Morphology Of Lead-free SnAgCu Solder Wetting On Cu With Artifical Non-wetting Zone

Surface Mount Technology (SMT) and Ball Grid Array are state-of-art techniques in fabrication of small size electronic products and miniature devices. To avoid solder bridging and accurately distribute solder liquid on aligned or arrayed solder pad during reflow process is always concerned in pad placement designing. In addition, application of lead-free solder as a substitution of traditional Tin-lead solder becomes mainstream in microelectronic industry. Due to a higher melting point and worse wettability of lead-free solder than that of Tin-lead solder, bridging take place more easily in the reflow processes. So it is of great important to conduct research on wetting morphology control for the reflow of the lead-free solder.The wetting morphology of SnAgCu solder, one of the most popular lead-free solder, was investigated from the following three aspects:(1) The basic wetting property of SnAgCu lead-free solder was studied. It was found that SnAgCu liquid solder can only spread on a Cu substrate, dewetting cannot be observed for the reflow of a SnAgCu solder ball or a paste. Intermetallic compounds formed during wetting of the liquid solder on the Cu substrate, which shows a natural characteristic of the reactive wetting.(2) The dewetting of the SnAgCu liquid solder on a non-wetting line was studied. Specific non-wetting lines with different width of 200μm, 270μm or 450μm were prepared on copper substrates. The dewetting of liquid solder coming from solder paste with different thickness on non-wetting lines was studied. By an approach of surface energy minimization for a proposed simple model, the dependence of the critical height of the solder liquid on the width of non-wetting line was proposed and the experimental data was comparable with the estimation. These results gave a clear hint on the origin of solder bridging.(3) The dewetting of the SnAgCu liquid solder on a non-wetting circle was studied. Specific non-wetting circles with different diameter of 520μm, 700μm or 960μm were prepared on copper substrates. The dewetting of liquid solder coming from solder paste with different thickness on non-wetting circles was studied. The numerical solution was employed to figure out the critical height of solder paste that liquid solder can dewet on non-wetting zone by using free energy minimization The calculated critical height of the solder paste was comparable with the experimental data. The results indicate that the interfacial energy plays a vital rule in shaping the solder liquid and the effect of gravitational energy can be ignored in the micro scale we studied.The research of morphology control of liquid solder on patterned substrate is not only helpful to reveal the limitation of the highest connection density, but also valuable for the development of new high-density connection techniques. It is significant in the miniaturization of electronic products and devices.