Study of soldering for VLSI/FLC spatial light modulators

Spatial light modulators (SLM) using ferroelectric liquid crystal over a very large scale integrated circuit (VLSI/FLC) are critical elements to high-definition displays, pattern correlation, and many other optoelectronic processing systems. A self-aligning soldering technology is being developed to package these high-performance modulators cost-effectively. It needs to achieve a uniform, /spl mu/m-level gap for FLC, and align the glass, the VLSI, and the substrate with an accuracy of 10 /spl mu/m. Two studies have been conducted to assist the technology development. The first designs the solder joints for the assembly. By modeling, 0.4 mm/sup 3/ is chosen for the volume of the large solder joints (1000 /spl mu/m in diameter) that would produce the maximum force pulling the glass against the VLSI for the gap control. Also, 11.4/spl times/10/sup 6/ /spl mu/m/sup 3/ is chosen for the volume of the medium solder joints (500 /spl mu/m in diameter) under the VLSI for low-cost self-alignments. The second study develops a fluxless soldering process with the measurements of the surface tension coefficients w.r.t. different reflow parameters. The optimum parameters are 280/spl deg/C of the hot paddle and 2.5 liter/min flow rate of the forming gas. The achieved surface tension coefficient is abound 280 dyne/cm that assures the quality of the developed soldering is as good as that of the soldering using flux. A glass/silicon test vehicle is soldered with the optimum parameters. The range of its gap thicknesses measured over the entire area is from 1.7 to 2.6 /spl mu/m. The developed soldering technology achieves the required /spl mu/m-level gap with sub-/spl mu/m uniformity.<>