The whitening phenomenon of wave soldering joints is often misjudged on the production line as Flux residue or excessively fast cooling, but in fact most cases stem from substandard purity of the Solder Bar alloy. GB/T 8012, 2013 explicitly requires that the mass fraction of the main component Sn in tin-based solder shall be no less than 99.3%, the upper limit of Pb content is 0.005%, and the total sum of As+Sb+Bi shall not exceed 0.01%. Once the Cu impurity in the solder bar exceeds 0.02%, it will accelerate oxidation in the high-temperature molten state, generating Cu6Sn5 intermetallic compounds that accumulate on the solder joint surface, forming a visible grayish-white matte layer.

We once conducted ICP-OES testing on solder bars of the same grade from three suppliers, and found that the measured Sn value of Factory A's batch was 99.27% with Pb reaching 0.007%; although Factory B's Sn reached 99.35%, its single-element Bi content was 0.006%, both violating the limit requirements of Clause 4.2 of GB/T 8012, 2013. The corresponding PCBA solder joints after wave soldering had white spot detection rates as high as 18.3% and 15.6% respectively in AOI inspection, far exceeding the 5% defect threshold allowed by IPC-A-610G Class 2.

Insufficient solder bar purity will also induce a chain reaction. When the Fe impurity in Sn exceeds 0.001%, hard FeSn2 particles will continuously precipitate in the wave soldering tin bath. These particles not only embed into solder joints to cause microcracks, but also accumulate on the surface of the tin pump impeller, leading to flow attenuation. A customer thus experienced a sharp rise in solder joint fillet lifting ratio from 2.1% to 9.7%, and the frequency of shutdown for tin pump cleaning increased from once a week to once a day, directly pushing up equipment maintenance costs by 37%.

On-site operations must implement a dual-track verification mechanism. Before new batches of solder bars are put into storage, a third-party SGS test report must be requested and its original data page checked; at the same time, samples shall be taken from the molten tin bath for a rapid titration test. If the titration endpoint color changes from bright red to light pink, it indicates that the Sn content is lower than 99.3%. At this point, feeding must be stopped immediately and the supplier quality traceability process initiated. It is strictly prohibited to avoid risks by the approach of "use first, inspect later".

The impurity removal process during solder bar smelting directly affects the final purity. Solder bars produced by vacuum induction melting have an average oxygen content 42% lower than those smelted in ordinary power frequency furnaces. In our comparative tests, we found that the white spot rate of solder joints for the former was stable within 0.8%, while that of the latter fluctuated between 3.2% and 6.9%. This shows that when purchasing, one cannot only look at the product's nominal grade, but must also confirm whether it has vacuum deoxidation qualification and furnace records corresponding to the production batch number.

Wave soldering parameter settings cannot compensate for defects in the solder bar itself. Even if the preheating temperature is increased from 120°C to 145°C and the chain speed is reduced from 1.2m/min to 0.9m/min, the improvement rate for solder joint whitening caused by excessive Bi is less than 11%, which instead exacerbates the risk of PCB substrate delamination. Therefore, process adjustments must be moved forward to the material access stage. Any solder bar that fails to provide a full-element test report from the past three months shall be rejected without exception.

To determine on-site whether solder joint whitening stems from solder bar problems, three dimensions must be investigated simultaneously. First, use XRF to perform area scan analysis on the solder joint area; if the Sn/Cu atomic ratio is found to be lower than 28:1, it is highly indicative of Cu contamination. Second, check the color of the sediment at the bottom of the tin bath; normally it should be gray-black dense lumps, and if it appears as yellowish-brown loose flocculent, it indicates a large amount of oxide formation. Third, observe whether there are serrated burrs at the edge of the solder joint; this feature is directly related to the abnormal increase in wetting angle caused by insufficient Sn purity.

Clause 5.4 of GB/T 8012, 2013 mandatorily stipulates that each batch of solder bars must be shipped with a declaration of conformity and test data. However, in reality, some small factories reuse the same report for multiple furnace batches. It is recommended to randomly select two packages of solder bars during acceptance, number them separately and send them for inspection. If the deviation of test results exceeds 0.03%, it shall be deemed data fraud, and a claim procedure shall be initiated in accordance with Article 40 of the Product Quality Law.