Tin Whiskers Formation on an Electronic Product: A Case Study

During qualification testing, a printed circuit board (PCB) of an electronic device for a drilling tool failed. The circuit board was exposed to a 120 h aging cycle at 180 degrees C followed by 10 thermal cycles between -40 and 180 degrees C before a failure was noticed. During inspection numerous white whiskers were observed over a lead-free solder surface. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) were used for microscopic examination and material characterization of the whiskers, end-cap metallization, and the solder materials. The tin whisker formation was attributed to the compressive stress in the tin solder material, which was caused by diffusion of the end cap metallization, formation of intermetallics, and thermal cycling of the soldered components. Recommendations are given to mitigate/control whisker formation on the lead-free solder materials.