Miniaturization — Solder paste attributes for maximizing the print & reflow manufacturing process window

Current technology drivers span a broad spectrum that include miniaturized hand-held devices where real estate is at a minimum as well as high density server assemblies that have high-reliability requirements. SMT (Surface Mount Technology) assembly consequently involves overcoming challenges that include-complex components such as sub-0.4mm CSPs, 01005s, LGAs; Area Ratios of 0.6 and below; Stencil apertures sub-250 microns dia, Stencil thicknesses between 75–110 microns; Solder-Mask vs Non-Solder-Mask defined pads; varied surface finishes such as OSP, ImAg; and long reflow profiles with reflow in air. With miniaturization and smaller stencil apertures, some of the key manufacturing challenges include (a) achieving a good print brick deposit, maximizing paste transfer, minimizing print-to-print deviation; (b) eliminating HiP (head-in-pillow) which is the biggest issue facing the industry; (c) achieve complete solder coalescence and prevent clumpy/grainy solder joints for the small paste deposits that see longer reflow profiles; (d) achieve low voiding even for micro via-in-pad designs. These conditions impose heavy demands on the solder paste flux chemistry. To add to the complexity, halogen-free flux requirements in a Pb-free process are pretty much becoming the norm. Pb-free solders mean higher temperatures and therefore the flux needs to do more work to reduce oxide. This is more difficult for the smaller paste deposits because of the higher surface area to flux volume ratio. Halogen-free fluxes mean that the fluxes have lesser “juice” when compared to normal halide-containing fluxes, but still need to address higher temperatures associated with a Pb-free process. This study details the development of a flux technology platform to (a) achieve print consistency (maximize paste transfer, minimize deviation) for small apertures (sub-300 microns diameter, area ratios of 0.5–0.6); (b) eliminate head-in-pillow with an enhanced oxidation- barrier approach as opposed to just making a flux more active; (c) achieve complete solder coalescence and prevent clumpy solder joints; (d) achieve very low voiding. The flux in the solder paste is a complex optimized chemistry and is truly the “secret sauce” that helps maximize the process window (print, reflow) towards achieving high yields.