At 3:17 p.m. last Wednesday, the AOI after the SMT line A placement machine continuously triggered an alarm for 12 boards, where the 0402 resistor solder joints appeared matte grayish-white. Under a microscope, it was observed that the solder had not climbed to the side of the component end caps, while the same spool of Solder Wire showed no abnormalities on line B. We immediately stopped the line and took samples for comparison, finding that the measured Flux Content of this batch of solder wire was 2.8%, far exceeding the process window requirement of 2.1% ± 0.3%. Excessive Flux caused high-temperature decomposition carbides to accumulate at the solder front, severely hindering solder spreading and metal interface bonding, which directly deteriorated solder joint wetting.

On-site engineers used alcohol cotton swabs dipped in rosin-based cleaning agent to repeatedly wipe the solder joint surfaces, then performed local rework soldering with a hot air gun at 800°C for 3 seconds, barely restoring partial wetting morphology, but the Solder Joint Strength did not meet the IPC-A-610E Class 2 standard. This emergency treatment is only applicable to small-batch rework and must be accompanied by simultaneous X-ray testing to confirm whether the internal void rate exceeds the standard, because thermal expansion of flux residue will induce microvoid aggregation, while simply relying on strong hot air blowing will instead exacerbate the risk of solder ball splattering.

The root cause was identified as the absence of flux content testing items in the incoming inspection process. The COA document provided by the supplier only stated "compliant with JIS Z3197" without providing measured values, while our company's IQC only performed melting point and spread rate tests, and did not conduct flux content titration sampling inspections in accordance with Q/SPC-SMT-023-2022. More critically, the warehouse did not verify the consistency between the outer packaging batch number of the solder wire and the COA number during material receipt, leading to the mixing in of a high-content batch that was concessionally accepted by a contract manufacturer last month.

We have included solder wire flux content as a mandatory incoming inspection item, adopting the ethanol extraction-acid-base titration method specified in Appendix B of GB/T 9491-2021. Each batch is sampled with no fewer than 5 spools, and 3 cross-sections are tested per spool to take the average value. If the result exceeds 2.1% ± 0.3%, the entire batch is rejected. At the same time, suppliers are required to laser-print real-time content values on the inner paper label of each spool to eliminate the disconnect between COA and physical products; line operators must use a calibrated handheld refractometer to initially screen the flux outflow state of the solder wire before daily startup. If the flux exudation length exceeds 1.5 times the solder wire diameter, use must be suspended and the sample sent for inspection.

Preventive measures also include adjusting the soldering iron temperature curve, changing the conventional 320°C constant temperature to a three-stage temperature control starting at 300°C, peaking at 340°C, and returning to 290°C for 2 seconds. The temperature gradient is used to delay the violent volatilization rhythm of the flux, giving the solder more sufficient time to complete interface diffusion and wetting spreading; all newly introduced solder wire brands must undergo a 72-hour continuous soldering aging test. Those with a cumulative statistical solder joint wetting angle < 30° pass rate below 99.2% are directly eliminated, and no vague expressions such as "trial evaluation" are accepted.

Last month, after line F replaced with a new brand of solder wire, wave soldering bridging occurred. The investigation found that the flux content was only 1.6%, causing the solder surface tension to be too high to smoothly detach from the nozzle. Ultimately, wetting uniformity was improved by adding 30 seconds of nitrogen purging time in the preheating zone. This shows that the solder wire flux content is not the higher the better, but must strictly match the equipment thermal field characteristics and PCB pad solderability decay cycle. Especially when using OSP process boards or PCBs stored for more than 90 days, the lower limit of flux content needs to be increased to 1.9% to ensure solder joint wetting stability.