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High thermal conductivity die attach epoxy is commonly required for power packages to dissipate heat and maintain stable electrical performance. This is especially critical for packages with only passive cooling and require high power envelop. Such packaging is always needed in growing market application like LED, automotive MCU and digital networking chip. Current industry trend for high thermal conductive epoxy easily exceeds 15W/mK. For such demanding requirement, die attach material supplier has to load very high silver filler content into their material. This has compromised the workability and dispensability of the material, besides affecting die bond robustness in mass production due to small process window. Consistent die bond process outputs like Bond Line Thickness (BLT), epoxy coverage and minimum void are usually difficult to achieve, due to poor epoxy dispensability and unstable material properties over short floor life. As such, thorough material characterization is required besides normal process optimization for high thermal conductive die attach material. Analysis such as Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA) was being performed to understand material behavior under different cure profiles. The results are then used to optimize the most cure profile for highest die shear strength and minimum void formation. In this effort, emphasize was placed to eliminate narrow worm voids, which were usually formed in the interface between die back and silver spot plated lead frame surface. To further understand the impact of different metallurgy impact, a comparison study was performed to compare lead frame surface of bare copper versus silver plated. In another time-phase study, we also attempt to understand the mechanism of the void formation and validate our idea to reduce worm void formation by minimizing epoxy out-gassing This was achieved through oven cure profile optimization like temperature ramp up and soaking time. In the second part of this paper, we look into the die bond process characterization to improve epoxy coverage, achieve consistent fillet height and target BLT. Typical material related problems like filler segregation and viscosity change was also being fully assessed through time study. Besides, factors such as dispensing needle, various dispense parameters and epoxy patterns were being evaluated to improve dispensability over a narrow process window. Slower writing speed and smaller needle diameter are recommended to improve dispensability within the limited material floor life. In summary, a lot of efforts are needed to address die bond process challenges of high thermal conductive epoxy, which has to be take into account are environmental variables such as die bond equipment condition and epoxy batch-to-batch material variability. |
