Lead-Free Trends and Requirements and the Need for Global Co-operation

Transcription

1 Lead-Free Trends and Requirements and the Need for Global Co-operation Kay Nimmo, Research Director Soldertec at Tin Technology Ltd, Uxbridge, UK Abstract Much of the required background technical work to achieve successful implementation of leadfree soldering has been carried out, or, is now underway. Process issues are also being addressed successfully through work with an ever-increasing range of lead-free product releases. Future work is expected to focus more on the development of standards, test methods and supply chain issues that are affected by the move to lead-free, and this paper outlines some of the activities underway in the US and Europe regarding, e.g. tin whisker and solderability test methods for lead-free, lead-level definition and component MSL. The need for global co-operation for successful outcome from all work in this area is highlighted and examples provided of activities designed to develop lead-free technology roadmaps outlining guidelines for introduction timescales and technical issues requiring resolution. Key words Lead free, legislation, roadmap, alloy 1. Introduction Market demand, waste disposal and treatment concerns and direct legislation regarding hazardous materials have all provided a stimulus for the development of lead-free electronics. The proposed EU Directives concerning Waste Electronic and Electrical Equipment (WEEE) and Restriction of Hazardous Substances in Electronic and Electrical Equipment (RHS) are likely to introduce significant changes to product requirements by target dates yet to be agreed. However, it is likely that the use of hazardous materials, lead, mercury, cadmium, hexavalent chromium, and certain brominated flame retardants will be banned from January 2007 at the latest, or most likely from While certain essential uses are exempt it should be noted that other materials of concern, such as PVC, could be added to the ban in the future. It should also be noted that the scope of the Directives is limited to certain consumer and IT goods. Similar legislation requiring collection and treatment of waste electronics in other regions has also encouraged the development of lead-free products although no direct ban on the metal is in place. The most significant example is the Japanese Home Electronics Recycling Law that came into effect for 4 The International Journal of Microcircuits and Electronic Packaging, Volume 24, Number 4, Fourth Quarter, 2001 (ISSN ) 304

2 product types in April No attempts to introduce national regulation are in place in the US although the Environmental Protection Agency (EPA) has strengthened controls on lead through recent toughening of Toxics Release Inventory (TRI) emission reporting limits [1]. Overall, lead-free assembly is directly linked to, and is essential for, the successful development of efficient recycling systems, specifically the reduction of cost of, and problems associated with, hazardous material treatment. Limiting hazardous material use should be considered at every stage of product design. A significant amount of the required background technical work to achieve leadfree soldering has been carried out, or, is now underway. Process issues have or are also being addressed successfully; mass production of lead-free units has been in progress in Japan since 1998 and is now in widespread use. It is now recognised that there is a need to focus more on the development of standards, test methods and supply chain management models applicable to specific lead-free issues, however, many differences and contradictions exist in the approaches to these questions in various regions of the world. Such differences are clear in, for instance, technology roadmaps for the electronics industry and there is now a requirement for industry, and representative associations, to work together in order to define a common approach. 2. Industry Cooperation 2.1 Regional technology roadmaps One of the most influential roadmaps was that produced in Japan by JEIDA in This reported an expectation for full use of leadfree solder for all new products from 2003, with leaded solders only to be used exceptionally after Many large Japanese OEM s have targets that match or better these deadlines [2]. In the US, the IPC roadmap does not set target dates for the US industry but aims to provide information on expected implementation timescales in other regions [3]. However, guidance is being provided for timescales for product introduction for market leaders, market match companies and, market followers in three standard product class categories [4]. In the first category of Class 1 products all manufacturers are recommended to have a lead-free product available for release by December The North American Electronics Manufacturing Initiative (NEMI) aimed to have the capability for North American companies to produce lead-free products by 2001, with an eye toward total lead elimination by 2004 (although timing of deployment is left to participating companies to determine) [5]. Debate continues in many organisations in the US questioning the need for lead-free processing despite significant research spending and a commitment for development from some sectors. It is therefore expected to be sometime before lead-free becomes the norm in domestic production for the US market, although, the first announced lead-free product (mobile phone i85) was released by Motorola in December Roadmap development in Europe has recently been initiated by Soldertec through publication of a document summarising industry attitudes to lead-free technology and expected production targets. On average, the first lead-free units should be expected in January 2003, use of lead-free in all new products expected July2004 and all products to be lead-free by the legislative deadlines of January 2006 [6]. Figure 2 shows some of the expected introduction timescales. 305

