The first step of incoming inspection for Solder Wire is always the combination of naked eyes and calipers. After receiving a new roll of solder wire, we first tear open the outer package to check whether the entire roll is neatly arranged, and whether there is obvious oxidation blackening or oil stain attachment on the surface. Then we use a vernier caliper to measure the diameter at three different positions and check the tolerance against the nominal value. For example, if the measured value of 0.8mm Solder Wire is 0.72mm or 0.86mm, it must be intercepted, because the gap of the wave soldering nozzle is usually only 0.05mm larger than the nominal diameter of the solder wire, and out-of-tolerance will cause feeding jamming or uncontrolled solder amount at the solder joint.

Next, cut a 3cm solder wire, lay it flat on a white porcelain plate, and use a magnifying glass to observe the uniformity of Flux distribution. The flux of high-quality solder wire should be translucent amber and continuously covered along the axial direction. If there are intermittent stripes, local dryness or milky white turbid lumps, it indicates that the thermal stability of rosin is insufficient or the proportion of halogen activator is unbalanced. Such solder wire is prone to problems such as increased smoke, blackened solder joints and even PCB surface corrosion when soldering above 250℃.

The third step is to do a simple wetting test. Take a clean unplated copper sheet and heat it to 260℃, gently touch the tip of the solder wire to the copper surface to observe the melting spreading speed and edge integrity. Qualified products should complete wetting within 1.5 seconds and form a smooth fan-shaped pad. If there is solder shrinkage, weak solder climbing or serrated edges, it indicates that the active agent is invalid or the solder matrix contains impurities. Such solder wire will cause bridging, cold soldering and increased solder joint void rate during actual furnace passage.

Finally, it is necessary to spot-check the rosin residue state. Scrape a small amount of flux residue, drop it into ethanol and stir it. Clear and transparent means that the halogen content is compliant. If it is milky white turbid or layered, it indicates that the chloride ion exceeds the standard. After such solder wire is stored in a high temperature and high humidity environment for 72 hours, white crystalline corrosion traces visible to the naked eye will appear on the surface of the PCB pad, which conventional IQC inspection will not do at all.

All inspection actions must be closed within 30 minutes after receipt, and cannot wait for the warehouse to send for inspection uniformly. Because once the solder wire is mixed into the production line and then traced back, it will involve the rework cost of the whole batch of PCBA. We once found in a batch abnormality that the same batch of solder wire from the same manufacturer was OK on line A and NG on line B, and finally locked that it was caused by moisture absorption and hydrolysis of the flux due to moisture during transportation. Therefore, each unpacking must simultaneously record the ambient temperature and humidity and cross-compare with the storage conditions.