Over three consecutive days on a production line at a customer site in Dongguan, we observed excessive spike formation on PCB surface solder joints after wave soldering, accompanied by localized bridging. Cross-sectional analysis revealed irregular grayish-black inclusions inside the solder joints. Synchronized composition testing of the current batch of solder bars showed a copper content of 0.082%, far exceeding the 0.05% upper limit specified in IPC J-STD-006B, while iron impurity reached as high as 0.015%, indicating that this batch of solder bars was contaminated with copper-containing alloy scrap and equipment wear iron filings during smelting or recycling. Copper in solder impurities preferentially forms Cu6Sn5 intermetallic compounds with tin. These hard phase particles not only impede the wetting and spreading of molten solder on pads, but also continuously accumulate in the solder pot and adhere to the inner wall of the titanium alloy nozzle, causing distorted jet flow patterns and unstable wave height. We observed obvious grayish-brown deposits on the nozzle edge using an endoscope, with a thickness of approximately 0.15mm, directly resulting in insufficient solder supply in some areas and extended tailing. More seriously, once arsenic impurities exceed the limit (>0.001%), they volatilize at high temperatures and react with Flux residues to form white powdery As2O3. This substance not only contaminates the oven, but also condenses on the solder joint surface in the cooling zone to form a hydrophobic film, increasing the AOI false rejection rate by over 40%. At a factory in Zhongshan, we scrapped two consecutive batches of USB interface boards due to this issue. If lead content in Solder Bars fluctuates beyond ±0.03% due to uncontrolled mixing, it will cause eutectic temperature shift. We measured that the liquidus temperature of an abnormal batch of solder rose from 183℃ to 185.7℃, resulting in dewetting on all QFP device pins after wave soldering. Under a microscope, solder was observed to cover only 30% of the pin side area. In daily control, spectral composition sampling must be conducted for each batch, with particular attention to the four key impurities: copper, iron, arsenic, and lead. Relying solely on supplier factory inspection reports is not acceptable, because peeling of the surface oxide layer on solder bars during transportation may expose underlying high-impurity layers. We once sent samples from the upper and lower layers of the same unopened package of solder bars for testing, and the copper content differed by 0.021%. Before replacing with new solder bars, the old solder pot must be completely emptied and the nozzle cleaned with a dedicated dross removal agent; otherwise, residual high-impurity solder will quickly contaminate the entire pot of new material. A factory once skipped this step, resulting in copper content rebounding to 0.071% just 4 hours after new solder bars were loaded into the pot.

Wave soldering defects caused by solder impurities are never isolated phenomena. They often manifest simultaneously as three signals: decreased solder joint gloss, deteriorated solder fluidity, and accelerated dross generation rate. If dross volume increases by more than 3 times the normal value, it can basically be confirmed that the solder bar itself has excessive impurities. At this point, production should be immediately stopped for sampling and sent to a third-party laboratory for full-element ICP testing, rather than blindly adjusting wave height or conveyor speed. We found that after high-impurity solder bars were left at a constant temperature of 260℃ for 2 hours, surface floating dross accounted for 12.7% of the total weight, while qualified solder bars only had 3.1%. This difference is visible to the naked eye on the production line: the floating dross appears as dark brown lumps that are difficult to skim, entraining large amounts of unmelted metal particles. In addition, solder bars with high impurities show significantly increased spattering during wave soldering. We recorded with a high-speed camera that under the same parameters, the number of spattered droplets was 2.8 times that of qualified material. These tiny solder balls cool and adhere to the back of the PCB, creating a potential short-circuit risk, which is especially hazardous for 0.4mm pitch BGA substrates. Customers often mistakenly believe that increasing flux spray volume can improve wetting. In reality, the reaction activity of high-impurity solder with flux has been severely degraded, and excessive spraying instead exacerbates PCB surface whitening and excessive ionic residue. A medical customer received a nonconformity finding during a UL factory audit due to this issue. Therefore, to determine whether a solder bar is qualified, one cannot rely solely on melting point and spreadability. A comprehensive judgment must be made by combining actual wave soldering performance, dross condition, and solder joint cross-sectional morphology. Any single-dimensional data may mask the true risk.