3 Figure 1. Motorola i85 mobile phone, lead-free process in co-operation with Indium Corp. Figure 2. Target dates for lead-free technology introduction in Europe 306

4 2.2 Global Technology Roadmap It is recognised that each regional roadmap developed uses an individual format that makes comparison of industry status difficult. Various initiatives hope to improve this situation and provide global roadmaps or comparative roadmaps from major production areas. As an example JEITA, IPC and Soldertec are working in close co-operation to generate industry surveys in their respective areas through development and agreement on a series of comparative questionnaires. This activity is designed to produce guidelines regarding average industry progress towards lead-free and requirement timescales for leadfree or lead-free compatible materials and components. It is also focused on key technical issues and resolution of those specific issues that have been identified. One example is to obtain data on average industry processing temperatures and profiles for leadfree to provide guidelines for component and material manufacturers. The GECI initiative also hopes to summarise roadblocks and timescales in each region and publish a document to cover the industry on a global basis. The Global Environment Coordination Initiative (GECI) was formed on the proposal of the HDPUG consortia and aims to maximize knowledge and minimize duplication of efforts[7]. The organizations supporting the initiative include the IPC, JEDEC, MEPTEC, NEMI, SAC, SEMI, Soldertec, and ITRI Taiwan. The mission of GECI is to; 1) Agree on a voluntary plan (roadmap) for the electronics industry to replace tin-lead solder with a lead-free alloy 2) Agree on one or, at most, two alloys for the majority of applications 3) Identify key standards that will need to be developed or modified including, for example, design, performance and test standards and 4) Facilitate global cooperation aimed at making the transition as smooth and cost-efficient as possible. GECI is also ready to work on other issues of importance for the electronic industry for compliance with market and legislative requirements. The first draft of the GECI roadblock document can be found on the relevant website [7]. 2.3 Voluntary company agreements In an excellent example of industry cooperation, Europe s three largest semiconductor manufacturers Infineon Technologies, Philips Semiconductors, and ST Microelectronics have announced a combined proposal for what is thought to be the world s first standard for defining and evaluating lead-free semiconductor devices. This inter-company agreement aims to accelerate the use of lead-free packages and to stimulate the further development of leadfree technologies [8]. The initiative shows the three companies commitment to work towards the elimination of lead in electronic systems to improve environmental protection, e.g. in recycling or disposal processes of electronic devices. By sharing knowledge and enthusiasm the companies can make huge steps towards meeting environmental goals. Co-operation on standards and evaluation can continue in tandem to pursuit of individual company research programs to find the most economically and technologically effective ways to remove lead. The three companies have addressed a common definition of leadfree, assessment of solderability and reliability. The collaborators state that the market is confused because there are no rules or standards so far for evaluating alternative technologies. They believe that their cooperation will help accelerate progress, and 307

