Severe oxidation of Solder Bars is not a problem caused by a single process step, but a concentrated manifestation of uncontrolled conditions across the entire workflow from incoming storage, soldering iron operation to wave soldering. Our production line once experienced a large number of solder balls and bridging defects for three consecutive days, and the root cause was eventually identified as excessive humidity in the warehouse and failure to store opened solder bars under nitrogen protection, which led to the formation of a dense oxide film on the surface. When this oxide layer enters the solder pot, it cannot be completely removed by Flux, and instead accelerates the imbalance of surface tension on the new molten solder, further causing splashing and poor solder joint wetting.

When dealing with severely oxidized solder bars, simply heating and stirring is absolutely not allowed. Instead, a stainless steel scraper must first be used to thoroughly remove dross, and the machine must be shut down to cool down to below 180°C, then new solder bars are added in batches for gradient melting replacement. The amount added each time must not exceed 15% of the total original molten solder, otherwise thermal shock will cause a large amount of oxides to be trapped inside the solder joints. At the same time, the entire set of flux must be replaced synchronously, because the active ingredients in the old flux have been completely consumed, and the residual halides will instead intensify the electrochemical corrosion of the solder bars.

The key to reducing the oxidation rate of solder lies in blocking the oxygen contact path. We require all solder pots to be equipped with an adjustable-speed nitrogen protection device, and the outlet pressure must be stably controlled at 0.03MPa. This not only prevents excessive nitrogen flow from disturbing the solder surface, but also ensures that the molten solder surface is always covered by inert gas. In addition, an infrared temperature gun must be used to calibrate the actual temperature of the solder pot before daily operation, to prevent the temperature controller from showing a falsely high reading that causes the actual temperature to remain above 260°C for a long time, because every 10°C increase in temperature will raise the oxidation rate of solder bars by approximately 2.3 times.

Fluctuations in the inherent quality of solder bars are also often overlooked. Under the same process conditions, the oxidation degree of solder bars from different manufacturers within the same batch can vary by up to 40%. Through three months of batch tracking, we found that solder bars with a bismuth content higher than 1.2% produce significantly less dross under 255°C operating conditions, but the preheating temperature must be adjusted synchronously to prevent cold soldering. In addition, all solder bars must be vacuum-sealed with moisture-proof aluminum foil before storage, and the relative humidity of the warehouse must be long-term controlled at ≤45%RH. Once opened, they must be used up within 24 hours. Solder bars that are not used after the time limit must go through three processes: ultrasonic cleaning + alcohol wiping + infrared drying before they can be put back into use.

The condition of the scraper also has a hidden impact on solder oxidation. When the blade edge wear of the scraper exceeds 0.1mm, micro-turbulence will be generated when the scraper passes over the wave peak, causing deep molten solder to frequently surge to the surface, which is equivalent to artificially increasing the oxidation interface area. Therefore, we stipulate that the scraper gap must be checked with a feeler gauge every shift. If intermittent bright spots are found in the contact area between the scraper and the solder surface, it indicates that the blade edge has been dulled and must be replaced immediately, otherwise the amount of dross will continue to rise even if the nitrogen and temperature conditions meet the standards.