We found that the tombstoning rate of 0201 resistors on the production line had risen abnormally for three consecutive days. During troubleshooting, we observed that the same can of refrigerated Solder Paste that had been opened for 72 hours showed obvious darkening at the edge of the stencil aperture and a dragging, sticky feel on the squeegee, while unopened solder paste from the same batch printed normally. This indicates that solder paste oxidation has begun to affect the surface activity of the metal particles, thereby disrupting the wetting balance between the solder paste and the pads.

If solder paste is taken out of the refrigerator and the lid is forcibly opened without allowing it to warm up to room temperature in time, condensation will directly adhere to the paste surface. Combined with the fact that workshop humidity often exceeds 65%RH, the copper-tin alloy particles rapidly form a stannous oxide film under the synergistic action of moisture and oxygen. This oxide layer not only reduces the activation efficiency of the Flux, but also causes the solder paste to exhibit characteristics such as sudden viscosity increase and reduced fluidity under the shear force of the squeegee, ultimately reflected in a 12% rise in stencil clogging rate and a 0.8μm deviation in the printing thickness CV value.

A more hidden problem is that some operators habitually loosen the solder paste can lid by half a turn before putting it back into the refrigerator, believing this prevents gas buildup. In reality, this allows the humidity inside the cabinet to fluctuate frequently with door openings and closings, causing the refrigerated solder paste to repeatedly undergo condensation and evaporation cycles. The oxygen content of the air at the top of the can remains consistently high, and the oxidation depth of the solder paste surface layer can reach 15–20μm within 48 hours, far exceeding the 5μm safety threshold specified in IPC-J-STD-005.

We once conducted comparative tests on three sealing methods: solder paste with a normal screw lid not fully tightened showed an average 23% drop in thrust test after 7 days of refrigeration, while solder paste vacuum-sealed to -80kPa before refrigeration showed only 4.6% thrust attenuation within 14 days. This indicates that solder paste oxidation is not a univariate problem of time, but rather the result of the coupled interaction of three factors: the airtightness of the refrigeration environment, the residual oxygen content inside the can, and temperature gradient changes.

The production line has now enforced the "double-lid method": the inner lid must be fully tightened with no gaps, the outer lid is fitted with a silicone sealing ring and then locked a second time, and after each use, the residual solder paste on the can mouth must be wiped with an alcohol pad before closing the lid to prevent dried residue from prying open the sealing surface. This procedure may seem cumbersome, but it has extended the average effective service life of a certain type of lead-free solder paste from 96 hours to 142 hours, and reduced the scrap rate caused by oxidation by 0.07%.

It is worth noting that not all refrigerated solder pastes are equally sensitive. Low-temperature solder paste containing bismuth elements has a denser BiO? oxide film, making it approximately 1.8 times more resistant to sealing failure than the SnAgCu system. However, once oxidation initiates, its reflow recovery ability is actually worse. Therefore, sealing control of refrigerated solder paste must not be relaxed simply because of short-term apparent stability.