Fluxless and substantially voidless soldering for semiconductor chips

A soldering method for reliability fabricating a composite structure is presented. This method, which need not use flux, provides a substantially voidless layer of solder that has low thermal resistance and excellent mechanical strength. A detailed description of soldering procedure, which uses a square, washer-shaped solder preform, is given. The experimental results show that the voids in the solder layer were reduced to less than 5% in bonds of silicon chips with a size of 1 cm ². When 3-in silicon wafer chips were used, a similar void reduction took place. Moreover, pure solder flowed into the vacant region within the preform in such a manner that the native oxide film at the surface of the preform was removed. This resulted in an average solder bond shear strength that was greater than that obtained in the conventional manner using flux. The method was successfully applied to the fabrication of a multichip module. It was also used to make a high-voltage diode composed of ten silicon chips piled up vertically and connected with solder