Method of aligning and mounting solder balls to a substrate

Abstract

A process for fabricating a plurality of solder joints from a low melting point solder paste and high melting point solder balls is described. An abbreviated process flow is as follows: First, the solder balls are placed into cavities in an alignment boat, a vacuum is initiated to hold the boat and solder balls in place. Second, an amount of solder paste is applied directly onto the solder balls in the boat. Third, an alignment plate is placed over the alignment boat to provide a means of roughly aligning the solder ball/solder paste combination to the substrate. Fourth, the substrate is placed through the alignment plate on top of the solder balls in the boat. Pressure is applied to wet the solder paste to the conductive pads on the substrate. Finally, the substrate and alignment boat assembly is processed through a furnace for solder paste reflow. No expensive alignment tools are necessary as the substrate can at least partially realign itself to the array of solder balls during reflow because of the surface tension exerted by the molten solder paste as it wets the entirety of each conductive pad on the substrate.