Engineers in the defense and aerospace industries are complaining that the testing suppliers are using to verify their lead-free components will be free of tin whiskers is inadequate. They also claim that recent testing conducted by the IPC – or the Association Connecting Electronics Industries – is insufficient to prove a tin/silver/copper alloy will mitigate whisker growth.
Engineers point out that the tin lead blend that has been used over the past few decades was proven over time and that lead-free variations will need many years of testing before companies can realistically claim their products will be free of whisker growth. “In plain English, we do not know what is necessary to slow or stop whisker growth. Therefore the testing to date is inadequate,” says a Lockheed Martin engineer who spoke on the condition of anonymity. “Some NASA documents show that whisker growth can occur up to 20 years after the soldering or plating process was performed.”
The engineer believes there are far too many unknowns that remain regarding tin alloy and its ability to restrain whisker growth. “We don’t know what impurity proportion is the minimum to inhibit tin whisker growth,” says the Lockheed Martin engineer. “Will the new alloy formulation have stress-cracking problems that are yet unforeseen? Will there be other problems such as silver leaching or electro-magnetizing?”
The engineer also questions whether the IPC tests were thorough enough to prove tin alloys inhibit whisker growth beyond laboratory testing conditions. “How do we relate these accelerated tests to real-world application conditions? Are the physics co-relatable? Will the tests themselves break off whiskers before they have a chance to grow to their maximum length?”
The engineer also notes that minimum thickness guidelines are needed for tin alloys. “Whiskers will grow through thin conformal coats, and nobody knows what the effective thickness is for retarding or inhibiting whisker growth.” He explains that the appropriate minimum thickness of the coating varies from coating material to coating material, suggesting that new thickness specification will need to be identified for an alternative to coating to the traditional tin/lead.
His skepticism about the effectiveness of tin alloys is prompted by experience with speedy solutions that later proved inadequate. “I’m an engineer, and I have seen too many quick fixes that failed at a later date,” says the engineer. “So I’m skeptical that the problem has been solved. It would be great if [tin with silver and copper] worked, but the question remains: Why does it work?”
He points to alternative solutions that are being floated by engineers, such as using a hard layer of epoxy or enamel to stop whisker growth where traditional conformal coats won’t. “The epoxies and enamels are hard enough that whiskers cannot penetrate them. The drawback to epoxies and enamels, however, is that during space usage, they may outgas.”
Defense and aerospace industry engineers have voiced their concerns about tin whisker growth in lead-free components since the RoHS directive was first announced. So far, none of the lead-free alternatives have quieted their skepticism. Many believe the environmental concerns behind the RoHS legislation are misguided, arguing that system failure due to tin whiskers is a greater danger than the small amount of lead that makes its way into the environment.