Lead whisker growth characteristics on satellites in Earth orbit

Abstract

This study was undertaken to facilitate the use of lead-free, tin-based, parts in satellites using an exposed experiment handrail attachment mechanism on Kibo, the outboard platform of the International Space Station, and to investigate the growth mechanism of whiskers on satellites in Earth orbit. Pure tin-plated, lead-free parts soldered with a eutectic tin-lead alloy formed lead whiskers in orbit. The lead whiskers were generated specifically from the lead areas on the tin plating of the electrodes owing to the wetting of the solder during soldering. The lead-tin whiskers were also observed on the tin plating. These whiskers were nearly 20 µm long or less; however, their morphology was thin and straight. For comparison with the orbital experiments, thermal cycling tests were performed on the ground in air. The lead-tin whiskers were observed on the ground test samples but were thicker and shorter than those in orbit and were curved. It was confirmed that the growth characteristics of these whiskers in orbit differed from those generated on the ground in air. The presence or absence of oxygen influenced the shape of these whiskers. In addition, striation rings perpendicular to the growth axis were observed on the surface of the whiskers in orbit. This indicates that these whiskers on satellites in orbit were generated by thermal cycling. At the higher temperature of the thermal cycling, the tin grains in tin plating expanded, and the lead in the tin-plated grain boundaries was compressed and squeezed out, leading to the formation of lead whiskers.