An overview of molded underfill in flip chip packaging applications

Abstract

Flip chip packaging is a wide-spread process used in high frequency, high IO count IC devices. It consists of joining the IC chip with the substrate using solder alloys or Cu pillars to form a continuous interconnection for the electronic signals to travel through. However, because during operation these devices dissipate heat, the temperature at the bumps interface rise and drop in cycles which cause thermal stress to build up due to the different thermal expansion of the materials. If left unchecked, these stresses will weaken and eventually cause fatigue failures. Therefore, stress absorbers are dispensed or underfilled to fill the gap between the substrate and the die. Underfills, as these materials are commonly known, are typically dispense as liquids using a variety of techniques like spraying, jetting, injection dispense, etc. In this paper we'll discuss the benefits and challenges of a new type of process that integrates the overall encapsulation of the package with the underfill process in one single process step, called molded underfilling. We'll discuss this application for simple devices consisting of one IC chip and then extend the discussion towards more complex package configurations consisting of IC chips and passive components to demonstrate the flexibility and robustness of this new process solution.