The key to the success of flip-chip technology lies in the availability of sucessful underfill materials. However, the reliability of flip-chip technology using current underfill materials is generally found to be lower than that of conventional wire-bond connection packaging materials such as epoxy molding compound (EMC) because of the high coefficients of thermal expansion (CTE) and moisture absorption of cured underfill material. In this study desbimide (DBMI), which has a low melting point (about 80degreesC), was used in the underfill materials as a cohardener. As a result, DBMI-added underfill can show excellent thermal reliability, which is due to the superior properties of the CTE, the elastic modulus, and water resistance. When the properties of a 2 wt % DBMI-added underfill were compared with those of a typical underfill (epoxy/anhydride), the CTE value was reduced to less than one-half at the solder reflow temperature (about 200degreesC), the elastic modulus was reduced to less than one-half in the temperature region below the glass-transition temperature, and the water resistance was improved twofold.