If Solder Paste is forcibly opened after being taken out of cold storage without sufficient thermal soaking, it will immediately cause an imbalance in the internal temperature gradient of the paste. The Flux will locally concentrate or precipitate within the paste due to thermal expansion and contraction, while the interfacial bonding force between the metal powder and the carrier resin decreases. This further leads to poor fluidity of the solder paste under the squeegee during printing and reduced ability to fill the pads. Especially on fine-pitch components such as 0201 and 0.4mm pitch QFN, it is extremely prone to the contradictory phenomenon of both missing printing and bridging.

We have repeatedly reproduced on the production line the printing results of solder paste that was opened after only 1 hour of thermal soaking. Under a microscope, increased burrs on the edges of the solder paste particles and a significant decrease in surface gloss can be observed. This is because the viscosity of rosin-based flux increases sharply at low temperatures, making it unable to effectively coat the tin powder to form a stable suspension system. After printing, the residual amount at the bottom of the stencil openings increases by more than 30%, and stringing and breakage easily occur during the demolding moment. Ultimately, the deviation in solder joint height consistency exceeds the allowable range of IPC-A-610 Class II.

A more hidden problem is that insufficient thermal soaking of solder paste can also induce explosive voids during the reflow phase. Since the solder paste absorbs trace moisture from the air after being opened at low temperature, the moisture rapidly vaporizes to form microbubbles during the heating process in the preheating zone. These bubbles are trapped inside the molten solder and are difficult to escape. After final solidification, the void area ratio in the solder joint cross-section generally exceeds 25%, far exceeding the 10% upper limit required for automotive-grade components. In a batch failure analysis of a customer's BGA packaged chip, the root cause was traced back to the operation where the Solder Paste Storage administrator skipped the temperature and humidity record sheet and directly opened the container.

In actual operations, the thermal soaking time of solder paste must be strictly controlled within the 2~4 hour range. It is not allowed to judge whether it is "about right" by hand feel, nor can methods such as hot air blowing or hot water bath be used to accelerate the thermal soaking. This is because external heating will cause the surface temperature of the paste to be higher than the inner layer, exacerbating the risk of phase separation. We once used an infrared thermal imager to measure and found that after 10 minutes of blowing, the surface temperature had reached 22°C while the center was still at 8°C. This non-uniform thermal soaking is more harmful than not soaking at all.

In addition, after the solder paste has completed thermal soaking and before opening the lid, be sure to visually inspect the can mouth and the contact surface of the stirring rod for condensed water droplets. If any are found, the standing time must be extended until the moisture naturally evaporates. Otherwise, the mixing of condensed water into the solder paste will directly destroy the proportion of active flux components, leading to an increase in the solder wetting angle and insufficient solder joint climb height. This is particularly significant for OSP surface-treated PCBs. The frequency of such problems in the plum rain season with humidity greater than 60%RH is three times higher than on normal days.