Whether expired Solder Paste can still be used cannot be judged solely by the expiry date marked on the package; it must be comprehensively evaluated based on its actual storage temperature, number of times the container has been opened, stirring method, and warm-up time. For example, a batch of solder paste with a nominal shelf life of 6 months, if stored throughout in a cold storage below 10°C and not repeatedly opened, may still have sufficient activity to meet the printing requirements for 0402 components. In contrast, another batch stored on a shelf at room temperature of 25°C for 15 days beyond its expiry date and frequently opened for use, even if it has not reached the end of its shelf life, is highly prone to slumping and poor release.

We once continuously tracked three batches of Sn63Pb37 Solder Paste that were 7 days past their expiry date at an OEM factory in Dongguan. We found that after 4 hours of warm-up, two of the batches showed a significant decrease in printing tension, and the squeegee pressure had to be increased by 0.2 MPa to maintain thickness uniformity. The third batch, however, had been mistakenly placed near an air conditioner vent during refrigeration, and a micro-gel-like floating layer had appeared on its surface. Under a microscope, local separation of the Flux and alloy powder was visible, and this batch was directly scrapped.

The key action to determine whether solder paste can be used beyond its expiry date is to conduct a small-batch process verification. First, use the same stencil on the same DEK printer to trial-print 10 boards each, then run them through the reflow oven using a standard reflow profile. Subsequently, use AOI to compare the solder joint bridging rate, number of solder balls, and solder paste climb height. If the fluctuation of any of these three indicators exceeds 15% of the average value of a normal batch, the solder paste must be discontinued and the material return process initiated. Under no circumstances should saving a few kilograms of solder paste cause the entire production line to be plunged into a batch rework crisis.

Purchasing personnel often overlook the logic behind when the solder paste expiry date starts. In reality, counting from the date of manufacture only applies to unopened containers stored under constant temperature and away from light. Once opened for use, even if 80% of the content remains, its effective window will be shortened to one-third of the original shelf life. For example, solder paste with a nominal shelf life of 6 months is recommended to be used up within 60 days after opening; otherwise, even if there are no visible abnormalities, the degree of aging of its rosin resin will be sufficient to affect the stability of the wetting angle.

The most easily overlooked detail on the production floor is the warm-up procedure. If expired solder paste is taken out of cold storage and forcibly stirred without being allowed to stand for a sufficient period, internal air bubbles will not be able to escape, resulting in void-type solder deficiency during printing. The correct procedure is to place the solder paste container flat in an environment of 22°C ± 2°C and let it stand for more than 8 hours, then stir it at a low speed (≤30 rpm) for 3 minutes at a constant speed. Finally, use a viscometer to measure whether its apparent viscosity falls within the manufacturer's recommended range; if the deviation exceeds 5%, it cannot be put into production.