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Survey on Lead Free Solder Systems

2 Background

The electronics and electrical industries are facing increasing pressure from legislation to remove lead from its products. This is because of the hazards to health during manufacture of the materials and environmental damage and human exposure caused by their end of life disposal. The second draft of a proposal for a directive on Waste from Electrical and Electronic Equipment was drafted in the middle of 1998. The proposal should become an EU directive for enforcement in all member States by July 2006. More information is available on the below web address:

http://www.lead-free.org/download/files/pdf/Directive_Section_WEEE_Final
_Proposal.pdf.

When the European Commission ratifies the legislation, there will be a major impact on all industries using electronic and electrical components.

The incentive to design lead-free solder assemblies lies in the fact that there is no recycling capability for the lead content of solder and that the current fluxing technology is based on VOC (volatile organic compounds) carriers and solvents, which are vented to the atmosphere during soldering. The development of a complete environmentally acceptable soldering technology is thus of vital importance to industry.

A double-sided plated-through-hole FR4 glass-epoxy type test board was supplied in two different board finshes, immersion gold and OSP. Selected components, 25 types are all supplied from the Bang & Olufsen stockage, and 20% were found to have lead free component finish.

Three experimental cases were included in the project:

1. Double reflow soldering followed by hand soldering (at Bang & Olufsen)
2. A combination of reflow (at Bang & Olufsen) and wave soldering (at HYTEK)
3. Finally a production case including reflow, wave and hand soldering was carried out jointly by SIMRAD, Grundfos and HYTEK.

The first two cases were based on a double-sided plated-through-hole FR4 test board while the production case used a multilayer FR4 with two different board finish variants (SnPb HASL and immersion tin solder).

Design and assembly of test boards was performed by Bang & Olufsen and DELTA in collaboration. Automatic assembly including screen printing of solder paste, component pick & placement, and lead free reflow soldering was successfully realised at a maximum temperature of 240°C, and a soldering time of 60 seconds at >217°C using the present manufacturing equipment. Lead free hand soldering of leaded components was found to require much heat and full wetting of the plated-through-holes was problematic. Special solder iron set-up is required compared to tin-lead soldering and further work is required in order to reach consistent hand soldered joints.

Robustness to solder heat is a key issue, and component selection/specification have to match the resistance to the solder heat guideline of 260°C, and soldering time of 60 seconds at >217°C.

OSP board finish is found to result in incomplete wetting of the solder lands, which is the case for both of the lead free solders, and in that respect the immersion gold board finish is preferred.

Qualification testing has been performed by high temperature ageing, temperature cycling and damp heat testing, and the properties of lead free joints compared to similar tin-lead joints have been evaluated by visual examination, X-ray examination, component shear testing, microsectioning and optical microscopy. Temperature cycling testing is found to be the major accelerating factor in joint degradation and results from destructive physical analysis show that the degradation takes place in the solder bulk material of the lead free joints.

In general, the performance and reliability of the lead free reflow soldered joints is found to match standard tin-lead soldered joints, and both of the lead free solders tested are found to show very similar performance.

A considerable R&D effort is currently under way in different parts of the world in a bid to find suitable alternatives to lead containing solders. The major alternatives for the volume production seem to be based on lead free solders and conductive adhesives. Some of the ongoing research activities in lead free solder development are listed below:

1. National Centre for Manufacturing Science (NCMS), Michigan, USA. A consortium based research programme evaluating lead free solders for electronics manufacturing. www.ncms.org. The programme was carried out by a consortium of AT&T, Lucent Technologies, EMPF, Ford Motor Company, GM-Delco Electronics, GM-Hughes Aircraft, NIST, Renssekaer Polytechnic Institute, Rockwell International Corp, Sandia National Laboratories, TI and Hamilton Standard. The goal of the project was to determine whether safe, reliable, non-toxic and cost effective substitutes existed for lead-bearing solders in electronics manufacturing.
2. A research consortium funded by the Department of Trade and Industry (DTI) in which Multicore, the Tin Research Institute and GEC Marconi among others are the participating partners. www.itri.co.uk. The consortium selected alloys with two criteria in mind: suitability for electronic assembly and supply potential. Thus the following alloys were short-listed for trials: Bi-42Sn: Eliminated due to poor mechanical properties and low melting point. Sn-9Zn: Eliminated due to corrosion of the zinc phase. Sn-5Sb: Eliminated due to the melting temperature being too high. Sn-3.5Ag and Sn-0.7Cu was found to have adequate properties.
3. TWI, UK runs a project on the characterisation of lead free solders. www.twi.co.uk.
4. BRITE/EURAM project on "Improved Design Life and Environmentally Aware Manufacture of Electronics Assemblies by Lead Free Soldering". BRPR960140 headed by GEC Marconi, UK. www.cordis.lu. Conclusions are that optimum solders are based on Sn-Ag-Cu. The Sn-3.8Ag-0.7Cu alloy was recommended for general purpose use, possibly with the addition of antimony. Sn-3.8Ag-0.7Cu-0.5Sb was recommended for wave soldering. Sn-Bi-Ag could be used for single sided wave soldering.
5. BRITE/EURAM project on "Development of Adaptive Solder Technology for Reliability and environment Compatibility of electronic assemblies. BRPR980683 headed by Thomson CSF, France. www.cordis.lu.
6. IVF programme on "Integrity of Lead Free Solders - Manufacturability, failure mechanism, etc." www.ivf.se.
7. Norther Telecom, Canada: Development of novel lead free alloy for mobile telephones.
8. Nokia with Multicore is doing research to replace lead containing solder in cellular phones.
9. Loughborough University, UK investigates fundamental characterisation of lead free systems using DSC, TGA, etc.
10. Japan Institute of Electronics Packaging: Consortium work finished in 1996 i9ncluded: Senju Metals, Harima Chemicals, Alpha Metals Japan, Nihon Handa, Ishikawa Metals, Uchibashi Estec, Speria Japan Solder Coat, Univ. of Tokyo, Osaka Univ., ICS, SC Lab, Tokyo Electric Univ., Tabai Espec, Mitsui Mining, Nippon Mining & metals, Nippaku, Ishihara Chemicals, Tanaka Electronics Industry, Furukawa Electric Industry, TDK, Murata, Rubicom, Sony, Fujitsu, NEC, Hitachi, Toshiba, Panasonic, Fuji Electric and Omron
11. Japanese Universities: Many universities are involved with the research and development of lead-free solders/soldering such as University of Tokyo, Osaka University, Yokohama National University, Shibaura Institute of Technology, Kohnan University and National defence Academy.

