A Sn Whisker Assessment for a Commercial Sn-plated Chip Capacitor

With the increased push to use commercial off-the-shelf (COTS) parts for high reliability applications due to cost savings, there is the ever-present concern for whisker growth from their pure Sn finishes. Metal whiskers pose serious risks for failures in electrical components, causing shorts that could potentially lead to plasma arcing in space applications. While the mechanism behind whisker growth is still under investigation, it is possibly induced by residual stresses and commonly occurs in pure metals such as Sn, Zn, Cd, Ag, Sb, and In. This study looks at chip capacitors with pure Sn finishes that were attached with eutectic Sn-Pb solder at 240°C to Cu pads. The intent was to achieve sufficient reflow to produce a homogeneous Sn‑Pb mixture on the terminations, as a risk mitigation for whisker formation. However, Sn nodules 1-2 micron-long with Pb grains as well as intermetallic needles were seen. To gauge whether the Sn nodules have enough residual stress to continue growing, and to confirm that there is no driving force for the IMCs to grow, a growth test was conducted by exposing the mounted capacitors to thermal cycling between -20 and 95°C and aging at 57°C. The results of this nodule growth test will be discussed.