11.1 Introduction

Since the publication by Tsukada et al.1 in 1992, the electronic packaging industry has witnessed an explosive growth in the research and development efforts devoted to solder bumped flip chip on low-cost PCB or organic substrate. Theoretically speaking, the solder-bumped flip chip on organic PCB/substrate1 36 is one of the most cost-effective electronic packaging techniques. However, because of the very large thermal expansion mismatch between the silicon chip (2.5 10 ⁶/ C) and the epoxy PCB (18.5 10 ⁶/ C), underfill encapsulant is usually needed for solder joint reliability.1 36 Unfortunately, due to the underfill operation, the manufacturing throughput is reduced and the rework of an underfilled flip chip on PCB is very difficult, if not impossible.

Even though the research efforts on fast-flow, fast-cure, rew orkable underfill materials are very active,1 8 ^,11 ^,13 15 ^,27 ^,28 most of them are using solvent chemical, and most of the chips (such as passivation) and substrates (such as solder mask, via, and copper pads and traces) are degraded or even damaged after rework. These issues indicate that more work needs to be done in these areas. In this section, the reworkable underfill materials from two different vendors are considered.

As to the manufacturing throughput issue, fast-flow and fast-cure underfill encapsulants are on their way.19 ^,20 ^,22 25 However, the material properties of these underfills can be degraded (due to excessive or large voids, too...