Study of solder bridging for the purpose of assembling three-dimensional structures

Abstract

Recently, soldering has been used to assemble three-dimensional microscale structures. Solder is deposited on adjacent metallic faces of planar polyhedral patterns, bridging the small gaps between individual faces. When all but one face of a polyhedral pattern are freed from the substrate and solder is reheated to a liquid state (reflow), the free faces of the pattern fold upward, out of the plane, to form the desired polyhedron. The wetting of solder with regards to coverage of metallic faces has been described previously, but the lateral bridging between the metal faces remains relatively unexplored. The goal of this work is to characterize the parameters influencing the bridging and folding process for two different ways of dip soldering: face and edge soldering. Face soldering refers to the complete wetting of metal faces, whereas edge soldering refers to selectively applying solder on the edges of a face that come in contact with other faces when folded. Our work explores bridging yield for various gap spacings and face thicknesses for eight different polyhedral patterns. Experiments show that the thickness and gap spacing strongly influence successful bridging. Experiments also show that improved control over the bridging process increases the yield of folded structures. In particular, gap spacing is positively correlated to face thickness for successful folding. Moreover, face soldering results in higher yields than edge soldering for all patterns.