Analysis of a dip-solder process for self-assembly

Abstract

Dip-soldering is a crucial step in forming certain self-assembled metal structures. However, this particular use of dip-soldering is not well described in the literature. The goal of this work is to characterize the thickness and roughness of solder layers deposited by dipping metallic films into solder melt over a range of temperatures. Control of the solder thickness and roughness will improve the yield of structures whose self-assembly is driven by surface area minimization during solder reflow. Film thickness and overall film roughness for four solder alloys, each with different melting points, were measured on unpatterned and patterned copper films. Additionally, two variations in flux treatment were investigated: flux maintained at room temperature and flux heated to 98 °C. Findings include the determination of critical temperatures, particular to each alloy, above which the roughness and thickness of the deposited solder dramatically decreases. Preheating the flux improves the nature of the deposition below these critical points. Above the critical points, thickness and roughness of the solder vary little and heating the flux does not provide significant improvements. This study provides insight into designing a process flow that optimizes the folding characteristics of self-assembled metal polyhedra by controlling the volume and quality of the solder layer.

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

“…Process parameters for dip soldering were based on previous experimental results and are given in Table II. 25 The sacrificial oxide, underneath the faces, was etched with buffered hydrofluoric acid. The etch freed the surrounding faces from the substrate, whereas solder hinges kept the faces attached to the fixed base.…”

“…25 Although yields were improved by controlling thickness and roughness, these factors were not the only ones affecting yield. Some remaining failures in folding were due to incomplete solder bridging between the faces and the base.…”

“…In addition, surface area greatly influences solder deposition thickness, with increased area leading to thicker solder layers. 25 Therefore, thicker metal pads should lead to more solder deposition on the sides of the pads. Increased solder thickness on the pad sidewalls should also positively affect bridging.…”

“…25 A set of dip soldering parameters were found that resulted in improved folding yields. However, we believe that other factors apart from consistent dip coating affects the successful construction of SBSA structures; these factors are explored in this paper.…”

Demonstration of electrical connectivity between self-assembled structures

“…[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.…”