5 that the joint initiative shows the inevitable transition to green assembly on a global scale. 3. The future depends on international agreement; Materials, processes and test methods Despite regional differences on many details, identified technology roadblocks to lead-free production are usually in basic agreement. It is therefore hoped that international cooperation will be possible in order to achieve industry-wide agreement. These roadblocks cover several issues the most significant of which are described in the following sections. 3.1 Agreement on Solder Composition SnAgCu The SnAgCu alloy family is the favoured mainstream lead-free solder system of the global electronics industry. However, an exact optimum composition has yet to be agreed due to a number of influencing factors, not limited to technical issues, but also covering cost and patent restrictions. Resolution of opinions from various regions has yet to be achieved in order to provide a precise composition recommendation, but the following guidelines are currently proposed; Ag: % Cu: % Sn: balance Further work is required but it is possible that an alloy of approximately central composition in this range may become favoured taking all currently known factors into account. Use of alloys within the composition range defined above is feasible, the exact eutectic is not required in practice. NIST has carried out work that has demonstrated a negligible pasty range between Ag and Cu. The Brite-Euram IDEALS project carried out significant work on the SnAgCu system during the research programme Soldertec [9] recommended SnAgCu as the alloy for mainstream use in October 1999 in the composition range Sn-[ ]Ag-[ ]Cu. NEMI has recommended an alloy of Sn3.9Ag0.6Cu with a +/- 0.2% tolerance on each element since January A significant amount of work has also been carried out in Japan over the last few years on this alloy family with a recommendation of Sn-3.0Ag-0.5Cu solder in the later versions of the JEIDA Lead-free Roadmap. It should be noted that some compositions are subject to patent/licence restrictions in some countries but solder suppliers should be aware of, and be able to provide information on this issue. Other alloys The choice of lead-free solder is more application specific than has been the case in the past with tin-lead. While SnAgCu solder is the recommended choice it is appreciated that in certain circumstances a lower temperature alloy, or, one of lower cost may be preferred. The recommendation of SnAgCu therefore does not preclude the use of other alloys and it is anticipated that users able to make informed choices on the differences between these alloys will prefer to use one or more for specific purposes. Sn-0.7Cu is a low cost alternative to SnAgCu for wave soldering if required. However, there is a growing trend towards the use of small 308

6 alloying additions to this composition to improve production or service performance of this basic solder. An example of such an alloy is Sn-0.7Cu-0.5Ag. In the US, NEMI recommend an alloy of Sn-0.7Cu with a +/- 0.2% tolerance on each element for use in wave soldering. SnBiAg, with the possible addition of low levels of other elements such as Cu, remains a possible alternative to SnAgCu for surface mount products that do not experience high temperatures (e.g. above 90 C) during service. This alloy is in use for the production of consumer goods. A suggested alloy composition range for this solder family is Sn- [ ]Ag-[ ]Bi. Significant interest remains in the development of low cost, low temperature SnZnBi systems for surface mount that overcome the well publicised drawbacks of this alloy. However, development has not yet reached a stage where general use of this alloy is possible. For low temperature soldering the alloy Sn- 57Bi-1Ag can be used. 3.2 Agreement on Lead Level Definition Acceptable levels of lead in an assembly must be defined through consideration of each part of the final product (typically in weight percent). Although an overall average lead content of a product is sometimes proposed this is thought to be an unworkable definition. Solder Permitted lead content relates to lead as an impurity in solder alloys. In the recently released IPC/J-STD-006 definitions for both 0.2% and 0.1%Pb solder alloys are available according to specific customer designation, although 0.2% is the most common. Variation E designation demands a Pb content below 0.10% and an Sb content below 0.20%. The current version of ISO standard 9453 (1990) specifies lead content limits for each lead-free alloy, as Pb impurity control is more important for certain lead-free alloys than others to ensure compatibility problems are minimised. Example: Sn-5Sb, Sn-1Cu, Sn- 4Ag type solders are permitted to contain up to 0.10%Pb. However, Bi-43Sn and Sn-50In are permitted to contain only 0.05%Pb. This document is currently under review. Further discussion and development of these definitions is required and will continue. Components In the previously mentioned proposal from Infineon, Philips and ST the definition of a green package is where Pb, Br, Cl, Sb are not intentionally added but where the elements may be present in the finished units as impurities [6]. The agreed maximum level of lead represents the concentration in the packaging materials, like lead finish, molding compounds or substrates. Definition for Pb content of lead finish and solder balls in components is proposed as Pb < 1000 ppm for the following reasons; 1) Anodes have 500ppm Pb, but Pb can fluctuate locally during plating: the level of 1000 ppm prevents the necessity for process control on Pb content in the terminations, 2) Soldertec, IPC and HDPUG recommend: 1000ppm. It is an achievable and sustainable level, 3) Although legislation has not yet been consolidated, a maximum level of 1000ppm is mentioned in the law of some countries, 4)Hazardous substances at levels below 1000 ppm do not need to be declared, 5) Reduction of Pbcontent below the current low level would mean a refining step that is costly and has a substantial impact on the environment. 309

