The mechanical reliability of electronic packages is one of the critical problems in the reliability of electronic products in general. Estimating the warpage of an electronic package is useful for increasing its mechanical reliability. The warpage of an electronic package often shows a hysteresis curve during a thermal cycle. However, this hysteresis is difficult to simulate. We measured the master curves of the relaxation modulus using a Dynamic Mechanical Analyzer (DMA) before and after the first heating. Measured equilibrium elastic modulus after heating was two times higher than before heating. Curing rate of the resin before heating was already more than 99%. Change of elastic modulus in the range over 99% curing rate was much stronger than expected according to the conventional theory of rheology. We then analyzed the warpage of the specimen considering the change in the master curve of the relaxation modulus of the underfill resin. The hysteresis of the warpage of the bonded specimen was successfully predicted using the proposed method. In this study, we extended this method to a package on package (PoP). The PoP package also showed temperature hysteresis of the warpage. We considered the same viscoelastic material properties for the underfill resin. We also took the multi layered print circuit board and the viscoelastic material properties of solder resist into account. Simulated thermal hysteresis of the warpage of a PoP successfully corresponded with the measured warpage.

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