Excessive impurities in wave soldering bars are not an accidental phenomenon, but an inevitable result of lax procurement control, supplier material mixing, or uncontrolled repeated smelting of recycled tin. When the copper content in the tin bar exceeds 0.3wt%, the long-term contact between the solder and copper nozzles at a wave soldering temperature of 255℃±5℃ will accelerate the formation of Cu6Sn5 intermetallic compounds, leading to increased solder viscosity and poor fluidity, which in turn causes solder joint spikes and trailing. Once the iron content exceeds 0.02wt%, a large number of hard FeSn2 particles will precipitate at the bottom of the tin bath. These particles enter the wave with the solder circulation and embed into the surface of PCB pads, causing local wetting failure and cold solder joints. In our analysis of three consecutive batches of defective products at an OEM factory in Dongguan, we detected that the iron particle enrichment areas highly coincided with the positions of open circuit points.

The most intuitive manifestation of excessive impurities is that the PCB surface turns black after soldering and cannot be removed by conventional cleaning agents. This often stems from the complexation reaction between metalloid impurities such as arsenic and antimony and flux residues at high temperatures, forming an insoluble black film with a thickness of up to 0.8～1.2μm. When the arsenic content is >0.005wt%, this phenomenon is particularly prominent in an environment with a nitrogen protection concentration of <100ppm. At the same time, if the total amount of low-melting-point impurities such as lead and bismuth exceeds 0.15wt%, it will widen the solidification range of solder joints and induce microcracks under a cooling rate of >2.5℃/s. Such defects are almost impossible to identify by AOI at the bottom of 0.4mm pitch QFN packages, and must be detected by X-ray sampling inspection.

In daily control, the composition of tin bars must be sampled and inspected for each batch. When using an XRF fluorescence spectrometer for detection, the number of sampling points shall be no less than 5, and the single-point test time shall be no less than 45 seconds to ensure stable data. The Sn purity of qualified tin bars shall be ≥99.3wt%, Cu≤0.25wt%, Fe≤0.015wt%, As≤0.004wt%, Bi≤0.05wt%. We once ignored the retest of a batch of tin bars containing 0.072wt% bismuth, which caused the FTL first-pass yield of the entire line to drop sharply from 99.2% to 94.6%, and the rework man-hours increased by 3.8 hours per thousand pieces. Wave soldering parameters must be tightened simultaneously. The tin bath temperature is recommended to be lowered to 252℃±3℃, and the conveyor speed is adjusted from 1.4m/min to 1.2m/min to extend the immersion time and compensate for the loss of wetting force. At the same time, the wave height is finely adjusted from 8.5mm to 7.2mm to avoid excessive solder accumulation and aggravate the risk of bridging.

Abnormal troubleshooting cannot only focus on the furnace temperature curve. First, use a stainless steel spatula to take samples from the middle, bottom, and wave outlet of the tin bath respectively, cool them to prepare metallographic test pieces, and observe under a 500× metallographic microscope whether there are obvious heterogeneous phase particles. If gray-white lumps with a size >8μm are found and the EDS spectrum shows that the Fe/Sn atomic ratio is close to 1:2, it can be determined as FeSn2 precipitation. Secondly, check the tin slag morphology. Normal oxidized slag is loose and flocculent and easy to salvage, while tin slag with excessive impurities appears as dense lumps with a blue-purple luster on the surface and a pungent sour smell. At this time, it is necessary to immediately shut down the machine, clean the bath, and replace it with new tin. It is strictly forbidden to add so-called "purifying agents" to cover up the problem. A factory once caused two pump motors to burn out continuously within three weeks due to this.

The experience and skill lies in establishing a tin bar life archive. After the cumulative use of tin bars of the same brand and batch exceeds 1200kg, they must be forcibly retired even if the composition test is qualified, because multiple thermal cycles will continuously enrich trace impurities. Our tracking test shows that the actual Cu equivalent of the tin bar after using 1500kg has reached 0.29wt%, which is close to the critical value. In addition, before each tin replacement, be sure to use a 300-mesh stainless steel filter to hot-filter the old tin, and control the filtration flow rate within 0.8L/min. If the weight of the residue after filtration is >0.7% of the initial tin weight, it indicates that the impurities have seriously exceeded the standard. At this time, even adding new tin cannot reverse the deterioration trend, and the entire bath must be replaced.