7 However, in contrast, JEDEC has proposed 0.2%Pb to be an acceptable level. Potential legislative requirements There is some indication of lead impurity level definition within EU legislation and potential levels to be expected in the RHS Directive. Through examination of a draft document relevant to a separate Directive on End of Life Vehicles, quoted below, the intention to define 0.1%Pb as a threshold level for lead-free in vehicles can be seen. It is logical to assume that a similar threshold level will be proposed in the RHS Directives at a later stage of its development, unless specific technical differences can be demonstrated between the case of lead in vehicles (including electronics) and lead in products covered by the RHS proposals. It is in fact evident that a zero level of heavy metals is in some instances impossible to achieve. It is therefore proposed to insert a paragraph in Annex II stating that a concentration of up to 0,1% by weight and per homogeneous material, of lead, hexavalent chromium and mercury and of 0,01% by weight per homogeneous material of cadmium shall be tolerated, provided these substances are not intentionally introduced. Under the same condition, a maximum quantity of 0,4% of lead in aluminium shall also be tolerated. The above thresholds are in line with EC legislation on hazardous substances and preparations and also take into account the need not to hamper the recycling of waste (such as secondary aluminium). [10] 3.3 Agreement on Process Standards Process issues have been identified by several groups as requiring adaptation, or development, to account for the introduction of lead-free technology. These topics include model assembly process specifications at peak temperatures between 235 and 260 C, model board repair processes, and improved understanding of the effects of profiling thermocouple attachment methods to obtain improved temperature measurement. Other issues being addressed by the IPC and others include alterations to joint inspection standards. Components and Assembly Process Temperatures In the soldering process surface mount plastic encapsulated devices must withstand the effect of high temperature and the moisture absorbed in the moulding compound. High internal water vapour pressure may be developed with the possible resultant delamination of package interfaces, and in the worst case, inner and outer cracks [8]. It is possible that Moisture Sensitivity issues, rather than termination finish or lead free solder choice, or lead free solder joint reliability will be the largest problem to overcome for implementation. In the US a JEDEC committee is surveying the industry for information about lead-free processing in order to make changes to the IPC/JEDEC J- Std-020A Moisture-Reflow Sensitivity Classification [11]. In Europe, Infineon, Philips and ST Microelectronics have agreed on common specifications for MSL assessment, with the following criteria; 1) preconditioning levels the same as in the existing J-STD-020A, with the same MSL, 2) delamination criteria more severe than the J-STD-020A so that any crack detected by acoustic microscopy or microsection and any die-top delamination is enough to downgrade the MSL to the next level, and 3) reflow profile defined in a narrower range of values for relevant temperatures, times and ramp up/ramp-down rate, with two possible peak temperatures: 310

8 245 C and 260 C. Full details of the reflow profile parameters are available from the companies involved. This specification is intended to limit the number of variables and to make the assessment of MSL more reproducible and consistent. It must be highlighted that moving to higher reflow temperature can have a remarkable impact on device reliability. It is also necessary to deal with differential peak temperatures for devices with large and small thermal inertia. Preliminary data has shown that while soldering in the range of C has limited impact on the package integrity, the range C can cause severe delaminations and cracks in a large number of devices, with down stepping of their MSL of one or two levels. This underlines current experience and the benefits of maintaining a reflow temperature in the C range, by suitable choice of furnace maximum temperature and belt speed. To become compatible with the C peak temperature, a number of devices must be re-designed by improving one or more of the factors known to affect MSL. However, many Japanese component suppliers are currently able or plan to make this type of temperature resistant product available for lead-free production. 3.4 Agreement on Mechanical and Reliability Testing One barrier against eliminating the use of lead has been a lack of internationally agreed standards and methodologies for evaluating the quality and reliability of lead-free technologies. Mechanical testing of solders has been inconsistent in terms of its performance, and has produced results of limited value. The absence of agreed standards with regard to test-piece geometry, rate of straining and prior history of the materials has led severe difficulties in comparing data between different workers and laboratories. The situation is gradually changing now that the significance of the above factors is recognised. For example, The Japanese Society of Material Strength (Solder Strength Working Group) has produced guidelines for both tensile and low cycle fatigue testing. Equivalent recommendations are emerging from the USA, but unfortunately quite distinct differences exist between the two proposals. This problem applies equally to traditional and lead-free solders and global agreement is urgently needed [12]. The fatigue performance of the more creep resistant lead-free alloys may show significant benefit over results from the less creep resistant tin-lead alloys due to the accelerated nature of many thermal cycling tests. Differences in reliability may be less in service and in test regimes using long dwell and relaxation times, and higher maximum temperatures. It appears that little work has so far been carried out on the adaptation of reliability tests to lead-free solder alloys. This could be achieved through increased understanding of the deformation behaviour of the alloys [13]. The European Structural Integrity Committee (ESIC) aims to bring established and new expertise into the electronics field. Purely empirical testing of boards through techniques such as thermal cycling is of use in the short term, and for product test, but more fundamental approaches are required to underpin reliable design and life prediction which can only succeed through a spirit of global collaboration [14]. 311

