Meeting wrap-up

Third lead-free summit offers new data


SAN JOSE, CALIF. – The Microelectronics Packaging and Test Engineering Council (MEPTEC), in association with San Jose State University (SJSU), presented its third Lead-Free Solder Implementation Summit in late August. Approximately 90 people attended the summit in Sunnyvale, Calif., representing manufacturers, suppliers and industry consortia. Two hot topics – appropriate reflow profiles for solder pastes and tin whiskering in matte tin plating – were discussed during the technical talks and explored in depth during the panel discussions.

Keynote speaker Dr. Jan Laskowski (IBM) said that while the movement to lead-free products is being driven by market forces, trade restrictions and customer perceptions (rather than environmental realities), it can't be turned around. He discussed the various lead-free alloys under consideration, using phase diagrams to compare the Pb/Sn system to the Sn/Ag/Cu system. Dr. Guna Selvaduray's (SJSU) subsequent talk provided further tutorial on the metallurgy of the Sn/Ag/Cu systems. Most U.S. and European groups support the use of Sn/Ag/Cu alloys for surface mount applications. Laskowski listed 15 companies worldwide that have existing or upcoming lead-free consumer products. IBM's approach is company-wide and covers solder pastes, wave solder and solder balls. Laskowski mentioned the difficulty in finding a solder hierarchy for flip chip BGAs, with one “lead-reduced” solution using high-lead solder to attach the chip and a lead-free alloy for the BGA balls.

Collaborative Findings

Vivek Gupta (Intel), representing the High Density Packaging User Group (HDPUG) and the Global Environmental Coordination Initiative (GECI), gave updates on the progress of these industry groups. GECI is a worldwide network of organizations that includes HDPUG, NEMI, MEPTEC, IPC, JEDEC, SAC, SEMI, ITRI UK and ITRI Taiwan. The mission of GECI is to develop a roadmap for conversion to lead-free technology, focusing on one or two mainstream alloys and determining PCB manufacturing specifications and standards. The initial version of the roadmap was due out in October.

Kim Hyland (Solectron) discussed NEMI's lead-free task force, which has a goal of total lead elimination by 2004. NEMI has recommended Sn/3.9Ag/0.6Cu as an industry standard for lead-free solder paste (with Sn/0.7 Cu for wave soldering) and is currently assessing its manufacturability and reliability. Hyland presented results to date on tin whiskering and on moisture sensitivity levels (MSL) as a function of reflow temperature, and described thermal cycling testing in progress. Data is being generated by NEMI member companies and Sandia National Labs; the resulting report will be available in November.

Karl Tierfert (Agilent) discussed challenges to lead-free implementation for optoelectonics and presented data showing catastrophic failures of optoelectronic components exposed to 260°C for 45 seconds. He suggested a maximum reflow temperature of 230°C to maintain reliability, because the transparent plastics used in the manufacture of optoelectronic products lose important properties above 225°C.

Hari Setiawan (AIT Batam) discussed the status of AIT's use of matte tin as a lead finish, providing an introduction to the afternoon panel discussion on whiskering guidelines and test methods. Matte tin is the preferred lead-free plating material, with Sn/3Bi used to meet Japanese customer requirements. It was emphasized that whiskering can occur in all tin-bearing alloys, including tin-lead, and that the return to matte tin merely brought attention to the issue. Ni underplating was discussed as an effective method to remove compressive stresses that cause whiskering. Drawbacks are increased cost, processing difficulties, and potential solderability problems with Ni. Annealing can also reduce whiskering, but nothing guarantees a whisker-free process. Test methods and criteria have not been standardized, though NEMI has an ongoing project to address this issue.

Reflow Profiles

The remaining panel topics were lead-free components and board level implementation. The question of appropriate reflow profiles for Sn/Ag/Cu solders came up during both panel discussions. Jim Raby (Soldering Technology International) presented his view during his technical talk entitled “Why Not Solder Lead-Free at 230°C?” He showed wetting balance measurements and cross sections that demonstrated excellent solderability of 96.5Sn/3.5Ag at 230°C in a nitrogen atmosphere. (Raby strongly favors the use of nitrogen). He conceded that specific configurations (e.g., BGAs) may require slightly higher oven temperatures, but rejected the requirement of testing at 260°C.

During the panel discussions, concern over MSL at 260°C was discussed, and it was noted that JEDEC is revising their moisture level standards to include lead-free reflow profiles and may recommend changing the peak temperature to 240 rather than 260°C. HDPUG is currently working to find an optimal reflow profile. Several panelists expressed a desire to keep peak reflow temperature in the range of 230 to 240°C, with 260°C considered an upper boundary. Because today's reflow ovens control temperature across a board much better than older ovens (2 to 10°C compared to 30°C), reflow temperature doesn't have to exceed the solder melting point by as much. Reflowing at 240°C may also allow a single reflow profile for boards containing both Pb/Sn and lead-free solder, which may happen as some component manufacturers make the conversion before others. AP


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