Void-Free Fluxless Solder Bonding of CPV Solar Cells

In order to obtain low thermal resistance and high reliability for thermal management of concentrating photovoltaic (CPV) modules, vacuum solder reflow technologies for void-free and fluxless bonding processes have been reviewed in this paper. Surface oxides of solder alloys that utilize elements in the top side of the Ellingham Diagrams are much easier to be decomposed in vacuum. Surface oxides of solders that are alloyed with multiple elements are easer to remove than pure-metal oxides. Without the very low oxygen partial pressures and corresponding high vacuum levels that are required by the Ellingham Diagrams, void-free soldering results using various types of solders such as AuSn, InAg, InSn, SnAgCu and SnAgInCu have been obtained. Without any special surface treatment such as forming gas, formic acid gas, plasma, ultrasonic, nitrogen or chemicals, this technology is very promising and reliable. It will meet the thermal, mechanical and reliability requirements of CPV or other high-power optoelectronic packages.