Wafer Level Underfill Processing

Underfill materials play a major role in the reliability of flip chip packages. These adhesives have been the subject of much research and development in the last few years, and much improvement in material performance has been obtained. However, the assembly method still remains unchanged, with the underfill being dispensed at the individual die level after flip chip reflow. Even with the arrival of “no-flow” underfills, assembly still requires depositing the underfill material onto the flip chip site prior to positioning the flip chip die. Processing underfill at the wafer level brings in a new paradigm shift to the area of flip chip packaging. Precoating the wafer with the underfill will create significant savings in both time and money. The application cycle time of the wafer level process becomes equivalent to a single dispensing operation for all the good dies on the wafer. This paper will present results obtained with screen printing used as the application method for the wafer level process. An experimental underfill was printed onto un-bumped silicon wafers and cured, and the resultant films were analyzed. Process conditions affecting film thickness and surface roughness were evaluated. Preliminary results with bumped wafers are also discussed.