Many more national projects than listed here are going on.

2.1 Legislation

There is a continual world-wide environmental movement away from the use of lead towards ‘non-toxic’ products. Various alternatives have replaced the traditional use of lead in wine bottle capsules, fishing weights, casting alloys for toys, as well as solders for certain plumbing applications. Once considered a joke, lead-free ammunition (bullets and shot) is now available and experiencing a significant growth in demand, particularly in the USA where the possibility of litigation against environmental contamination or employee exposure to hazardous materials is high.

Added to this, there are now a series of initiatives worldwide that outline targets for electronic equipment re-use and recycling. In such initiatives, the use of hazardous materials such as lead is often limited in order to improve the ease of recycling.

Legislation directly affecting the solder and electronic assembly industries has been agreed by the EU system outlining targets for electronic equipment re-use and recycling (Directive on Waste of Electronic and Electrical Equipment, WEEE). At the same time agreement was reached to limit the use of hazardous materials (Directive on Reduction of Hazardous Substances, RoHS) to improve the ease of recycling and to limit the hazards of certain substances including lead. The RoHS directive temporarily exempts lead in solders. These two directives, entering into force by 1 July 2006, will impact not just on solder alloys but component finishes and temperature ratings, board finishes and flame retardancy issues.

The European Commission has also passed a Directive on End-of-Life Vehicles which again is aimed towards recycling and re-use targets and prohibiting the use of certain hazardous materials. Lead in solders for automotive applications have a temporary exemption from the lead ban. This Directive enters into force 1 July 2003.

The Danish Environment Agency has also taken action against products containing lead compounds and metallic lead in a number of product areas.

The Japanese Ministry of Trade (MITI) has drafted a recycling law for electrical appliances with a 2001 deadline. This does not yet include lead phase-out but it is expected to come.

Although there is no federal legislation yet in the US, there are a number of State electronics recycling initiatives to consider. In addition, the EPA has recently proposed a crack-down on lead emissions from plants that may impact the soldering industry.

A permitted residual lead content in "lead-free" solders has yet to be defined. The US EPA defines lead-free plumbing solders as those containing less than 0.2% Pb although this may be reduced to 0.1% in the future. It is this level of impurity limit that may also be expected in "lead-free" solders for electronics.

IPC – Association Connecting Electronics Industries Environmental Health and Safety (EHS) Committee has announced the Environmental Protection Agency's (EPA) finalization of a lead reporting rule could impact U.S. PWB and EMS companies. The final legislation changes the Toxics Release Inventory reporting threshold for lead and lead compounds from 11,000 kg per year to 50 kg per year.

This new legislation is retroactive to January 1, 2001, requiring companies to begin tracking and reporting all lead and lead components used during the 2001 calendar year. EPA calculates the rule will cost the industry $80 million for the first year, and $40 million per year in subsequent years. The electronics industry alone is expected to submit more than 3,500 reports at a cost of more than $25,000 per company for the first year, and more than $12,000 per company each year following.

Companies with 10 or more employees that use or process 100 pounds of lead, or more than 350 pounds of eutectic tin-lead solder per year, will be effected the most. IPC is currently pursuing all available avenues in it continued efforts to halt the implementation of this rule. However, facilities are advised to begin immediately tracking their lead use for the record-keeping process. The full text of EPA's final rule can be found on the EPA's Web site at www.epa.gov/tri.