Over three consecutive months of mass production of an automotive-grade MCU module on Line A, we found that after switching to a new batch of the same model of SN63Pb37 no-clean Solder Paste, the residual amount on the stencil after printing suddenly increased by more than 15%. Meanwhile, SPI inspection showed that the volume deviation worsened from ±8% to ±13%. Visually, this batch of solder paste was distinctly grayish-brown and darker than previous batches, but the manufacturer's COA did not specify chromaticity parameters. This indicates that the batch color difference of the solder paste is not merely an appearance issue, but rather the combined result of changes in the thickness of the oxide film on the metal powder surface and aging of the rosin carrier, which in turn alters the rheological properties and thixotropic index of the solder paste.

In actual testing, we used the same DEK printer with identical squeegee pressure (5.2 kg) and speed (45 mm/s) to compare the two batches of solder paste. At a room temperature of 32°C, the measured viscosity of the darker batch increased by 900 Pa·s, leading to a stronger tendency for solder paste stringing during release. Continuous bridging occurred particularly at the openings of the 0.3 mm thick laser-cut stencil, and the post-reflow AOI false alarm rate increased by 22%. The root cause is that the increased tin oxide particles weakened the surface tension regulation ability of the molten solder, making it difficult for the solder to rapidly spread and wet the edges of the pads.

More critically, after 7 days of storage in a constant temperature and humidity cabinet, the condensate adsorption of the color-different batch was 0.18 g/100g higher than that of the lighter batch. This indicates that some hydrogenated rosin derivatives in the rosin system had undergone mild hydrolysis, which directly led to a decrease in Fluxing activity during the initial reflow stage. Under a nitrogen atmosphere, we measured that the wetting angle increased from 28° to 39°, corresponding to a sharp rise in the void rate at the bottom of the QFN from 8% to 26%. This change was completely undetected by the three conventional incoming inspection indicators: viscosity, metal content, and solid content.

Therefore, procurement cannot focus solely on paper data in the COA such as lead content, acid value, and specific gravity. It is necessary to incorporate the color difference of batch solder paste into the IQC visual comparison items. It is recommended to use a D65 light source for side-by-side observation with standard sample blocks. Any batch showing a distinct yellowish-brown or grayish-black tendency should be immediately suspended from production. At the same time, the process team needs to synchronously update the solder paste welding parameters, reducing the ramp-up rate in the preheating zone from 1.8°C/s to 1.3°C/s to extend the solvent evaporation window. Otherwise, even adjusting the peak reflow temperature cannot compensate for the loss of fluxing activity.

We once conducted a trial run on Line B using the same batch of dark solder paste. By simply adjusting the stencil aperture ratio from 85% to 82%, the solder bridge rate dropped from 147 per million points to 69. This proves that the welding performance degradation caused by solder paste batch color differences must be dynamically compensated through printing parameters, rather than simply being returned to the supplier. Because the oxidation process is irreversible, process adaptation can secure a 3 to 5 week mass production window.