| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系所 === 104 === When the electronic devices are in operation, the Wafer Level Chip Scale Packaging (WLCSP) interconnects are subjected to thermo-mechanical fatigue that is generated by temperature cycles reacting with a substrate and a chip that have different coefficients of thermal expansion (CTE). Usually, the locations of crack initiation are strongly related to the CTE mismatch between Si chip and solder alloy.
In general, there are some ways to enhance the thermo-reliability, including the selection of pad finish and Ag addition in solders. Adjusting Ag content in solders would influenced the performance of thermal cycling. Besides, the joint properties with dissimilar microstructures and IMCs formation will differ between joints with various surface finishes. By the way of pad selection, the situation of recrystallization and other phenomena occurred during thermal cycling are affected. Consequently, the solder joint reliability regarding Ag effects and substrate effects under thermal cycling and related mechanism are needed to be well addressed.
As for these critical concerns, this study investigated the correlations among crack, elemental distribution and grain structure in Ni(V)/SAC/OSP and Ni(V)/SAC/ENIG solder joints both with SAC396 and SAC105 solder alloy, respectively. The results of thermal cycling test shows that the joints with OSP substrate exhibit longer lifetime during thermal cycling test, as compared to that of ENIG substrate. Meanwhile, the joints with SAC396 solder alloy demonstrates superior thermal stability than that of SAC105 solder alloy.
To probe the mechanisms regarding the failure more in each condition, this study applied FE-SEM to observe the crack propagation, surrounding IMCs precipitates in solders and interfacial IMCs. Moreover, FE-EPMA and EBSD were used to detect the elemental distribution and grain orientation, near the cracks, respectively. It is demonstrated that the solder joints with OSP substrate and SAC396 solder content exhibit superior thermal stability. The related mechanisms were proposed and addressed.
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