Design of Solder Connections for Self-Assembly of Optoelectronic Devices

Abstract

Abstract The cost of optoelectronic assemblies is significantly higher than that of electronic assemblies due in large part to the method of assembly. A typical computer circuit board is built by screen printing solder paste onto a printed wiring board, placing components on the board at rates of several thousand per hour, and then reflowing the solder paste in a conveyor oven. By contrast, optoelectronic assemblies are built up in a sequential process in which epoxy is dispensed for a single component, which is placed and held in position until the epoxy is cured. Many minutes are required to build an optoelectronic assembly, such as a laser module, by this approach, also the precision robotic placement tool needed for this process costs in excess of a million dollars. The demand for all types of optoelectronic components in communications, computing, automotive, medical and aerospace applications is great, but the high cost of manufacture is constraining growth, so clearly a better method of assembly is needed. In surface mount assembly, advantage is taken of the high surface tension of molten solder to self-align ball grid array packages and flip chip die. However, in these applications, the volume of solder applied as paste by stencil printing is not sufficiently well controlled to achieve the precise alignment required for optoelectronic devices. We believe that the requirement on solder volume control for assembly of optoelectronic devices can be relaxed by designing the bond pads so that the height or alignment of connections is controlled by surface tension of the solder rather than its volume. Our design approach to accomplishing this is to connect auxiliary pads to the primary attachment pad, which act as solder reservoirs. Surface tension causes solder to be redistributed among these pads to achieve a uniform pressure throughout the solder volume. This phenomena is governed by the Young-Laplace equation, ΔP = γκ, in which ΔP represents the difference in pressure within and outside the solder, γ the surface tension of the solder and κ the local curvature of the solder surface. Thus, the design of the set of primary and auxiliary pads is critically important to realizing the desired control of joint height. In this paper we describe the use of the Surface Evolver software package in combination with analytical models, to analyze the behavior of various connection configurations with respect to variations in printed solder volume. Specifically, we calculate the equilibrium shape of the solder surface over the connected set of pads and examine how control of joint height is affected by the number, size and geometry of auxiliary pad configurations. We also discuss results from some preliminary experiments that we are conducting to validate our modeling results.