The Novel Failure Mechanism of the Polymer Ball Interconnected CBGA under Board Level Thermal Mechanical Stress

Abstract

The polymer cored solder interconnect has been investigated in board level strain concerned ball grid array type IC package for many years. The novel solder ball with a polymer core is designed to compensate the board level reliability weakness of current metal alloy interconnect such as mechanical and thermal-mechanical fatigue endurance due to its unique spherical polymeric core able to absorb the strain energy generated during the reliability test, as well as the service in field. In terms of large size BGA packages such as Ceramic BGA and FCBGA, which suffer more significant solder joint strain on the package corner, the polymer cored solder interconnect is adopted to be capable of withstanding solder joint deformation between PCB and package under board level stress due to its more ductile characteristics. In the study, the daisy chained ceramic substrate based BGA with 29x29 mm square, 1.2mm thickness, 483 I/O, 1.27 mm pitch and 2 types of solder ball, one is SAC305 and another is polymer core coated SnAg, were ball attached through standard package assembly process. The assembled IC package was surface mounted on OSP finished PCB with SAC305 NC solder paste, then is subjected to accelerated temperature cycling test until 3000 cycles at 0/100 degree C with 10min dwell and 10min ramp. After that, the electrical failed sample was taken for various failure mode observations through dye and pry analysis and cross section under OM and SEM, respectively. The diverse typical failure modes across all solder joint was investigated for comparison in statistic between SAC305 and polymer cored solder and concluded polymer cored solder will outperform SAC305.