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Mitigation techniques for tin whiskers, part two
Rob Spiegel -- Design News, May 22, 2006
The first part of this article looked at the problems facing those industries that are exempt from RoHS laws. Defense, aerospace, medical equipment and parts of the telecommunications industry are not required to convert to lead-free components. However, these industries are still facing the prospect of using lead-free parts, since many of the leaded parts they have used in the past are being discontinued. Component manufacturers that have switched to lead-free versions will not long run two lines, one a high volume lead-free version for consumer products, and a second leaded version for the smaller exempt market.
This means the exempt industries will probably have to switch to lead-free parts, like it or not. The exempt industries suspected they would face this prospect and turned to industry groups to help develop policies for adopting lead-free parts while also trying to avoid tin whiskers. The International Electronics Manufacturing Initiative (iNEMI), put together teams to develop a list of recommendations for tin whisker testing and mitigation. The iNEMI teams took their research to the standard bodies JEDEC and IPC to create industry standard approaches to lowering the risk of tin whiskers.
The resulting standards include JEDEC’s JESD201, “Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes,” and the JEDEC and IPC joint document, JP002, “Current Tin Whiskers Theory and Mitigation Practices Guideline.” Joe Smetana, an Alcatel engineer and chair of iNEMI’s Tin Whisker User’s Group, notes that the documents don’t eliminate the risk of tin whiskers even as they reduce the risk. “Although it is not possible to guarantee that whiskers will not grow under field conditions, the mitigation practices detailed in JP002, combined with the test and acceptance criteria outlined in JESD201, form a cornerstone of a three-fold strategy of mitigation practices, process controls and verification testing that helps to reduce the risks of tin whiskers.”
A design engineer from Lockheed Martin who spoke to us on the condition of anonymity believes its time we learned more about what actually causes tin whiskers. At this point, whisker growth is still a mystery. There are mitigation strategies that reduce the risk – as in adding lead to pure tin solders and finishes – but we still don’t really understand the physics of whisker development. “Nobody is willing to fund a detailed study on tin whisker growth and the physics of the whisker formation,” says the Lockheed Martin engineer. “What is the acceptable overcoats, and what is the minimum overcoat thickness? What is the necessary stress to make a whisker grow? We don’t know the answers.”
The engineer believes the government entity best suited to funding and carrying out a study is the Department of Defense (DoD). “The DoD took the cheap way out and just said don’t use pure tin,” says the engineer. “But in the long run, it might be cheaper if the DoD and the electronics industry paid to solve the questions of whisker development.”
