Assessment of solder paste technology limitation at miniaturization for SIP and SMT application

For SiP or SMT assembly process, solder paste has been the choice of primary soldering material. In the meantime, miniaturization has been the trend of electronic industry. While the size of components, pads, stencil thickness, aperture, pitch, and solder powder has been shrinking continuously so far, it is about time to ask whether the solder paste technology is able to support this trend forever. In this study, solder powder oxygen content was observed to increase linearly with increasing powder surface area up to Type 5. Beyond Type 5, it is getting difficult to maintain oxide thickness. Beyond Type 8, the oxide thickness always higher than Type 5 or coarser. The flux burn-off rate increased linearly with decreasing flux size plotted in log scale. However, for solder paste, the paste residue showed a sharp drop initially with decreasing flux quantity, then levelled off at 3 mg flux (~30 mg paste), and maintained at around 30% residue through the remaining range down to 0.3 mg flux (~3 mg paste). The constant residue level at small sample size was attributed to the surface adsorption phenomenon, or “Flux Shell” phenomenon, where a layer of flux was strongly adsorbed on the solder powder surface. For solder paste, the flux workload increased rapidly with decreasing powder size. The flux capacity needed in removing oxide was derived as volume fraction of COOH functional group in flux residue, with 10% maximum being set as a guideline for no-clean applications. Type 8 paste may be the limit of SiP or SMT printable no-clean solder paste due to corrosivity consideration. Oxygen barrier and alloy dopants may expand the miniaturization potential. When reviewing the brittleness of solder joint formed, Type 6 may be the limit of fine pitch applications. Beyond that, the joints may be too brittle to be reliable, unless underfill is applied.