In Situ Study of Electromigration in Eutectic Tin-Bismuth Planar Solder Joints

The motivation for an in-situ study of microstructural evolution of eutectic tin-bismuth alloy was to explain the observation that in the early stage of electromigration stressing, and under certain conditions, there is a detectable drop in solder joint electrical resistance before the resistance begins to rise. At lower solder temperatures and electric current densities, the period of decreasing resistance is longer. A tin-bismuth solder joint under high electric current stress develops three distinct microstructural regions: At the anode end, a bismuth layer accumulates; at the cathode end, bismuth particles get swept away leaving behind the tin-rich phase, while the middle section is expected to remain essentially the eutectic microstructure. In the early stage of electromigration stressing, before any substantial amount of bismuth has piled up at the anode and bismuth particles swept away from the cathode, two phenomena occur that potentially affect the joint resistance: One is the microstructure driving towards thermodynamic equilibrium predicted by the Sn-Bi phase diagram. The other is the coarsening of the microstructure with the enlargement of the bismuth particles dispersed in the tin-rich phase matrix. Both these phenomena can potentially decrease the joint electrical resistance. At a later stage, when the bismuth layer at the anode becomes continuous, the solder joint resistance begins to rise. The paper will describe an in-situ electromigration study of planar geometry, 30-μm thick eutectic tin-bismuth solder in a scanning electron microscope. The microstructural changes caused by the current stressing were studied to shed light on the physics of solder joint resistance changes during the early stage of solder joint electromigration. In particular, the contributions of the two phenomena, the microstructure driving to achieve thermodynamic equilibrium and the microstructure coarsening, to the lowering of the solder joint resistance, are reported and discussed.