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.

Demonstration of electrical connectivity between self-assembled structures

“…The yield is substantially higher for the truncated square pyramid than the truncated pyramid and is consistent with our previous findings. 39 These results suggest that the SBSA process is a robust method and in the future can be extended to provide high yield 3D structures on metal-filled TSVs.…”

“…We also studied solder thickness and roughness after dip soldering 38 as well as solder bridging. 39 Based on our previous work, a low melting point solder was chosen for this work. Process parameters for dip soldering are shown in Table I.…”

“…[37][38][39] These structures offer a solder reservoir that can be exploited in stacking device layers. 3D structures are formed by SBSA using conventional microfabrication techniques and leverage surface tension of molten solder.…”

A Solder Based Self Assembly Project In An Introductory IC Fabrication Course

“…SBSA is a simple and inexpensive method to create 3D structures (Rao, Lusth, & Burkett, 2009;Kong, Jeon, Au, Hwang, & Lee, 2011). Highly parallel fabrication of 3D structures is made possible by the SBSA approach (Rao, Lusth, & Burkett, 2012). The microscale 3D structures find various research applications (Gracias, Tien, Breen, Hsu, & Whitesides, 2000;He, Guan, & Lee, 2006;Howe, 1988;Mirkin, Letsinger, Mucic, & Storhoff, 1996;Randall, Leong, Bassik, & Gracias, 2007;Park, Kim, Sung, & Pak, 2003;Rao, Lusth, & Burkett, 2011;Oraon, Kumar, Srivastava, & Rao, 2013) that may encourage students to continue research in the 3D microelectronics field in the future.…”

“…The process traveler, described to students in completing the SBSA project, is shown in Table 1. The process traveler for SBSA was derived in our previous work (Rao et al, 2012). This paper discusses the results of teaching IC fabrication principles with an integrated laboratory project at a university using two primary processes: evaporation and photolithography.…”