Fine Pitch Bumping and Flip Chip Joining with Sn-Bi Based Solders by Injection Molded Solder Technology

In this study, we aimed to optimize process conditions of fine pitch bumping ranging from 20 to 100 micrometer size with eutectic Sn-Bi and Sn-58wt%Bi-2wt%In with less than 1wt% of Pd (referred to as Sn-Bi-In-Pd) solders by Injection Molded Solder (IMS) technology. Because IMS is a flux-less soldering process, firstly solder wetting behavior was characterized at various temperatures by measuring contact radius of molten solders on Cu without flux. Feasibility of bump fabrication by IMS was evaluated at various temperatures and time. Bump robustness characterized by bump shear testing correlated to degree of intermetallic compound (IMC) formation. In addition, microstructure of flip chip joints was investigated. Just after flip chip joining, both eutectic Sn-Bi and Sn-Bi-In-Pd joints showed Sn-Bi solder layer between the upper and lower IMC layers. Subsequential baking at 165°C for 2 hours produced the joint with higher re-melting temperature consisting of Cu/IMC/Bi layers with Sn-Bi-In-Pd solder, while Sn-Bi solder potion still remained for eutectic Sn-58Bi solder.