Microscale Electrical Contacts for Self-Assembly

Self-assembly is an alternative means of integrating smaller parts into structures.

An experimental study of self-assembly processes in which small, heterogeneous components become spontaneously aligned with each other and bonded through soldering of mating electrical contacts was performed to determine and, to the extent possible, to extend the lower limits of, contact sizes for which such processes can be utilized successfully. The issue of self-assembly arises because advances in microelectronic circuitry and microelectromechanical systems pose significant challenges in the construction of threedimensional structures and the building of integrated systems made of parts from incompatible microfabrication processes. Although robotic “pick-and-place” techniques are now used to integrate parts made by different processes, the ability to efficiently handle individual parts diminishes as their sizes decrease below about 300 μm. Self-assembly is attractive as an alternative means of integrating smaller parts into structures.

Figure 1. In this Self-Assembly Process, one part from a randomly agitated collection of parts comes into contact with a molten-alloy-coated binding site. Capillary forces in the molten alloy then pull and align the part in place.

A self-assembly process of the type considered in this study exploits fluidic agitation and capillary forces from a molten alloy. In a typical process, depicted schematically in Figure 1, parts to be integrated onto a template are suspended over the template in an agitated fluid. When an alloy (e.g., solder) coat on an electrical contact or other metal region of the template is liquefied by heating and comes into contact with an electrical contact or other metal binding site on one of the parts suspended in the fluid, capillary forces bind and align the part with the template.

Heretofore, processes like this one have been used for joining parts having contact sizes ≥200 μm. For a given process, the minimum useable contact size is dictated by the following considerations: Acid flux needed to clean oxides off the bonding alloy surfaces removes some of the alloy, and some of the contact base metal is consumed in the formation of an intermetallic compound between the bonding alloy and the contact base metal (the formation of this compound is essential to wetting and bonding). Hence, the contacts must be initially large enough that sufficient contact material and alloy remain at the conclusion of the process.

Figure 2. These Images Show Results of self-assembly experiments in which 40-¼m-square, 20-¼m-tall parts were bonded in place at 85-percent yield and 20-¼m-diameter, 10-¼m-tall parts were bonded in place at 15-percent yield.

This study involved consideration of (1) five alloys and one pure metal having various melting temperatures from 47 to 154°C and (2) nine different suspension fluids having boiling temperatures >160°C. Tin-based alloys were generally found to be highly susceptible to corrosion at temperatures above the alloy melting temperatures, tin being the primary component to corrode and react with the contact base metal.

Of the alloy/fluid combinations tested, one was found to be useful for extending the lower size limit: In experiments using a eutectic Sn-Bi alloy (melting temperature 138°C) and glycerol (boiling temperature 290°C) heated to temperatures between 180 and 200°C, self-assembly of 1,500 parts having 100-μm square contacts and 500 parts having 40-μm square contacts in a process time of about 21 ⁄2 minutes was demonstrated. Self-assembly of round parts of 20-μm diameter was also achieved, albeit at substantially reduced yield (see Figure 2). Thus, the lower practical size limit under the conditions considered in the study seems to be about 40-μm. Further study would be needed to determine whether and how the size limit could be reduced further through changes in contact design and/or processing.

This work was done by Christopher J. Morris of the Army Research Laboratory.

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Microscale Electrical Contacts for Self-Assembly

(reference ARL-0034) is currently available for download from the TSP library.

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