Comparison between Experimental Measurement and Numerical Analysis of Warpage in PBGA Package and Assembly with the Consideration of Residual Strain in the Molding Compound

Abstract

The purpose of this study is to experimentally and numerically investigate thermal and residual deformations of plastic ball grid array (PBGA) package and assembly. In the experiments, a full-field shadow moire with a sensitivity of 29.2 mum/fringe is used for measuring their real-time out-of- plane deformations (warpages) during heating and cooling conditions. The elastic moduli (Es) and coefficients of thermal expansion (CTEs) for epoxy molding compound (EMC) and organic substrate are measured in terms of temperatures by dynamic mechanical analyzer (DMA) and thermo-mechanical analyzer (TMA), respectively. And a finite element method with the input of these measured material properties is employed to numerically simulate the thermally-induced deformations of them, in order to understand the insight of mechanics. The full-field warpages of the PBGA package and assembly from shadow moire were documented during the temperature cycling (from room temperature to 220degC). It is interesting that the maximum warpages with concave shape for both the package and assembly occur at near 155degC during this thermal cycling, (rather than with convex 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 with the residual strain (stress) in the 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 more possible stress relaxation of the EMC during the first solder reflow of attaching the solder balls. Furthermore, the thermal deformations for both the package and assembly were fully addressed by the finite element models. Both consistent results indicate that the more severe coplanarity with the value of about 275 mum at the corner of the substrate with the size of 40 times 40 times 1.7 mm occurs at near 155degC, due to the Tg and residual strains of the EMC, rather than at the room temperature. This severe coplanarity 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 PCB through less-pronounced creep solder balls.