Reworkable no-flow underfills for flip chip applications

Underfill is a polymeric material used in the flip-chip devices that fills the gap of IC chip and organic board, and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of the flip-chip devices as compared to the non-underfilled devices. No-flow underfill is a new type of underfill that allows simultaneous solder bump reflow and underfill cure, which leads to more efficient no-flow underfilling process as compared to standard underfilling process. Reworkable underfill is another type of underfill that allows the faulty chip to be replaced individually. It is the key material to address the non-reworkability issue of current underfills. Both underfills are very important to the continuous advancement of flip chip technology. The combination of these two good features in one underfill is a novel idea. However, currently no such kind of research is carried out. Our goal is to develop reworkable no-flow underfill, an underfill that not only can be used in no-flow underfilling process but also makes the flip-chip device reworkable. We are taking two approaches for this new type of underfill. The first one is to add special additive into a no-flow underfill formulation to make it reworkable. The second approach is to develop no-flow underfill based on a new epoxy resin that decomposes around 260°C. Two underfills are developed based on these approaches. Comparing to the standard no-flow underfill, these two had similar properties including curing and fluxing, but they showed reworkability while the standard no-flow underfill was not reworkable