Effect of the Thermomechanical Properties of No-Flow Underfill Materials on Interconnect Reliability

The demands on flip chip packaging are increasing as the requirement for miniaturization and thinner silicon chips rises. In encapsulating flip chip packages it is important to not only maintain the mountability, but also to control the warpage of the package. In this paper, three types of underfill material with different thermomechanical properties are prepared. The requirements for achieving both longer interconnect life and the mountability with lower warpage are investigated. Underfill material with higher Young´s modulus shows a longer interconnect life. From the results of failure analysis after thermal cycling, cracking at the edges of the bumps is observed. It is found from finite element analysis that underfill material with higher Young´s modulus can effectively reduce the strain accumulating inside the bump. Underfill material with a low coefficient of thermal expansion reduces warpage after chip assembly. Therefore, optimization of both Young´s modulus and the coefficient of thermal expansion of the underfill material are essential in order to achieve higher interconnect reliability while maintaining higher mountability.