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Void-Free Fluxless Solder Bonding of CPV Solar Cells
Keywords: PVC solar cell reliability, void free, lead free, flux free
The increasing demand for high-efficiency solar cells such as Concentrating Photovoltaic (PVC) has proposed new challenges on the effective heat dissipation. In such applications, normally the higher power chip has to be cooled effectively in the harsh outdoor environment by the natural convection. The thermal resistance of the whole package plays the key role to dissipate the heat from the chip to the ambient. The design, selection and assembly of the thermal interface material between the chip and heat spreader becomes the most important technique to enable the desired thermal performance and mechanical reliability. As required from thermal, mechanical, cost, manufacturability and reliability aspects, the PVC cell is recommended to be bonded with highly thermal conductive solder onto the heat spreader with: 1. Void-free in the solder bonded layer to get the best thermal performance and reliability. 2. Fluxless soldering process to protect the optical structures on the chip. 3. Acceptable CTE stress, reliability, manufacturability and cost. At SST International Inc, we successfully developed the void-free fluxless solder reflow process in the vacuum reflow oven. Different to the common fluxless soldering in which solders were costively pre-coated onto the chip or heat spreader followed with the Au flash for surface protection, a piece of commercial solder perform is sandwiched between the chip and heat spreader, the oxides of the solder surface could be broken after certain baking in vacuum, and eventually resulted in void-free bonding after reflow. No special surface treatment such as forming gas, formic acid gas, plasma, ultrasonic, nitrogen or chemicals is needed. This technology has been proved to be very reliable to meet up all the cooling requirements by PVC or high-power electronic devices.
Wei Shi,
SST International
Downey, CA

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