Evaluation of reworkable underfills for area array packaging encapsulation

Underfill is a polymeric material used in flip-chip devices to fill the gap between the IC chip and the organic board, encapsulating the solder joints. It enhances flip-chip device reliability by distributing thermo-mechanical stresses caused by the coefficient of thermal expansion (CTE) mismatch between chip and board evenly over the whole package. Current underfills are mainly epoxy-based materials that are not reworkable after curing, which is an obstacle to flip-chip technology development. Not only is reworkable underfill a key material to address the nonreworkability of flip-chip packages, it can also be used to enhance the reliability of ball grid array (BGA) and chip scale package (CSP) devices without sacrificing their good reworkability feature. The objective of this study is to evaluate process viability, material performance, and reliability of reworkable underfills for board level encapsulation of BGA, CSP and flip-chip packages. Both commercial and in-house developed reworkable underfills are also included in this study. For comparison, three commercial nonreworkable underfills are also included as baseline materials. It is found that certain reworkable underfills can provide similar thermal shock reliability to the flip chip test vehicle as to the baseline underfills. The effect of the underfills on board-level BGA and CSP reliability is largely dependent on the internal structure of the BGA/CSP components, but in general reworkable underfills improve the reliability of these components. In this paper, details of reliability assessment and failure mode analysis are presented and discussed.