If the rosin core hollow of the 0.5mm Solder Wire we use daily on the production line is made to 0.12mm, the solder joint forms stably, with less splatter and thin, uniform residue. Once the hollow expands to 0.18mm, rosin bursting and splashing is clearly visible as soon as the soldering tip touches the pad, especially in the dense pin area at the bottom of QFN packages; the sprayed rosin oil quickly carbonizes and wraps around the root of the solder joint, creating a false cold solder joint appearance. When the hollow shrinks to 0.08mm, the molten solder flow becomes noticeably sluggish at the same temperature, the surface tension of the solder joint is out of control, and it is easy to pull out thin solder whiskers, resulting in insufficient solder pile-up, with the AOI false alarm rate rising by more than 12%.

The rosin core hollow is not better when thicker, nor more stable when thinner; it must form a closed-loop match with the Solder Wire outer diameter, alloy composition, and soldering iron temperature curve. For example, for 63/37 Eutectic Solder wire at a soldering iron temperature of 330℃, a 0.12mm hollow releases approximately 0.0042mg of rosin per millimeter, which just meets the wetting and coverage requirements for a 0.3mm pad. If switching to SAC305 containing 0.3% silver, due to the 15℃ increase in melting point, rosin carbonizes prematurely under the same hollow size; in this case, the hollow needs to be fine-tuned to 0.11mm to avoid carbon residue accumulation.

Most customer feedback of blackened solder joints is not a problem with rosin quality, but rather that an excessively large solder wire hollow causes rosin to volatilize excessively in the high-temperature zone and then re-deposit on the edge of the solder joint; a black ring-shaped carbon film is visible under a microscope. This phenomenon is particularly prominent after continuous soldering of more than 8 points. After switching to solder wire of the same specification with a 0.10mm hollow, no obvious carbonization is observed even after continuous soldering of 20 points. However, a hollow below 0.09mm will cause difficulty in initial tinning, especially for gold-plated pads with heavy oxidation, requiring an additional 0.5 seconds of preheating time to achieve effective wetting.

We tested three types of 0.5mm solder wires from different manufacturers, all with a nominal rosin content of 2.2%, but the measured hollow tolerances reached ±0.015mm, ±0.022mm, and ±0.031mm respectively. For the one with the largest tolerance, after continuous operation on the automatic soldering machine for 2 hours, the frequency of wire feed wheel jamming increased by 3 times; upon disassembly, it was found that rosin carbon powder formed layered accumulation in the wire feed channel. In contrast, the solder wire with tolerance controlled within ±0.015mm showed no abnormalities throughout the process, indicating that the stability of the hollow size affects equipment adaptability more than the absolute value.

When changing solder wire batches on the production line, hollow size sampling inspection must be conducted; use a pin gauge with an accuracy of 0.005mm to measure at least 10 cross-sections, and do not rely solely on the manufacturer's factory report. Because the hollow size of the front and rear sections of the same roll of solder wire may differ by 0.02mm, this difference is not easily noticeable during Manual Soldering, but will directly cause nozzle blockage or flow mutation at the fine-pitch nozzle of Selective Wave Soldering.