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Cohesive Zone Modeling of Failure Underfilled BGA-PCB Assemblies
Cohesive Zone Modeling of Failure Underfilled BGA-PCB Assemblies
A model was developed to simulate the delamination behavior of multilayer PCBs assembled with BGA components reinforced with an underfill epoxy adhesive.
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Authored By:
Saeed Akbari, Amir Nourani, Jan K. Spelt
Department of Mechanical and Industrial Engineering
University of Toronto
Toronto, ON, Canada

Summary
A cohesive zone model (CZM) was developed to simulate the delamination behavior of multilayer printed circuit boards (PCBs) assembled with ball grid array (BGA) components that were reinforced with an underfill epoxy adhesive. Two different delamination modes were observed in the bending specimens: delamination at the interface between the solder mask and the first conducting layer of the PCB, and PCB subsurface delamination at the interface between the epoxy and glass fibers of one of the prepreg layers. The cohesive parameters for each of the two delamination interfaces were determined from fracture tests of bending test specimens consisting of PCB substrates bonded with the underfill adhesive. The model was able to accurately predict the fracture load and failure mode of the underfilled BGA-PCB assemblies. Key words: multilayer PCB, cohesive zone modeling, delamination, finite element analysis (FEA), underfill adhesive

Conclusions
Two different subsurface delamination modes, were observed in multilayer PCBs assembled with BGAs and tested under bending loading conditions. A CZM was presented to predict delamination initiation and propagation in these PCB-BGA assemblies. The cohesive parameters were determined from fracture tests of DCB specimens consisting of PCB substrates bonded with the underfill adhesive. The model was then coupled with an FEM to predict the transitions in failure mechanisms and the bending strength of underfilled BGA-PCB specimens fabricated in an SMT line, and fracture tested under bending configurations.

The CZM could also successfully predict the change of the failure mode with underfill fillet size; i.e. near-surface delamination for relatively small fillets, and subsurface delamination for larger fillets. This was attributed to the lower stress concentration in PCB-fillet interface for larger fillets.

Overall, it was demonstrated that this model could accurately predict the fracture loads of these underfilled BGA-PCB assemblies. The model was also able to predict the correct crack path as it changed with fillet size.

Initially Published in the SMTA Proceedings

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