The Influence of Die Tilting on the Thermal Response and Die Attach Stress of a Bottom Exposed Package

In this paper, a study on thermal and mechanical performance of a power discrete package with different die sizes and non-uniform bond line thickness (BLT) variations is conducted by finite element analysis and experiment. Two different die sizes of 4.14x3.04x0.2mm (big die) and 1.78x0.88x0.2mm (small die) in IPAK package are used in this study to exam steady state junction-to-case thermal resistance (R-thJC). Meanwhile, the effect of die tilting variations on R-thJC is also analyzed. Based on the results, the influence of the die tilting on R-thJC is 10% and 8% between non-tilting and worst tilting on the big die and the small die respectively. The difference of R-thJC from experiment shows the quit close results. Transient thermal performance on different die tilting variations is also studied. The results show that the worst tilting case has about 8% larger R-thJC in junction temperature than non-tilting case. In addition to the thermal resistance, the Von Mises stress of soft solder used for die attach for the worst tilting case is 15% more than the non-tilting case, and the largest stress is located in the area where the solder becomes thinnest. The stress exceeds the solder tensile strength of 26-34Mpa in both cases. The worst tilting case has 2.5 times more plastic strain than the non-tilting case, and the largest strain is also concentrated in the thin solder area. The strain value is still bellow the elongation limitation of 0.23-0.33. While the TC500 testing for this two groups shows both chip and solder are safe.