Microstructural Evaluation and Comparison of Solder Samples Processed Aboard the International Space Station

Samples from the In-Space Soldering Investigation (ISSI), conducted aboard the International Space Station (ISS), are being examined for post-solidification microstructural development and porosity distribution. In this preliminary study, the internal structures of two ISSI processed samples are compared. In one case 10cm of rosin-core solder was wrapped around a coupon wire and melted by conduction, whereas, in the other a comparable length of solder was melted directly onto the hot wire; in both cases the molten solder formed ellipsoidal blobs, a shape that was maintained during subsequent solidification. In the former case, there is clear evidence of porosity throughout the sample, and an accumulation of larger pores near the hot end that implies thermocapillary induced migration and eventual coalescence of the flux vapor bubbles. In the second context, when solder was fed onto the wire. a part of the flux constituting the solder core is introduced into and remains within the liquid solder ball, becoming entombed upon solidification. In both cases the consequential porosity, particularly at a solder/contact interface, is very undesirable. In addition to compromising the desired electrical and thermal conductivity, it promotes mechanical failure.