9 3.5 Agreement on Solderability Test Parameters Consideration of changes to the standard wetting balance test for bulk solder, spread test and wetting balance test for solder paste is also required. In Japan a project is underway to establish recommendations for change. In the US the ECA Soldering Technology Committee and the IPC 5-23b Component and Wire Solderability Test Task Group are also aware of the possibility that a change in test conditions may be required to accommodate lead free. There are three efforts currently underway to recommend changes to the IPC/EIA J Std-002A Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires [11]. In Europe, the Infineon, Philips, ST cooperation is also considering changes to test conditions, and has agreed a common solderability test which will be used in parallel with the existing IEC and -69, for the assessment of solderability of both Pbbased and Pb-free package terminations. The solder alloy is Sn-[ %]Ag-[ %]Cu. The bath setting is 245 C, compared to 215 or 235 C for SnPb, with a dip time of 3 seconds. This applies to both bath and globule testing. Progress in the IEC will be taken into consideration by these groups. 3.6 Agreement on Tin Whiskering Test and Acceptance Reliable lead-frame device packages are an essential part of any electronics assembly, and one of the major technical concerns within the entire lead-free soldering issue is that of the potential for tin whiskering from termination coatings [15]. While electroplated tin-lead has been used in the past, the requirement for a completely lead-free system means that this must be replaced with other suitable metal coatings. Candidate systems include binary systems such as SnBi, SnCu and SnAg and also pure tin, all of which may be more prone to tin whisker growth if applied in uncontrolled conditions. While whisker growth may be a minor concern in relatively large pitch components such as through-hole packages, it is of much greater potential significance in finer pitch surface mount devices, with the increased risk of short circuit. Although whiskering is not thought to be a great concern with the current tin-lead coatings, it does occur and has been seen in the past. Concerns surround the comparative performance of newly developed tin based electroplating systems, the best method for tin whisker formation testing, and, the acceptable level of whisker occurrence during device lifetime. Resolution of these issues requires the co-operation of several bodies worldwide. The definition of a suitable whisker test is being addressed by projects at Soldertec UK, NEMI US as well as work in other regions. Methods of accelerated growth are mainly based on various temperature/ humidity/cycling conditions, although further work is required to correlate results with activity under normal ambient storage. Importantly, other methods are also being developed to investigate the propensity for whisker growth by examination of coating grain structure [16]. The Infineon, Philips, ST statement also describes plans to guarantee whisker-free components for 2 years as this time corresponds to recommended storage time for devices [8]. The group also plans to define the acceptable length of a whisker. For example, isolated whiskers with a maximum length of 50µm are not expected to impact on 312

