Once the rosin core of Solder Wire breaks, the most immediate manifestation during soldering is intermittent Flux release, which leads to a lack of effective activation the moment the soldering iron tip contacts the solder joint, preventing the solder from fully wetting the copper foil or pins, and subsequently causing typical defects such as dull solder joints, shrinkage cavities, and insufficient edge climbing. We consecutively found 3 batches of cold solder joints on 0805 resistors of the same model at the manual rework station for SMD LEDs. After disassembly, it was confirmed that all issues stemmed from microcracks in the internal rosin core of the solder wire at approximately 2 meters from the end of the spool, which exactly corresponds to the stress concentration area of the crimped tail of the previous solder wire roll.

If the rosin core breakage is not detected in time, the operator continues to feed the wire at the normal speed, resulting in continuous melting of the solder wire body without flux. The solder joint surface rapidly oxidizes to form a gray-black film layer, while the tension imbalance of the molten solder causes obvious spike formation and spherical splashing. This phenomenon is particularly prominent in manual rework of QFN devices with 0.3mm pitch, because the heat conduction path is short and the temperature response is fast, and the wetting lag caused by flux deficiency is sharply amplified.

We have reproduced similar problems in the flux pre-coating process of SMT wave soldering. When solder wire is used to make flux dilution liquid, if rosin core debris enters the nozzle, it will cause fluctuations in atomization pressure and uneven spraying. At this time, the false alarm rate of AOI before the furnace increases by 12%, which is not caused by abnormal solder joints themselves, but by the decrease in flux coverage leading to unactivated oxide layers in some areas, and subsequent wave soldering wetting failure is misjudged by the system as an open circuit.

Production line verification shows that rosin core breakage mostly occurs in the cold work hardening zone at the end of solder wire drawing. Especially for fine solder wires with a diameter below 0.6mm, when the high-speed winding tension exceeds 1.8N, insufficient interfacial shear strength between rosin and tin matrix easily leads to axial microcracks. Such defects are almost invisible to the naked eye, but when observing along the axial direction of the solder wire with a 10x magnifying glass, local enhanced light transmittance or slight dents can be found. It is recommended that IQC sampling inspection add an axial torsion test step, that is, take a 30cm solder wire, fix both ends, apply a torque of 0.5N·m for 10 seconds, and if the rosin core shows displacement or fragmentation, it is judged as unqualified.

If similar problems still occur after replacing with a new batch of solder wire, it is necessary to simultaneously check whether the pressing force of the wire feed roller is too large, because overloaded pressing will aggravate the extrusion deformation of the rosin core. Especially when using high-activity solder wire containing halogen activators, the increased brittleness of the rosin matrix makes it more prone to cracking under mechanical stress. At this time, even if the solder wire looks intact, its internal rosin core already has hidden delamination, which will suddenly disintegrate when heated during soldering.