Evaluation of laser solder ball jetting for solder ball attachment process

Due to the perpetual push in microelectronic industry for miniaturization and better performance, the density of input/output counts on the electronic packages is multiplying within a given area. Conventional flux-based solder ball attachment process is fast reaching its bottleneck in satisfying the more restrictive pitch tolerances, and assembling challenges in optoelectronics and micro-electromechanical systems (MEMS) packages. To meet the new packaging requirements, a new flux-less laser solder ball jetting technology has been developed. Despite the various advantages which laser solder ball jetting can offer, it has not been extensively reported. In this paper, fine pitch laser solder ball jetting at 200μm pitch was demonstrated using 120μm SAC305 solder spheres. The reliability of the laser jetted bumps was evaluated and compared against the flux-based reflowed bumps, by subjecting the bumps under high temperature storage (125⁰C for 24hrs, 500hrs and 1000 hrs) and multiple reflow (5 and 10 times). The quality and reliability of the solder joints were quantified through the solder ball shear test, cross-sectioning, energy dispersive x-ray (EDX) spectroscopy analysis and scanning electron microscopy (SEM) imaging. From our results, laser jetted bumps showed high initial average shear strength of 10.70g/mil², which eventually decreased to 6.96g/mil² after 24 hours. Comparing the laser jetted bumps against the flux-based reflowed bumps after 1000 hours of thermal aging and 10 times of reflow, the average shear strength values were persistently higher and the measurements of the IMC thickness were constantly lower. Hence, laser solder ball jetting has proven to be an attractive and alternative solder ball attachment method for strong and reliable solder interconnections.