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A New Fan-Out Package Structure Utilizing the Self-Alignment Effect of Molten Solder to Improve the Die Shift and Enhance the Thermal Properties

We propose a new process utilizing back-side under bump metallurgy to improve die shift and enhance the thermal properties of advanced fan-out package applications. Using a back-sided UBM pad and molten solder, the pick-and-placed dies were spontaneously aligned on the substrate pad during reflow. Four kinds of UBMs were prepared on glass dies, and SAC305 solder was formed on the Cu and ENEPIG pads of an FR-4 based substrate. The die shift value ranged from 20 to 50 µm after the pick-and-place step. After reflow, the max die shift value was less than 1 µm. The initially misaligned die with the large shift was aligned with high accuracy during reflow due to the surface tension of the molten solder. The thermal diffusivity and thermal conductivity on the Cu and the ENEPIG substrate ranged from 10.0 to 10.3 mm2/s and 11.2 to 11.5 W/mK, respectively. The measurement complied with the standards of the American Society of Testing Materials and the laser flash analyzer method. This newly proposed process can improve the die shift in fan-out wafer level packaging and can be extended to panel level packaging embedded with an active die. Additionally, this interconnected solder with a back-side UBM can be used as a heat-transfer path for advanced AP package applications.