Current underfills for CSP and BGA

Chip scale packages (CSP) and ball grid arrays (BGA) are now major package types. In the introduction of CSP/BGA to the market, package reliability was one of the main issues and underfill was required to improve board level reliability (BLR). Flip chip underfill was utilized in applications, but it was relatively slow, required a high temperature cure and had to be kept in a freezer. This paper describes the concepts, development and performance of underfills for CSP/BGA, which the authors have developed recently. The concept of the new underfill for CSP/BGA was fast, low temperature curing with sufficient reliability, and that can be kept in a refrigerator for convenience in handling. Reworkable and nonreworkable underfills were discussed. Reworkability of the underfill is a general requirement in the market for replacement of defective CSP/BGA devices due to the many devices present on multilayer printed wiring boards in current integrated assemblies. One approach to reworkable materials is thermal degradation of the chemical linkages. A few examples, which were found in the current developmental work, are described in terms of degradation and reworkability.