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, μm-level gap for FLC, and align the glass, the VLSI, and the substrate with an accuracy of 10 μm. Two studies have been conducted to assist the technology development. The first designs the solder joints for the assembly. By modeling, 0.4 mm³ is chosen for the volume of the large solder joints (1000 μm in diameter) that would produce the maximum force pulling the glass against the VLSI for the gap control. Also, 11.4×10⁶ μm³ is chosen for the volume of the medium solder joints (500 μm 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°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 μm. The developed soldering technology achieves the required μm-level gap with sub-μm uniformity