Low Temperature Formation of SAC-SnBi BGA Interconnections using Solid Liquid Inter-Diffusion (SLID)

We propose a novel approach to adopt SLID to low temperature BGA interconnections that enables both ball attach and card attach at SnBi eutectic reflow temperatures and enables the formation of a homogeneous, stable hypoeutectic SnBi connection of considerably lower Bi content than its eutectic counterpart. The SLID mechanism and its diffusion kinetics were investigated by selecting appropriate sized SAC spheres and eutectic Sn58Bi solder paste volumes then evaluating the resultant ball attach connections. It was determined that hypoeutectic SnBi connections in the range of 18-26% Bi could be effectively formed as homogeneous or near-homogeneous compositions under temperature excursions representative of typical cumulative Sn58Bi reflow profiles. Microstructural observations after reflow (t0) and aging at 100 degrees C showed some differences between the hypoeutectic connections and the eutectic or near eutectic (Sn57BiAg) connections within the bulk solder regions. The most significant difference was observed at the solder-ENIG interface where a thick layer formed with aging of the eutectic and near eutectic connections whereas no such layer formed with the hypoeutectic connections. Shear testing appeared to correlate the interfacial differences to mechanical integrity. Hypoeutectic connections exhibited mostly ductile fracture within the bulk solder while eutectic and near-eutectic connections failed in a brittle fashion near the solder-Ni interface and at lower shear strengths.