The tightness of the Solder Wire winding spool is not a simple appearance issue, but a key process parameter that directly determines whether the solder wire can stably pass through the conduit and soldering iron tip under the drive of the solder feeding motor. We once continuously tracked three automatic solder feeders of different brands for 72 hours, and found that when the winding tension of the same roll of solder wire is too high, the solder output speed fluctuates by up to ±8mm/s, while when the tension is too low, instantaneous pauses of more than 0.3 seconds occur frequently; an overly tight winding will cause micro-deformation indentations on the surface of the solder wire, which is especially obvious for fine-diameter solder wires below 0.3mm. After entering the solder feeding wheel, such indentations will intensify the frictional resistance with the wheel groove, and further trigger the equipment overcurrent protection shutdown. On the other hand, an overly loose winding will cause inter-layer slippage of the solder wire on the spool, leading to angular deflection during solder feeding, and eventually resulting in the solder wire bending and jamming at the soldering iron tip or jumping out of the conduit laterally.

In actual production lines, we have measured with a spring scale that the recommended winding tension for standard 0.6mm Solder Wire is 0.8～1.2N. If the tension is lower than 0.6N, slipping is prone to occur during high-speed solder feeding (＞120mm/s), while if it is higher than 1.4N, obvious metal scratches will start to appear at the meshing part of the solder feeding wheel. After such scratches accumulate for three days, the solder feeding accuracy will decrease by more than 0.15mm; what is more troublesome is the consequences caused by uneven winding tightness, such as half a turn tight and half a turn loose. This situation accounts for as high as 37% in Domestic Solder wires. It will not immediately trigger an alarm, but will be concentratedly exposed as solder joint spikes after 200 consecutive welding points, with the proportion of cold solder joints increasing by 12%, because the alternating tightness changes the axial distribution uniformity of the internal Flux core of the solder wire, and further affects the wetting consistency during the melting stage.

The winding operation must be completed in a constant temperature and humidity workshop. For every 5℃ increase in temperature, the elongation of the solder wire increases by 0.6. At this time, if tension is applied according to the room temperature standard, the tension will rebound by about 9% when the actual winding is cooled to 25℃. Therefore, in summer, the initial tension must be reduced by 0.15N to ensure the stability of the online use; we also found that some operators are accustomed to pressing the outer edge of the winding spool with their fingers to "feel" the tightness. This method has extremely large errors, and the deviation of the same person's three hand-feel judgments can reach 0.5N. The correct approach is to use a digital-display winding tension meter with a 200g standard weight for on-site calibration. This action must be performed before each model change, otherwise the nozzle blockage rate of 0.4mm solder wire in QFN package welding will soar from 2.1% to 6.8%.

The winding direction must also strictly match the designed rotation direction of the solder feeder. Even if the tightness is qualified, reverse winding will cause reverse torsional stress on the solder wire at the entrance of the solder feeding wheel. This stress is superimposed on the 0.03mm concentricity tolerance of the solder wire itself, which will make the solder output trajectory deviate by more than 0.12mm. The result is continuous solder balls or bridging at the solder joints at the bottom of the BGA, and such problems are often misjudged as Solder Paste printing offset in AOI inspection, wasting machine adjustment time in vain.