Investigation of Repairable Underfill Materials for Reliability Enhancements

ABSTRACT Filling the interspace between package and the printed wiring board (PWB), namely, underfilling has been demonstrated to yield dramatic improvement in the thermal fatigue reliability of most Chip Scale Package (CSP) assemblies in Mobile Phone applications. Rework of defective CSPs can only be performed prior to the underfill operation. Thus, inability to rework assemblies has hindered the wide spread use of underfill materials. Reworkability of the packages with underfill is needed for Circuit Card Assemblies with CSP's failing during final test or those that are returned from field. The current investigation provides initial results of an ongoing study that is aimed at enhancing mechanical shock and thermal shock reliability of the CSP assemblies in mobile phone application with reworkable underfills and compare performance with non-underfilled, and snap-cure (non-reworkable) assemblies. Additional tests are used to determine the reworkable underfill application in a variety of different products and processes. Reworkable underfill materials from two sources were evaluated in order to find a suitable system for CSP assemblies. Material properties such as coefficient of thermal expansion (CTE), glass transition temperature (Tg), modulus, strength of adhesion, etc were obtained. Initial investigations consisted of determining the effect of moisture on the appearance, and adhesion of several assembly-level underfill materials using Pressure Cooker test (PCT). Tests that simulate the production process and product reliability tests have also been conducted with these materials. A comparative study of two reworkable underfill materials and snap-cure (non-reworkable) material are reported. Results of mechanical shock tests, cycle time studies, ease of reworkability, and cure times are discussed.