Correlation Between Measurement and Simulation of Thermal Warpage in PBGA With Consideration of Molding Compound Residual Strain

Abstract

The objective of this study is to experimentally and numerically investigate thermal and residual deformation of plastic ball grid array (PBGA) package and assembly. Shadow moire was used to measure their real-time out-of-plane deformations (warpages) during heating and cooling conditions. A finite element model with material properties characterized by dynamic mechanical analyzer and thermomechanical analyzer was established to simulate the thermally-induced deformations of test specimens for understanding mechanics. The full-field warpages of the PBGA package and assembly were measured during the temperature cycling. The results show that a maximum warpage with a concave (smiling) shape in both package and assembly occurred at the neighborhood of 155degC during the thermal cycling, rather than with a convex (crying) shape at room temperature by assuming the warpage-free (or stress-free) temperature is at 175degC of molding. This issue has been resolved through the finite element analyses by cooperating into the residual strain (stress) in the epoxy molding compound (EMC) of the package, which is obtained by measuring the residual and thermal deformations of the detached EMC/die bi-material structure. This residual strain of the EMC maybe comes either from the chemical shrinkage of the EMC curing, or possible from stress relaxation of the EMC during the first solder reflow of attaching the solder balls. Furthermore, the consistency of thermal deformations for both the package and assembly obtained from the finite element models indicates that the maximum warpage occurs at the corner of the substrate at near 155degC, rather than at the room temperature, due to the glass transition temperature (Tg) and residual strains of the EMC. This maximum warpage can be mitigated by lowering the residual shrinkage strain. And the warpage of the PBGA assembly coincides with the package at the temperature beyond 155degC (highly resulting from apparent creep of the solder balls), but is larger than that of the package below 155degC, due to constraint of the printed circuit board through less-pronounced creep solder balls.