Adhesion study on underfill encapsulant affected by flip chip assembly variables

Underfill material is a polymeric adhesive which is used in flip chip devices. It encapsulates the solder joints by filling the gap between silicon die and organic substrate. Within a typical flip chip assembly, there are interfaces between the various components, i.e. substrate, solder mask, flux residue, underfill encapsulant and die passivation layer, etc. Maintaining good adhesion conditions, both as-made and after aging, for these interfaces is vital for the expected performance of the flip chip device assembly, where underfill material is employed to enhance the reliability of the flip-chip device dramatically as compared to a nonunderfilled device. A large portion of a previous adhesion study was focused on the underfill material itself; however, this paper looks into the effects of the different assembly factors, such as solder mask, flux residue, underfill and IC chip passivation, etc., upon the adhesion strength between these interfaces. The influence of some accelerated aging tests on the adhesion durability is also investigated. This could provide device/board level insights into the interfacial adhesion of the flip chip assembly.