Last week, when we were mass-producing a certain medical motherboard on Line A, IC pin solder bridging and missing BGA solder balls occurred consecutively across three batches. It was not until we inspected the 17th board that we found the problem stemmed from the bucket of SN63/Pb37 Solder Paste, which was labeled with an expiry date of October 2023 but had actually been in use for 42 days past its expiration. After being opened, this bucket of solder paste was stored in a nitrogen cabinet for 68 hours, but was only allowed to warm up for 1.5 hours before being loaded onto the printer for stencil printing. This caused the measured viscosity to be 23% lower than the standard lower limit, and even after the squeegee pressure was forcibly increased by 18%, release integrity could still not be maintained.

The oxidation rate of metal powder in expired solder paste increases non-linearly over time. Especially for silver-doped solder paste, after 20 days of expiration, XRF testing showed that the surface AgO content rose to 0.78, which directly weakened the wetting and spreading ability of solder joints. Meanwhile, the acid value attenuation rate of rosin resin in the Flux decreases by 0.35mgKOH/g for every week of expiration under a 25℃ environment, leading to insufficient oxide film removal efficiency. During reflow, exposed copper on pads or solder balling are common defects.

The core of solder paste control is not simply checking the packaging date, but conducting three-dimensional verification by binding temperature and humidity recorder data, opening timestamps, and production line requisition ledgers. For example, in one case, it was found that although the solder paste batch provided by the warehouse had not exceeded its shelf life, it had been exposed to 40℃ direct sunlight for 3 consecutive days during transportation. Infrared thermal imaging showed that the peak surface temperature of the bucket reached 48.6℃. Within 30 minutes after opening this batch, the viscosity dropped below the 180Pa·s warning line, requiring immediate suspension of use and activation of an alternative plan.

On-site engineers must develop the habit of spot-checking Solder Paste Viscosity with a rheometer before the first article of each shift, while using a 50x microscope to observe whether the solder paste edges after stencil release show jagged burrs. Once the burr length is found to exceed 1.2 times the diameter of a single solder particle, it is necessary to immediately trace the cold storage temperature curve, warm-up environment temperature and humidity, and stirring speed records of that solder paste, because burrs are an intuitive manifestation of unbalanced flux volatilization and aggravated metal particle agglomeration.

A certain customer once accepted near-expiry solder paste gifted by a supplier, resulting in rework of 3200 PCBs during mass production. The root cause was that their solder paste control did not incorporate "cumulative exposure time after opening" into the system's automatic warning. When the solder paste was cumulatively exposed to 22℃ workshop air for 11.5 hours, 41% of the water-soluble activator had already decomposed. At this point, even re-sealing with nitrogen cannot restore the original reaction activity, and the entire bucket must be scrapped.