Solder Wire cold solder joints frequently occur during wave soldering touch-up and manual rework. They are not caused by a single factor, but are the combined result of fluctuations in the alloy composition of the solder wire itself, deviations in Flux Content, and the operator's wire feed angle, dwell time, and oxidation state of the soldering tip. Last week, after we replaced the 63/37 solder wire of the same batch but different reel number on Line A, the cold solder rate for BGA bottom touch-up jumped sharply from 0.8% to 3.2%. Re-testing found that the measured Flux content of this reel of solder wire was 1.8%, which is below the nominal range of 2.2±0.3%. At the same time, the soldering tip had been used for over 48 hours without replacement, and the heat transfer efficiency dropped by approximately 27%.

Cold solder joints exhibit an irregular matte contour under AOI, which forms a sharp contrast with the mirror-like reflection of normal solder joints. This difference cannot be accurately identified by visual experience alone; it must be observed under a 10x microscope to check whether the solder climb height reaches more than 75% of the pin height. Otherwise, even if the appearance is intact, there is insufficient internal wetting. If the solder wire diameter tolerance exceeds the standard by 0.02mm, it will directly cause jamming of the wire feed mechanism or uneven wire output, resulting in insufficient local solder supply. This is especially sensitive in large-area copper foil areas such as QFN thermal pads.

When troubleshooting, we never focus solely on the solder wire parameter sheet. Instead, we simultaneously measure whether the actual working temperature of the soldering iron is more than 15℃ lower than the set value. Because soldering irons of the same model generally experience temperature drift after two hours of continuous operation. In addition, when the workshop humidity is consistently above 65%RH, the flux on the solder wire surface easily absorbs moisture and fails. At this point, even increasing the soldering iron temperature cannot improve wettability; instead, it exacerbates PCB pad oxidation. Therefore, before each material change, we must use a temperature controller to actually measure the dynamic temperature curves of three commonly used soldering irons, and spot-check whether the silver content and measured melting point of the solder wire on shift fall within the standard band.

There is no "one-size-fits-all" solution for cold solder problems on site. For example, switching to a solder wire version containing 0.3% silver may alleviate cold soldering, but if the operator does not simultaneously shorten the solder joint dwell time, it will cause PCB delamination due to overheating, also leading to reliability failures similar to cold solder. Therefore, the solder wire batch number, soldering iron number, operator ID, and ambient temperature and humidity must all be recorded together, so that when batch abnormalities occur, we can quickly determine whether it is material variation or uncontrolled human operation rhythm.

Last week, cold solder on Line C was concentrated on 0402 resistors. It was ultimately determined that the solder wire winding tension regulator was loose, causing the wire feed speed to fluctuate by ±12%. This parameter was not listed as a routine item in the equipment inspection sheet. Later, we added a color-coded scale sticker next to the tension wheel and included it in the daily first article confirmation items. The cold solder rate dropped back below 0.3% on the same shift. This shows that many so-called material problems are actually illusions caused by missing process control points.