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3D stacked die packages are especially common today. With thinner chip and multi-layer, it requires higher reliability of such a package design. Besides this, with the increasing rigid requirements for prohibiting lead (Pb) and also the trend of green electronics, higher reflow temperature is required. All mentioned above pose a challenge towards the development of the stacked die package. And in this paper, the author presents a hygrothermal and analysis of a 4-layer stacked die package, with emphasis on the thermal induced stress and moisture diffusion characteristic. First, according to the JEDEC standard, the moisture diffusion characteristic under different conditions is investigated. Second, the thermal induced stress during reflow time is studied. Results show that the substrate and bottom adhesive endure large thermal stress and strain while have higher moisture diffusion rate. Both of the hazards will reduce the capability of the whole device, even cause the 'popcorn' cracking. Such results also indicate why the reliability of the bottom layers is relatively low in manufacture.