10 reliability. Evaluations are continuing in an attempt to define maximum allowable length, density and whisker morphology. At the same-time it must be noted that Japanese component companies are well on the way towards production of lead-free devices and have carried out significant work on the whiskering issue. Their co-operation on this topic is appreciated. Conclusions from a project underway on this issue will be available at a later date. 4. Supply Chain and Life Cycle Issues Methods for dealing with environmental supply network management (ESNM) are still developing. However, the importance of managing the flow of materials, funds and information between business functions or organisations is increasing important and can impact on cost reduction and customer satisfaction [17]. Methodology is likely to be discussed in other papers [18], but it is clear that barriers to development of an environmentally sound supply network include factors such as; complexity of global supply, high cost of information collection, perceived cost or lack of business case, and the difficulty in dealing with soft business issues such as corporate culture, or lack of awareness. These factors are directly applicable to, and can be seen within environmental supply networks (ESN) dealing with legislation on hazardous material reduction and development of leadfree products. Customer/supplier partnerships and alliances are vital for successful ecodesign and must stretch beyond regional barriers as the effects of environmental legislation do. It can be assumed that the cost penalty of hazardous material treatment during recycling is a significant driver towards the use of more environmentally materials (although this obviously depends on the type of product). Hazardous material disposal and treatment charges may vary between areas but as environmental awareness develops in all regions the cost benefits should become more evident. Those companies introducing hazardous material-free product as early as possible will benefit most substantially in the future as recycling regulation and requirements become more stringent. Agreement on component marking systems and other forms of identification is essential to ensure the success of future recycling processes. Agreement on methodology and results from Life Cycle Analysis (LCA) and toxicity assessment is also vital to ensure acceptance of environmentally sound products from one region to another. In Japan the Intelligent Manufacturing Initiative (IMS) initiated in June 2000, funded by the Japanese government and member companies, such as Hitachi, aims to assess the toxicity of alloying elements found in lead-free solders. In the US the EPA together with the IPC is currently in the process of planning a LCA on soldering materials. In Europe some work has been carried out, but, no full LCA is planned by the legislative bodies of the EU. Under the precautionary principle the responsibility lies with industry to demonstrate that chemicals used in their products will not cause significant harm to human health or the environment. This proof must be in the form of extensive toxicity information on the metal/chemical itself, and a detailed risk assessment of possible exposure. This principle is the basis of much proposed legislation, such as the draft EU Chemicals Policy described elsewhere [19, 20]. 313

11 5. Conclusion It is hoped that a global approach to the technical issues discussed above will facilitate environmental change (including lead-free assembly) for the electronics industry worldwide. Through co-operative efforts, such as those of the IPC, JEITA and Soldertec, it is hoped that agreement will be possible on all the issues described in this paper, as well as others yet to be fully considered. Publication of comparative regional and global roadmaps will also provide industry guidance on timescales for implementation of lead-free soldering. Due to the global nature of the business environment, legislative requirements of the WEEE/RHS Directives should be considered as global, rather than solely European deadlines. References [1.] EPA website, [2.] University of Tokyo website, July 2001, [3.] IPC website, July 2001, [4.] Bergman, D. Roadmap of Lead-free Soldering in the USA, JISSO/PROTEC Forum, Tokyo, [5.] NEMI website, July 2001, [6.] Soldertec European Lead-free Technology Roadmap, version 1, February 2002 [7.] GECI Website, July [8.] Press release, Top three EuropeanSemiconductor Manufacturers announce initiative to eliminate lead from semiconductor products, 12 July 2001 [9.] Soldertec website, July 2001, [10.] Draft Commission Decision revising Annex II of Directive 2000/53/EC, December 2001 [11.] M. Kwoka, Intersil, Personal communication, July 2001 [12.] W. Plumbridge, Reliability of interconnections, Training Course presented by Open University, UK, May 2001 [13.] G. Grossman, Swiss Federal Institute for Materials Testing and Research, Personal communication, July 2001 [14.] W. Plumbridge, Open University UK,Personal communication, August 2001 [15.] P. Harris, The Growth of Tin Whiskers, Soldertec Publication 734, 1995 [16.] I. Baudry, Focused Ion Beam in Microelectronics Packaging Applications, Lead-free Plating Analysis, Soldering and Surface Mount Assembly, September 2001 [17.] O. Young, Environmental Supply Network Management in the Eectronics Sector, Soldertec meeting, July 2001 [18.] Charter M. EcoDesign Conference, Tokyo, December 2001 [19.] M. Overview of EU Activities in the Feld of Products and the enevironment, EcoDesign Conference, Tokyo, December

12 [20.] European Commission, White Paper for Strategy for a future chemicals policy, COM(2001)088, Acknowledgements The author would like to note the continued support of the tin industry, through ITRI members, for work on lead-free soldering issues. Thanks are also given to those providing information to assist in the preparation of this paper. This paper is based on a previous publication in Proceedings of EcoDesign 2001, Tokyo, published by the Institute of Electrical and Electronics Engineers. 315