Mitigating Solder Beading in Non-Eutectic Low-Temperature Solder: Mechanism and Solution

Non-eutectic solder is recognized for its propensity to induce solder beading, thereby elevating the risk of bridging in fine-pitch packages. A profound understanding of beading formation is imperative for defect minimization. This study explores the solder beading mechanisms of Sn-35Bi-Cu-Ni low-temperature solder (LTS), taking into account material properties, PCB topography, and PCB/component warpage, to enhance defect mitigation strategies. Differential Scanning Calorimetry (DSC) analysis reveals that the Sn-35Bi-Cu-Ni alloy consists of two distinct solder portions melting separately at 142°C and 177°C, with dual solidification points at around 120°C and 143°C, respectively. The solder ball test confirms that the solder portion with a 177°C melting point fails to integrate into the main soldering region, as most solder particles melt at 142°C, resulting in the unmelted portion forming solder beads. The reflow simulator also observed solder that slumped outside of the Cu Pad is difficult to join back to the main region. The "smile shape" warpage of the component and PCB topography irregularity amplify the challenges of solder integration, further increasing the tendency for solder beading near the center of the BGA. 2D and 3D X-ray examinations further validate these findings, revealing pronounced solder beads and bridging in the test vehicle using this dual melting alloy, while no defects emerge when the identical package design is employed with single melting phase solder. Reducing the printing stencil opening area effectively decreases the solder beads phenomenon.