Improved bonding pad design for fluxless flip chip bonding process and low fracture strength substrates

Abstract

A fluxless flip-chip bonding process and a metallized glass substrate have been used for the development of an optical display module. Two major problems were encountered with this design and the bonding process that affected both the assembly yield and reliability of the modules. One was, in the absence of flux, holding the chip in place on the bonding pads until the solder reflow had been difficult. This resulted non-wets and low assembly yield. The other problem was, fracture of the glass around and underneath the bonding pads during flip chip bonding and subsequent reliability tests. Incorporating "donut hole" structure in the bonding pads provided excellent tacking, as the solder deformed into these donut hole structures, and held the chip in place until the solder reflow. This dramatically improved the assembly yield. Covering the bonding pad edges with passivation, which modified the stress state around the bonding pads, eliminated the latter problem (fracture of glass).