A comprehensive analysis of the thermal cycling reliability of lead-free chip scale package assemblies with various reworkable board-level polymeric reinforcement strategies

The low coefficient of thermal expansion (CTE), high modulus and high glass transition temperature (T_g) underfills with silica filler have been successfully used for extending the thermal fatigue life of flip chip to package substrate interconnection. And along with the explosive growth of portable electronics market in recent years, various polymeric materials are also applied for reinforcing the mechanical strength of board-level solder joint under drop and shock conditions. However, the expected properties of polymeric materials for board-level applications are quite different from those of package-level underfills due to their own specific roles. In addition, the features of good reworkability, high-throughput and low-cost require the board-level polymer materials should be designed with little to no filler loading as well. Therefore, the knowledge and experience with package-level underfills couldn´t be directly replicated to array-based package (ABP) assemblies. In this paper, the solder joint reliability of SAC305 chip scale package (CSPs) with various reworkable board-level polymeric reinforcement strategies (PRSs) were comparatively studied using a thermal cycling test. Two full capillary flow underfills (FCFUs), two partial capillary flow underfills (PCFUs), four edge-bond adhesives (EBAs) and two corner-bond adhesives (CBAs) with a great variety of material properties were considered. The test results show that all of the present PRSs shorten the thermal fatigue lives of CSP assemblies owing to the large CTE mismatch between polymers, solder joint, package substrate and PCB, especially for the FCFUs, followed by PCFUs/EBAs, and CBAs. The material with low CTE, moderate modulus and high T_g has the least negative impact on the thermal cycling performance regardless of which polymeric approach for strengthening solder joint was used. Finally, dye-and-pry and cross section techniques were also conducted to further understand the failur- locations and mechanisms of CSP assemblies with various PRSs.