Improving bend performance of lead-free plastic ball grid array assemblies with thermal curing epoxy and UV curing acrylic edge-bond adhesives

The introduction of lead-free (LF) solder metallurgy have contributed to a gradual reduction in the bend performance of board level interconnects (BLI) of ball grid array (BGA) packages. In this paper, bend performance of lead-free plastic BGA assemblies with thermal curing epoxy and UV curing acrylic edge-bond adhesives was investigated using 3-point bending test. The assemblies without adhesives were also tested for comparison. Four triaxial strain gages were mounted on the board to monitor the PCB strain and verify symmetrical loading. The failure criterion for this study was set as when the daisy chain resistance readings are more than 20% of the normal values. The results show that both the epoxy and acrylic adhesives improve the bend performance of BGA assemblies. The epoxy with higher modulus and adhesion strength compared to acrylic can provide better bend performance. After analyzing three components of each test leg that were dye penetrated using dye-and-pry, cross section, SEM, and EDX. Five failure modes have been identified: (1) PCB pad lift/cratering, (2) fracture at package pad/IMC interface, (3) fracture at PCB pad/IMC interface, (4) fracture within bulk solder, and (5) trace fracture.