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Vacuum Solder Reflow Achieving Void Free and a Flux FreeAttachment of Concentrated Photovoltaic (CPV) Solar Cells
Keywords: Void Free, Flux Free, Vacuum Reflow
The presence of voids reduces the reliability of the solar cells. Besides enlarging the probability of the components cracking, voids will increase the solar cell operating temperature and weaken the bonding interface. The main cause of voids in the solder bonds is the segregation and formation of materials such as oxides, carbon and silicon on the molten solder solution. Films formed by the oxides present will prevent wetting at the interfaces of the metallized surfaces. Most conventional methods used to breakdown this film utilize a scrubbing method. Although this method will activate the oxides, it also disturbs the molten solution and, consequently, reduces the homogeneity of the solution. This affects the compositions of the bonding material. Because of the existence of these oxides before bonding, they become enhanced in the bonding process and, as a result, the wetting angle of the molten eutectic solution is generally close to or above 90 degrees. The solid-vapor surface energy must be greater than the solid-liquid surface energy to achieve the contact angle less than 90 degrees. The ability to control the process parameters and to control the soldering process in an oxide free vacuum environment, without the use of flux, will help establish a void free attach which contibutes to the reliability and high performance requirement of the solar cell assembly.
Paul W. Barnes, Vice President
SST International
Downey, CA

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