The great irony of the switch to lead-free components is that the exempt industries – aerospace, defense, medical and portions of telecom – are having the hardest time adjusting to the change. Those industries exempt from the European Union’s Restriction of Hazardous Substances (RoHS) legislation are under no compulsion to quit using components that contain lead. Yet these industries are facing a world of lead-free parts that may hinder their ability to buy the high-reliability leaded parts. In response to this, industry groups have recently released documents that are designed to help companies reduce the risk of whisker growth in lead-free parts.

The rationale for RoHS exemptions is that lead mixed in tin for solder and finishes suppresses the growth of tin whiskers that can break off under stress and cause the part to fail. The lead-free parts in your DVD are fairly safe from the tin whisker problem as they sit comfortably in your living room for three or four years. But the same part in jet fighter that experiences pressure and temperature stresses over 20 or 30 years may not last.

There are high-reliability military-grade parts that are specifically designed to withstand stress over extended periods, but those parts are expensive and don’t cover the entire spectrum of needed components. For years, the exempt industries have been buying consumer off-the-shelf (COTS) components. They’re cheap and, until RoHS came along, they were also reliable.

With the RoHS deadline just a few weeks away, the components industry has switched to pure tin and tin alloys without lead for their consumer components. This is a natural progression since most of their parts are purchased for consumer products. Those component suppliers are expected to eventually shut down much of their production of leaded parts, as the exempt industries now represent what is effectively a niche market. End-of-life notices have predicatively started showing up for COTS leaded parts.

In response, the exempt industries have started to the process of learning to live with lead-free parts. This week JEDEC and the International Electronic Manufacturing Initiative (iNEMI) announced the availability of two documents intended to help manufacturers reduce the risk of tin whiskers in lead-free products. The first is JEDEC standard JESD201, “Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes,” and the second is a JECEC/IPC joint publication, JP002, “Current Tin Whiskers Theory and Mitigation Practices Guideline.”

The two documents began life within iNEMI, specifically in the Tin Whisker User Group that is made up of representatives from the exempt industries, including personnel from Lucent, IBM, Actel and Cisco. As group members developed their recommendations, they sought support from JEDEC and IPC in order to convert their draft ideas into industry standards. The recommendations include three different areas of attention to whisker mitigation. “Our approach has three pieces,” explained Richard Coyle, consulting member for the technical staff at Lucent Technologies. Coyle is also co-chair of iNEMI’s Tin Whisker User Group. The three pieces include mitigation practices, process controls and verification testing. “All three are necessary as a package,” noted Coyle.

Coyle said that even when a company adopts the recommendation standards, tin whiskers may still be a problem. “If you’re using tin, you need a mitigation practice, but this is no guarantee that you won’t get whiskers,” said Coyle. “It’s a constant education and re-education. By using the recommendations in the documents, you can still have whiskers, but we’re trying to reduce the risk.”

Some in the exempt industries believe tin whiskers are a big enough problem to warrant an in-depth study. “Nobody had determined the physics of tin whisker growth,” said an engineer at Lockheed Martin who spoke to Electronic News on the condition of anonymity. “Nobody is willing to fund a detailed study on tin whisker growth and the physics of the whisker formation.”

He noted that because we don’t know the physics of whisker growth, we also don’t know what acceptable overcoats are or the minimum thickness of an overcoat. “We also don’t know what the necessary stress is that makes a whisker grow,” said the engineer. “We don’t know how to relieve the stress and we don’t know metal is best to relieve the stress.” He concluded that the Department of Defense would be the appropriate governmental body to fund a study of tin whisker development.