Modeling of residual strain in BGA-PCB assemblies for pad cratering control

Post-reflow residual strain is a typical thermo-mechanical phenomenon of printed circuit board assemblies (PCBA) with a large ball grid array (BGA) component after the surface mount process. In general, the post-reflow residual strain may not directly lead to the failure of solder joints, but it may result in a potential risk for another reliability concern, namely, pad cratering. Without sufficient relaxation to reduce the residual strain after board level reflow soldering, the backplane assembly may become more likely to generate the pad cratering failure. Therefore, accurate estimation of the post-reflow residual strain in large BGA-PCB assemblies is in demand. This study proposes a model to estimate the post-reflow residual strain on BGA-PCB assemblies. For model validation, high temperature strain gauges are employed to monitor the board level warpage during the reflow cooling process. Based on this model, the residual strain accumulation during the reflow cooling process becomes predictable. A parametric study with different cooling rates and relaxation periods is conducted for PCBA with both plastic ball grid array (PBGA) and ceramic ball grid array (CBGA) samples. The present study demonstrates that the proposed model has the capability to evaluate the key factors of post-reflow residual strain for different types of BGA devices.

Nomenclature

pre-exponential factor, /s

strain rate sensitivity of hardening/softening non-dim.

hardening/softening constant, MPa

strain rate sensitivity, no-dim.

strain rate sensitivity for the saturation value of deformation resistance, no-dim.

activation energy, J/mol

gas constant, J/mol/K

deformation resistance, MPa

coefficient of saturation deformation resistance, MPa

absolute temperature, K

displacement, mm

equivalent stress, MPa.

strain, no-dim.

strain rate, /s

multiplier of stress, no-dim.