Thermo-mechanical sub-modelling of BGA components in PCB reflow

This paper details a sub-modelling approach to the thermo-mechanical characterisation of flip-chip Ball Grid Array (fcBGA) behaviour during PCB reflow assembly. The main advances in the outlined modelling methodology relate to: (1) accurate assessment of PCB material properties through image processing of the PCB Gerbers, (2) adoption of mesh element “birth” and “dead” technology to capture accurately the real interaction between the BGA and the PCB during reflow, and (3) detailed analysis of the mechanical behaviour of copper structures in the BGA substrate under the reflow thermal loads. The focus in the study is on the warpage behaviour of the package during PCB reflow assembly, the behaviour of 2^nd level solder joints, and on the evaluation of the stress response of internal BGA substrate structures such as macro vias and copper paths in and through substrate dielectric layers. Principal conclusion from this study is that the BGA warpage is significantly reduced as a result of the assembly formation with the PCB but reflow causes partial yielding of the substrate copper structure. The modelling study identified the BGA locations that are most susceptible to damage. Stress levels and damage parameters are obtained for package materials and at material interfaces, and these can be compared with known failure limits.