Influence of moisture absorption in uncured underfill materials

Underfill is used to fill the gap between the integrated circuit (IC) chip and the substrate to improve the solder joint fatigue life in a flip chip package. In order to improve underfilling process, wafer level underfill materials are being developed. Wafer level underfill materials will be applied onto silicon wafer, B-staged, and then individual chips are diced. A certain time later, the individual underfill coated chip is attached onto substrate and underfill is cured during assembly. Most underfills are based on epoxy cured with different hardeners. However, uncured or incompletely cured epoxy underfill can absorb moisture in the normal environment. This will be an important concern for wafer level underfill materials. This paper will present a systematic study on moisture absorption in uncured underfill based on epoxy cured with acid anhydride (methylhexahydrophthalic anhydride, MHHPA) and epoxy cured with non-acid anhydride curing agent. The influence of absorbed moisture on curing properties, thermomechanical property, and adhesion property of underfill after curing was investigated.