Estimated reading time: 14 minutes

Reliability Testing and Failure Analysis: Lessons Learned

Lead-free solderable finishes for PCBs included immersion ton, immersion silver, electroless nickel immersion gold (ENIG), copper-OSP, lead-free HASL and electroless nickel electroless palladium immersion gold (ENEPIG). Each had its own benefits and reliability risks. Dr. Anselm reviewed whiskering with immersion tin, champagne voiding and silver sulphide whiskering with immersion silver, “black-pad” issues with ENIG and dissolution of copper pads with HASL, illustrated with many real-life examples.

Dr. Anselm’s final topic was a review of recent research aimed at developing a meaningful test for solder joints that allowed the study of intermetallic structures within the joint following component attachment, to investigate the cause of premature failures when BGA components with nickel-gold surface finish were soldered to PCBs with copper-based pads. Whether the finish on the copper was OSP, immersion silver, immersion tin or HASL, some copper would be dissolved into the solder joint, and this could have an important effect on the intermetallic structure at the nickel-gold component pad which was not accurately replicated by current mechanical solder ball testing procedures.

He explained how the presence of copper affected the rate of nickel dissolution in lead-free solder, and the subsequent formation of copper-nickel-tin intermetallics at the component interface. Two ball-attach cycles had been used, with short and long times above liquidus, then hot-bump-pull testing and cold-bump-pull testing had been compared following addition of copper as powder in a second reflow. Intermetallic structures following hot-bump-pull testing were largely identical, and the process appeared to have eliminated the intermetallic variations observed during ball attach. Cold-bump-pull testing showed varying intermetallic behavior in both load-to-failure distribution and failure mode and intermetallic morphologies following second reflow with copper powder were vastly different to those produced in hot-bump-pull testing. It had been concluded that the current hot-bump-pull process was not comparable to cold-bump-pull following copper addition, but the benefits of hot-bump-pull were still attractive and additional development was planned, to further examine the procedure and to identify the conditions for ideal intermetallic formation.

The workshop concluded with an energetic session of interactive discussion and comparison of individual experiences. All present had a most interesting and rewarding day, and the benefit of a substantial learning and networking experience. Martin Anselm is to be applauded for the quality and content of his presentations, the reality of his practical examples and for his willingness to share his extensive specialist knowledge.

And the generous hospitality of Alent plc, the parent company of familiar industry names Alpha and Enthone, in hosting the event, is gratefully acknowledged.

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