the immersion gold process. It can lead to catastrophic failures due to the separation of the solder from the pad. [27] ENIG is the predominant surface nish in Japan and the EU. [28]
6.8.2 Immersion silver
I-Ag is a co-deposit of silver and organics. The advantages are excellent sol-derability, excellent for ne pitch and BGAs technologies, very good alternative to HASL and similar cost. It also has a good shelf life (6-12 months). [28, 40] Immersion silver is a less costly alternative, but the solderability and contact pad performance are not as good as ENIG. It is after all an adequate surface nish for most applications. However handling and storage needs to be carefully controlled. [27]
A test made by Sandia National Laboratories investigated the eects of storage environments on the solderability of immersion silver board nishes. SAC396 was used with solder temperatures of 245 and 260◦C. Contact angle less than 50◦ have predicted the successful use of lead-free solders on printed wiring as-semblies. The contact angles values were generally lower and the wetting rates generally faster at the higher solder temperature. There was a signi cant drop in wetting rate after 6 and 9 months. Figure 6.2 shows the storage eect on I-Ag board nishes. A process temperature of 245◦C is preferred in case of re ow soldering. The SAC solder would maintain a su cient solderability over that temperature for a shelf life of 12 months. If the nish were stored for 120 months acceptable solderability was predicted. Poor solderability could be expected between 12 and 120 months. It is therefore preferred to avoid aging times beyond 12 months. SnPb solder would maintain su cient solderability on immersion silver for aging periods of up to 24 months.
Steam aging did not have a signi cant eect on the wetting rate fore a solder temperature of 260◦C. It did aect the wetting rate of SAC solder tested at 245◦Cafter aging of 16 months. The wetting rate rebounded after 24 months. The contact angle and wetting rate showed that the solderability of the im-mersion silver was insensitive to steam aging. Only one exception, a signi cant decrease of wetting rate was found after 16 months. When the solder tempera-ture was increased to 260◦Cthe contact angle increased to 17◦. The conclusion was that steam aging is not appropriate for predicting the solderability storage life of immersion silver for either SnPb or SAC solders. [20]
