Analysis of A Larger Die, Copper Pillar Bump Flip Chip Package with No-Flow Underfill

Abstract

In the flip chip assembly process, no-flow underfill (NFU) has the advantage over traditional capillary-flow underfill because of the elimination of processing steps and the reduction of packaging cost. However, currently one of the major technical obstacles in applying no-flow underfill technology is the fillet cracking of no-flow underfill during the reflow after the moisture preconditioning. In this paper, comprehensive thermo-mechanical and hygroswelling models are established to study a larger die flip chip package with no flow underfill during reflow after moisture preconditioning. The adhesion strengths between no-flow underfill and die have been characterized. Based on the modelling results and the adhesion strength data, the reason why the crack on the no-flow underfill starts and propagates, leading to fillet cracking, is also explained. A series of parametric studies are also performed to eliminate or reduce the fillet crack. The results show that a lower coefficient of thermal expansion (CTE), a lower Young's modulus (E) and a higher cure temperature of no-flow underfill are desirable for the robustness of the package. The results also show that thinner die thickness is desirable for the robustness of the package. These findings form design guidelines for the design of larger die, copper pillar bump flip chip package with no-flow underfill.