Wafer level underfill - processing and reliability

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 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 one single dispensing operation. This paper will present and discuss the latest results obtained with stencil printing used as the application method for the wafer level process. Several experimental underfill formulations were tested as a function of various printing conditions. With the optimal process conditions, the desired coating thickness can be applied without damage to bumped wafers. Assembly challenges together with reliability data are presented.