Solder Paste warming is not simply taking refrigerated solder paste out and leaving it for a while; it must be left to stand for a full 4 hours in a constant-temperature workshop at 22℃±3℃ before being put into the solder paste usage process. This is because the metal powder and Flux system inside low-temperature solder paste have not yet reached a thermodynamic equilibrium state. If the lid is opened early for stirring or it is put on the machine for printing, problems such as the squeegee being unable to push, severe residue in stencil openings, and jagged trailing at the edges of the solder paste will occur.

Our production line once had an over 6% tombstoning rate for 0201 resistors for two consecutive days. Traceability found that night shift personnel, in order to meet the delivery deadline, put the solder paste just taken out of the refrigerator on the machine after only 90 minutes of standing. As a result, the measured viscosity of the solder paste reached 850Pa·s (the standard value should be 650±100Pa·s), leading to uneven printing thickness and small solder paste volume, which caused unbalanced wetting forces at both ends during reflow and thus tombstoning.

Insufficient warming will also exacerbate the risk of solder paste oxidation. Especially in high-humidity environments (RH＞60%), a micron-scale oxide film easily forms on the surface of solder paste that has not been fully warmed. This film will hinder intermetallic diffusion during solder melting, resulting in cold soldering, increased void rate, and failed thrust tests. During a customer audit, it was found that the X-ray void area of BGA solder joints reached 32%, and the root cause was that the solder paste warming time was compressed to 2 hours and 15 minutes.

In actual operation, a temperature and humidity recorder with a timestamp must be used to monitor the warming area; visual judgment or empirical estimation cannot be relied on. This is because the difference in substrate thermal conductivity between different batches of solder paste can reach 18%, and the internal temperature rise rate under the same environment is not consistent. There was once a batch of SN96.5AG3.0CU0.5 solder paste whose core temperature lagged behind the shell by 3.7℃ at room temperature of 23℃, and it did not truly meet the standard until the 217th minute.

After the solder paste warming is completed, a three-step confirmation action must also be performed: gently shake the can before opening the lid to observe whether the fluidity is uniform, check whether there is delamination or oil bleeding on the surface after opening the lid, and use a viscometer to take three spot measurements and take the average value before putting it on the machine. If any item is abnormal, it must be returned for re-warming; we must never let problematic solder paste flow into the printing process with the mindset of "good enough".

A common wrong practice in the workshop is stacking multiple cans of solder paste on the outer shelf of the temperature-controlled cabinet. Although they seem to be in the same environment, differences in air convection cause the outer cans to meet the standard early while the inner ones are still below 15℃. This uneven warming is more dangerous than no warming at all, because it is hidden and often not detected until the AOI missed inspection rate suddenly increases.