We have observed in actual production lines that repeatedly taking the same can of Solder Paste out of a -40°C freezer, thawing it at room temperature, equilibrating it, and then refrigerating it again significantly accelerates the viscosity degradation of the solder paste. Especially when the thawing time exceeds 4 hours or the ambient humidity is higher than 60%RH, the viscosity loss rate increases noticeably. Moreover, if thawed solder paste is not fully stirred and mixed before being directly used on the machine, it will also lead to poor printing consistency between the front and rear sections, with frequent occurrences of printing draw peaks and edge collapse.

A customer used SN63/PB37 lead-free solder paste (with a nominal viscosity of 750±150Pa·s). The measured viscosity remained around 720Pa·s within 24 hours after the first thaw. Under the same conditions, the measured value dropped to 610Pa·s after the second thaw, fell to 540Pa·s after the third thaw, and was as low as 405Pa·s by the fifth thaw. This degradation is not a proportional decline, but rather shows a trend of slow initial decrease followed by rapid acceleration, indicating that the interfacial stability between the metal powder and the Flux system inside the solder paste is gradually damaged under repeated thermal stress shocks.

We simultaneously compared the effects of three common thawing methods on Solder Paste Viscosity. Natural room-temperature thawing (25°C) resulted in approximately 9% more initial viscosity loss than rapid thawing in a constant-temperature oven (30°C/2h). Furthermore, repeated unsealing and taking portions exposes the solder paste to air, intensifying the oxidation of the rosin component in the flux and further weakening the thixotropic recovery ability of the solder paste. Therefore, once thawed solder paste is opened, it must be used up within 8 hours; otherwise, even resealing and refrigerating it cannot reverse the viscosity degradation.

Production line feedback shows that when the solder paste viscosity is below 500Pa·s, 0201 components with stencil openings smaller than 0.3mm often experience insufficient solder or even complete missing printing. Meanwhile, QFN-type devices with large pads exhibit bridging due to the enhanced ductility of the solder paste. This indicates that solder paste viscosity not only affects transfer efficiency, but also directly determines the rheological response of the solder paste under squeegee shear. Therefore, before each batch replacement or restart of old solder paste, the current viscosity value must be measured using a rotational viscometer, and the squeegee pressure, speed, and release distance parameters should be adjusted accordingly.

It is particularly important to note that different brands of solder paste have vastly different tolerances to the number of thaw cycles. Some Japanese solder pastes can still retain more than 85% of their original viscosity after three thaws, while certain low-cost Domestic Solder Pastes show obvious delamination after just the second thaw. Therefore, the procurement side cannot only look at the unit price, but must also request the thaw cycle test report provided by the supplier, focusing on the viscosity retention rate data after completing 5 complete freeze-thaw cycles under -40°C?25°C conditions.