Method for improved thermal performance of flip chip on flex power devices

A thermal solution for flip chip preamp die on flex applications was realized via a new flex design. This design incorporates an opening in the flex circuit under the center region of the preamp die thus removing the low thermal conductivity flex materials and exposing the aluminum stiffener plate. The surface of the aluminum stiffener plate is modified by zincation followed by electrolytic Ni, immersion Sn plating. This provides a solderable surface that readily forms a metallurgical bond with solder. Thermal solder bumps that are of order 5X the volume of the electrical bumps are added to the center region of the preamp die which resides above the opening in the flex circuit. The thermal bumps are formed at the same time the electrical bumps are formed with only a UBM and solder bump mask change. Using a conventional reflow assembly process, both the peripheral signal bumps and thermal bumps are attached to the flex circuit thus creating a high conductivity thermal path from the die directly to the metal stiffener plate. The assembly is completed using the standard underfill process. Prototype flex circuit assemblies with this new design as well as the standard design were assembled and characterized for thermal performance. Detailed thermal modeling was performed for both cases and the results are consistent with the experimental findings. Finally, a detailed construction analysis was performed utilizing cross-sectioning techniques and both optical and scanning electron microscopy to verify the assembly structure. This analysis revealed the importance of forming a metallurgical bond between the thermal bump and the Al-stiffener plate.