During actual troubleshooting on the production line, we found that under the same wave soldering machine, identical chain speed and preheating temperature, after replacing SAC305 Solder Bars from different batches, the solder joint void rate surged from 8% to 22%. The root cause was not process parameter drift, but that the measured copper content of this batch of Solder Bars was 0.72%, exceeding the standard range of 0.6±0.05%. This caused the viscosity of the molten solder to increase and the upward resistance for bubbles to rise. Meanwhile, trace residual chloride ions in the solder bars continuously release gas at high temperatures, further worsening the void morphology.

The metallurgical uniformity of solder bars has a more direct impact on voids than theoretical composition. We once used XRF to test three batches of solder bars with identical appearance, and found that the silver content fluctuation at different positions on the cross-section of one batch reached 0.04%. This micro-segregation raises the melting point of local areas and delays wetting, so water vapor generated by Flux decomposition cannot be discharged in time, eventually forming concentrated voids with a diameter over 150μm at the root of solder joints. For another batch of solder bars with a silver distribution standard deviation less than 0.01%, the void rate remained stable below 6% under the same working conditions.

Inadequate control of oxygen content in solder bars can also induce voids. When the protective atmosphere during solder bar casting is insufficient, stannous oxide (SnO) inclusions enter the base material. These oxides cannot be completely reduced by rosin-based flux during wave soldering, and instead become bubble nucleation centers. Especially in the narrow gap where QFP pins meet the PCB pads, oxide particles hinder solder spreading while trapping nitrogen and water vapor, forming chain-like void clusters. Such defects are often misjudged as cold solder joints in AOI inspection.

We compared the void performance of bismuth-containing solder bars and conventional SAC-series solder bars. Solder bars with 4.5% bismuth content lower the melting point to 216℃, but the bismuth phase segregates at grain boundaries at the end of solidification, disrupting metal continuity. This causes internal stress to be unable to be evenly released during solder joint cooling and shrinkage, and micro-pores that could originally be healed are pulled and expanded into elliptical voids. Their long axis direction is highly consistent with the conveyor movement direction, indicating a coupling effect between mechanical stress and metallurgical defects.

The SGS reports provided by solder bar suppliers cannot replace incoming material rapid verification. We require that after each batch of solder bars arrives at the factory, a surface tension test in the molten state must be conducted. If the measured data at 250℃ using the Wilhelmy plate method is lower than 480mN/m, this batch of solder bars has an extremely high risk of solder joint voids in wave soldering. Because excessively low surface tension is often accompanied by abnormal addition of active elements such as germanium and nickel. Although these elements improve wetting speed, they inhibit the bubble coalescence and upward floating dynamic process.