Oxidation of Solder Wire is essentially a slow reaction between the surface tin layer and oxygen and moisture in the air, forming a gray-black SnO? film. This film directly causes difficulty in tin wetting on the soldering tip, slow solder climb on joints, and obvious stringing and solder breakage during drag soldering. We encountered cold solder joints on 0402 resistors on the same PCB for three consecutive days at the SMT Manual Soldering rework station, and finally found that the 0.6mm Solder Wire placed open at the edge of the workbench for more than 3 months had been severely oxidized. After scraping off the surface layer, the exposed metallic luster was obviously dull and lacked ductility.

The most effective on-site method for treating oxidized solder wire is to first use fine sandpaper of 600 grit or above to lightly grind along the axial direction of the solder wire 2 to 3 times, with the force just enough to remove the oxide film without damaging the internal tin-copper alloy matrix. Immediately after that, use absorbent cotton dipped in 99.5% absolute ethanol to wipe repeatedly until there is no gray-black residue on the cotton swab. This process must be carried out in a well-ventilated area to avoid the accumulation of ethanol vapor and potential safety hazards. We once failed to turn on the exhaust fan in a customer's workshop, resulting in a strong ethanol smell for two consecutive hours, which affected employees' concentration.

The polished solder wire must be immediately placed in a sealed aluminum foil bag with silica gel desiccant, and marked with the treatment date. If it is not used up on the same day, it must be transferred to a constant temperature and humidity-proof cabinet with a temperature of 25±2℃ and humidity ≤40%RH. It must never be placed open again on the workstation rack. We have counted data from 12 production lines: for solder wire exposed to normal temperature and humidity environment, the oxidation rate increases by 3.2 times within 72 hours, while placing it in a moisture-proof cabinet can maintain effective soldering performance for more than 18 days.

It should be noted that silver-containing solder wires (such as SAC305) are more prone to oxidation than pure tin or tin-copper series, because silver elements accelerate electrochemical corrosion in humid environments. Even if such solder wires have just been opened, it is recommended to heat-seal them immediately after each use. We once left an unsealed roll of 0.3mm SAC305 solder wire next to the reflow oven overnight, and the next day during soldering, abnormal solder joints with BGA solder ball wetting angle >90° appeared consecutively for 5 times.

In addition, it is not recommended to use strong acid solutions such as hydrochloric acid or dilute sulfuric acid to soak and remove the oxide film. Although it works quickly in the short term, it is extremely easy to leave corrosive ions, which will inhibit the activity of the Flux in subsequent soldering and cause abnormal growth of the IMC layer of the solder joint. We verified this method in an OEM, and within three days, microporous etching marks appeared on the edge of the QFN device pad, and the precursor of tin whisker germination was clearly visible under the microscope.