| Summary: | 碩士 === 長庚大學 === 機械工程研究所 === 95 === The first purpose of this study is to quantify the residual strains of the EMC in the PBGA package during the manufacturing and IR reflow processes by combining experimental, theoretical and numerical approaches. In the experiments, a full-field shadow moiré with sensitivity of 30μm/fringe is used for measuring their real-time 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 202 ). 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 moiré results. And the finite element method (FEM) 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 moiré were documented during the temperature cycling (from room temperature to 260 ). The residual strains of the EMC for the specimens with low-Tg and high-Tg substrate are 0.059% and 0.134%, respectively, after the fully curing of manufacturing 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 are coming from the chemical shrinkage of the EMC curing and mold flow pressure, (which may be different with dissimilar substrates) and stress relaxations during the first solder reflow.
The second aim is to measure CTEs of the EMC and bending deformations of PBGA structure under thermal loading by a strain gage technique. By comparing with TMA and strain gage results, there exists about 40% difference for α2 (CTE above the Tg), but not for α1 in strain gage measurement. It was found out from FEM analysis that the cause of discrepancy is due to the softness of material properties (modulus dropping dramatically) above the Tg on the EMC. Moreover, the strains (in-plane) obtained from strain gages can be successfully transferred to the curvature by a simple equation. However, this technique plus the transformation still suffers the inaccuracy due to the softness of material properties on the EMC. If this inaccuracy can be improved, the strain gage technique can provide a simple alternative to quantify warpages of PBGA packages.
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