A novel temporary adhesive for solder ball attachment in fluxless reflow system

Except for the attachment of solder ball on soldering pads, flux is generally used to remove oxidation and clean the pad surface to be soldered for good wetting and form reliable joints. The upcoming concerns about fluxes are the residues on the solder balls and leading to voids after heating. For several years, a new reflow system, known as fluxless reflow system, claiming that flux is needless during reflow has been developed. In this new system, the functions of flux are replaced by formic acid or hydrogen in the form of gas, thus the residues and voiding issue can be mitigated. In the case of ball attachment in fluxless reflow system, it is critically significant to keep solder balls on the exact pads during the reflow process. Therefore, this paper will discuss the details about the applicability of a novel temporary adhesive in fluxless reflow system from the points of view of fundamental characteristics, including viscosity measurement by viscometer, wettability by measuring wetting angle, and residues from thermal analysis by thermogravimetric analyzer (TGA). As a result, low viscosity and wetting angle on a variety of pads showed applicable to be good wetting on copper pads in a jetting or spraying way. The characterization of ultra-low residues (<5wt%) was achieved by TGA. Reliability for fluxless reflow system were determined. Crosssection view of the sample with temporary adhesive applied showed good joints and no voids in solder balls as well as no side effects from reducing gas and temporary adhesive. In addition, joints strength was believed to be strong enough from the results of ball shear test. Hence, it is revealed that the novel temporary adhesive is a potential candidate for solder ball attachment in fluxless reflow system.