Co-design for thermal performance and mechanical reliability of flip chip devices

This paper presents the results of thermal performance and mechanical reliability co-design of flip chip devices by a comprehensive evaluation of package structure and material effects. The study provided valuable insights that can assist to identify potential package solutions and enhancement options by considering process, materials, thermal performance and mechanical reliability. With concurrent thermal and mechanical analysis, the thermal trends obtained from this study can be used to enhance the selection of thermal options and the reliability insights gained allowed further mechanical optimizations to identified solutions. The study evaluates both the larger sized FC-BGA and smaller sized FC-CSP using selected production, development and potential devices. A total of 6 FC-BGA and 5 FC-CSP test vehicles were selected for the study. The UTAC-patented XP (eXtra Performance) embedded heat spreader was also assessed and benchmarked against the other test vehicles. Thermal performance analysis, detailed package level stress and warpage analysis were conducted for all the identified test vehicles. For both FC-BGA and FC-CSP devices, analysis results showed that the bare die structure has the highest warpage and highest junction-to-ambient thermal resistance. The addition of mold compound is able to improve warpage for small die FC-BGA packages. However, mold compound results in insignificant warpage improvements when die-topackage ratio is large, such as in the FC-CSP. Concurrent thermal analysis shows that mold compound slightly enhances the thermal performance of both FC-BGA and FC-CSP devices under no/low wind velocities. Heat spreaders or heat sinks are comparatively more effective thermal enhancement solutions. The effects of heat spreaders were analyzed using several different package structures. Other than the thermal benefits reaped, heat spreaders brought about significant warpage control. Especially in the case of XP, the combined effect of the mold compound and the embedded heat spreader remarkably reduced warpage even in package with large dieto-package ratio. However, stress analysis showed a 10-25% increase in die stress for both small and large die packages when the heat spreader is used. With the recent interest and migration to the use of moldable underfill material (MUF), its performance was also benchmarked against the conventional capillary underfill (CUF) and mold compound. Thermal analysis results shows that MUF results in minor thermal enhancements compared to the capillary underfill (CUF). Mechanical analysis results showed that the MUF improves package coplanarity when the package exhibited smiling warpage behaviour at room temperature. The shift in critical die and underfill stresses was also observed with the change in material set. The thermal performance and mechanical reliability costudy adopted a holistic approach that obtained the trends and insights learnt that enabled informed choices to be made in materials selection and identification of potential packages. The approach and the insights obtained are thus presented in this paper.