Characterization of residual strains of the EMC in PBGA during manufacturing and IR solder reflow process

Abstract

The coplanarity of the plastic ball grid array (PBGA) packages is one of important issues related to the package assembly and solder ball reliability. This issue would become more severe, when the size of the packages is getting larger and the temperature of solder reflow getting higher (due to the application of lead-free solders). Recently published results indicated that residual strains (mainly involving curing shrinkage strains and stress relaxation) of the epoxy molding compound (EMC) play an important role on the warpage values and shapes of the PBGA packages. But it is still unknown about how these residual strains change during the manufacturing and IR reflow processes. The purpose of this study is to quantify the residual strains of the EMC in the PBGA packages during the aforementioned processes by combining experimental, theoretical and numerical approaches. In the experiments, a full-field shadow moire with a sensitivity of 30 mum/fringe is used for measuring their realtime out-of-plane deformations (warpages), during heating and cooling conditions, of two types of the PBGA specimens (without a silicon chip inside) with the same EMC but different substrates (with Tg=172 and 202degC). The elastic moduli (Es) and coefficients of thermal expansion (CTEs) for the EMC and organic substrates are measured in terms of temperatures by dynamic mechanical analyzer (DMA) and thermal mechanical analyzer (TMA), respectively. Timoshenko's bi-material theory is applied for extracting residual strains of the EMC from shadow moire results. And the finite element method cooperating with those determined residual strains is employed to numerically simulate the thermal-induced deformations of the PBGA specimens, in order to verify mechanics. The full-field warpages of the after-cured specimens from shadow moire were documented before and after post-mold curing, solder reflow and during the temperature cycling (from room temperature to 260degC). The residual strains of the EMC for the specimens with low-Tg and high-Tg substrate after post-mold curing were found to be 0.059% and 0.134%, respectively, which double those before post-mold curing, and further down to 0.035% and 0.08% after the first thermal cycling. After the first cycling, the residual strains keep almost constant during heating and cooling processes. This phenomenon was also observed at lead-free solder reflow processes. Therefore, the residual strains of the EMC induced by the chemical shrinkage of the EMC curing and possibly mold flow pressure are different between with low-Tg and high-Tg substrates, and they can bechanged during post-mold curing processes and stress relaxations during the first solder reflow.