Analysis and comparison of thermal stress and hygrothermal stress of SiP device by QFN packaging

In this paper, the model is built by using the SIP structure. The SIP structure is consist of two chips which were using QFN flip-structure. The finite element analysis is used to simulate and calculate the distribution of moisture diffusion under thermal and moisture condition, then to simulate the thermal stress and hygrothermal stress. The results showed that thermal stress are mainly distributed in the mismatch of CTE (coefficient of thermal expansion) between the materials. The largest thermal stress lie in the junction of EMC, DA and chips. We can see that it has a larger thermal effect to the SIP devices used QFN packaging inside than general QPN.The distribution of hygrothermal stress, which is similar as thermal stress in trend, is mainly in mismatch of CTE and CME Coefficient of Moisture induced Expansion. However, moisture gradient, which is caused by structure and the uneven diffusing of moisture become larger. It due to moisture difference that hygrothermal stress is much higher than the thermal stress, its stress concentration appears obviously at the junction between the materials. If the interface of the two materials exist defects, they would lead to cracks and cause the failure of the device. In the area of inside device there is less moisture diffused, its hygrothermal stress distribution is similar to general thermal stress, which is due to moisture transfer firstly through the EMC materials then into the device inside. Moreover, the internal bonding material (DA) is thin, only can absorp moisture from EMC, resulting in only a little moisture can diffuse into the DA material, then cause higher stress gradient. From the simulation results, we can see that the greater stress gradient will lead to warpage of the device, it will prone to more damage in the areas stress concentration. It is larger effect on reliability on SiP device in which used QFN packaging than the general QPN device. Therefore, the effect of EMC and DApsilas moisture on the devicepsilas stress can not be ignored. And the reaults show that the maximum hygrothermal stress is 1.56 times as thermal stress. Therefore, the effect of moistrue and thermal in reliability on the device canpsilat be ignored.