When accepting new batches of Solder Bars on the production line, the first thing we do is take a sample of about 20 grams and melt it on an electric furnace. If obvious smoke, excessive dross, or continuous yellowing and blackening of the surface occur during melting, it indicates that oxides or organic impurities exceed the standard. High-purity solder bars should quickly melt into a clear, silvery-white liquid at around 232°C, with strong fluidity and no obvious bubbling. After cooling and solidifying, cut it with a hacksaw to observe the fracture surface. Solder bars with a purity ≥99.3% have a fine, silver-gray luster on the cross-section, with no obvious graininess or dark inclusions. If grayish-white patches or matte areas appear, it is likely mixed with low-melting-point impurities such as lead, bismuth, or antimony.

The scraping test is equally critical. Use a hard scraper to scrape the surface of the solder bar at a 30-degree angle at a constant speed. High-quality solder bars will produce continuous, curled, silvery bright flakes, with a smooth feel and no obvious resistance. Inferior solder bars doped with copper or iron are prone to producing debris, dark scratches, or even tool slipping, because impurities change the ductility and grain structure of the tin matrix. We once encountered a batch of solder bars nominally rated at 99.7%, which continuously caused solder joint spikes and bridging in wave soldering for three days. Finally, on-site scanning with a portable XRF instrument revealed that the copper content was as high as 0.18%, far exceeding the 0.08% upper limit specified in IPC-J-STD-006, leading to an abnormal increase in solder surface tension.

Flux reaction is also an important clue. Drop an equal amount of rosin-based flux onto the surface of the molten tin liquid. Pure tin will quickly spread to form a uniform film with clear edges and no shrinkage. If local shrinkage, shrinkage cavities, or edge wrinkling occur, it indicates that the tin liquid contains metal impurities that affect interfacial activity. In addition, the change in solution turbidity after soaking the solder bar in a constant temperature bath for 2 hours can also assist in judgment. High-purity solder bars hardly discolor deionized water, while batches containing lead or cadmium will slightly yellow the solution, which is closely related to the dissolution rate of impurity ions.

It must be emphasized that visual inspection and feel are only suitable for preliminary screening. The key data that truly determines whether to accept the goods comes from the XRF inspection report. In particular, the actual readings of the four main control elements Pb, Cu, Fe, and Bi must be checked, and the COA documents provided by suppliers cannot be relied on solely. Our workshop stipulates that all solder bars must complete point measurements at at least 3 different positions before warehousing. If the single-point deviation exceeds ±0.02%, a re-inspection process will be initiated. A so-called "lead-free high-purity solder bar" purchased once was found to have an actual Sn content of only 98.41% in XRF scanning, with the rest being SnO? and trace Al, which simply failed to meet the Sn99.3 Grade A requirements in the JIS H 3201 standard, and the entire batch was directly returned.

Finally, a reminder: a slight oxide film on the surface of the solder bar is normal. However, if it has been stored unopened for more than 6 months and large areas of grayish-white spots appear, no matter how high the nominal purity is, it must be downgraded for use in non-critical stations. Because the oxide layer cannot be completely reduced during the melting process, it will significantly reduce the solder joint wetting angle and increase the void rate. This is an empirical conclusion we have repeatedly verified on hundreds of SMT lines over the past twenty years.