Critical aspects of reworkable underfills for portable consumer products

Filling the interspace between package and the printed wiring board (PWB), namely underfilling, was demonstrated to yield dramatic reliability improvement in mechanical shock and bending (flexing) stresses of most chip scale package (CSP) assemblies in mobile phone applications. However, rework of defective CSPs cannot be performed after the underfill operation. The need for the ability to rework underfills has, in recent years, resulted in developmental efforts to formulate materials that can easily be reworked as well as provide requisite product reliability. The implementation of reworkable underfills involves: choice of proper material, development of an acceptable process, and a verification of reliability. In this paper are discussed some of the critical issues that need to be considered in the evaluation of reworkable underfill materials and their application in portable communication products.