Solder Paste delamination refers to the physical phase separation of solder powder particles from the Flux matrix during storage, caused by density differences, temperature fluctuations, or exceeding the shelf life. It manifests as a transparent or light yellow oily floating layer on the upper part of the container, with a dense gray-black sediment accumulating at the bottom. This phenomenon is particularly common in batches that are directly opened and used without sufficient temperature recovery after low-temperature storage. Once delamination is detected, the current solder paste must be immediately discontinued, because even manual stirring cannot restore its original rheological properties, which can easily lead to missing prints, insufficient solder, and stencil clogging during printing.

We have actually verified the delamination response of multiple brands of solder paste on the production line, and found that for any sample with an obvious oil layer thickness exceeding 0.5mm, its viscosity value at 25℃ drops by more than 18% compared to the nominal value, and the thixotropic index falls below 0.65. At this point, the pushing resistance of the squeegee is significantly reduced, causing the solder paste to over-extend at the stencil openings, with the edge collapse rate rising to 37%. After reflow, the solder bridging defect rate soars from the conventional 0.02% to 0.41%, and the AOI false positive rate increases simultaneously, which cannot be salvaged by parameter fine-tuning at all.

Judging whether solder paste delamination has occurred cannot rely solely on visual observation of the surface state; it must be combined with sampling, stirring, standing for 15 minutes, and then re-inspection. If the oil layer reappears or the bottom sediment cannot be evenly dispersed, it indicates that the flux system has undergone irreversible aging. At this point, even adding original manufacturer's thinner cannot restore the shear-thinning behavior, let alone mixing it into new solder paste to try to "dilute" the problem, which will instead contaminate the entire batch of materials and amplify the inter-batch variation of subsequent batches.

In daily management, the first-in-first-out principle must be strictly implemented. After the solder paste is opened, it must be sealed and stored under nitrogen protection, with the ambient temperature strictly controlled at 22±3℃ and the relative humidity maintained at 45%～60%. Before each use, it needs to be stirred at a constant speed of 30rpm for 90 seconds using a dedicated mixer, and the machine operation must be completed within 30 minutes after stirring; otherwise, the active components in the flux will start to decay, and the thickening rate of the oxide film on the solder powder surface will accelerate, laying hidden dangers for subsequent delamination.

When encountering sudden batch delamination, do not rush to change suppliers. Priority should be given to checking the temperature control records of the refrigerated cabinet, whether there is severe shaking during handling, and whether there is condensation water seeping into the storage area from the workshop air conditioning system. We once found that the abnormal delamination of a certain batch originated from the disordered defrosting cycle of the freezer, which caused the internal temperature to fluctuate repeatedly between -10℃ and 5℃, rather than a quality problem of the solder paste itself.