The production line has recently received frequent customer complaints about dull solder joints and uneven reflection. Upon disassembling the boards, we observed significant gloss differences between adjacent solder joints on the same PCB—some bright as a mirror, others grayish-white and matte. The issue does not stem from preheating temperature or conveyor speed, but rather from a chemical mismatch between wave soldering Flux and Solder Bars. This phenomenon is particularly prominent on LED driver boards and automotive instrument cluster control boards, as these two product types have AQL=0.65 visual inspection requirements for solder joint appearance consistency.

We pulled line changeover records from the past three months and identified three mainstream fluxes: rosin-based RMA-218, no-clean NC-920, and water-soluble WS-703. These were cross-combined with 63/37 Eutectic Solder bars, 60/40 near-eutectic solder bars, and SAC305 lead-free solder bars for testing. On the same Heller 970 wave soldering machine, with fixed preheating temperature of 125℃, wave height of 5.2mm, and conveyor speed of 1.3m/min, 200 boards were run continuously. For each group, 30 solder joints were sampled and measured for L* value (brightness value) using a Konica Minolta CS-200 colorimeter. We found that the rosin-based flux paired with 63/37 solder bars had an average L* value of 82.6, which is 9.3 units higher than the no-clean flux paired with the same solder bar. In contrast, the water-soluble flux paired with SAC305 had an L* value of only 70.1, with white crystalline residue on the surface. This indicates that the natural resin components in the rosin system can effectively reduce the surface tension of molten solder. Combined with the rapid solidification characteristics brought by the eutectic composition, it enables solder joints to form a denser grain structure the moment they leave the wave, thereby enhancing mirror reflection capability.

Wetting angle measurement results further confirm this conclusion. The average wetting angle of the rosin-based + 63/37 combination on copper PADs was 28.4°, while the no-clean + 60/40 combination reached 39.7°, and the water-soluble + S AC305 combination was as high as 46.1°. A larger angle indicates slower solder spreading, and excess solder tends to accumulate at the edge of solder joints, forming burr-like protrusions. This microscopic morphology directly weakens macroscopic gloss performance. Especially in 0.3mm fine-pitch QFP pin soldering, the no-clean flux's activator volatilizes too quickly, leaving the solder bar's oxide film insufficiently stripped. The solder cannot fully wet the side surfaces of the pins, resulting in local gray spots.

The comparison of bridging defect rates is even more intuitive. The bridging rate of rosin-based flux paired with 63/37 solder bars was 0.17%, rising to 0.83% for no-clean flux paired with 60/40, and reaching 1.42% for water-soluble flux paired with SAC305. This is because the liquid protective film formed by rosin at high temperatures slows down the oxidation rate of the solder bar, allowing the solder to maintain fluidity for a longer period. In contrast, the organic acid salts in no-clean flux decompose rapidly above 180℃, losing their continuous reducing ability. SAC305 itself has a high melting point (217℃) and high viscosity; the combination of these two factors exacerbates the tendency of solder to remain trapped between pin gaps, ultimately being forcibly pulled apart during cooling to form matte short-circuit bridges.

In terms of cleaning difficulty, rosin-based flux residue requires 15 minutes of hydrocarbon solvent soaking to be completely removed. No-clean flux leaves no visible residue after 30 seconds of rinsing with pure water at room temperature. Although water-soluble flux is nominally water-washable, the zinc chloride byproducts generated by the WS-703 and SAC305 combination easily form hygroscopic crystals in PCB gaps. These must be scrubbed with 5% citric acid aqueous solution; otherwise, leakage current will exceed standards after 72 hours of damp heat testing. This means that if the production line is not equipped with a dedicated acid cleaning station, forcibly adopting this combination will instead increase rework costs.

We provide clear selection paths for different product types. Consumer motherboards that prioritize appearance delivery and have generous rework windows should directly select rosin-based flux with 63/37 solder bars; their L* value stability fluctuates by no more than ±1.2 after 8 hours of continuous production. Automotive-grade ECU modules that must pass AEC-Q200 vibration testing should switch to no-clean flux paired with 60/40 solder bars; although glossiness decreases by approximately 12%, the solder joint IMC layer thickness is more uniform, and shear force dispersion is controlled within 8.3%. Medical device PCBs containing a large number of OSP-treated fine-pitch BGA pads must use water-soluble flux paired with SAC305. In this case, the wave soldering temperature must be simultaneously reduced from 255℃ to 248℃, and the nitrogen flow rate increased from 15L/min to 22L/min; otherwise, dense pitting will appear on the solder joint surface. These pits, visible under a 100× metallographic microscope as numerous micropores, are a typical characteristic of secondary solder oxidation caused by insufficient nitrogen coverage.