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The coefficient of temperature expansion mismatch between materials in a thin plastic ball grid array package is primarily attributed to the warpage when mounting the package to a printed circuit board, especially when the moisture concentration reaches a specified value. The influence of variations in storage conditions on warpage was investigated in this paper. The effects of the reflow parameters on warpage were also examined. Taguchi’s method was implemented for experimental design and data analyses. A critical relative moisture absorption between 0.25% and 0.30% was found for a considerable warpage response.