When Solder Paste is taken directly from cold storage and loaded onto the printer for stencil printing, the Flux is stirred immediately after the can is opened without sufficient temperature equilibration. The low-boiling-point solvents in the flux have not yet diffused uniformly, and they violently vaporize as soon as they contact the high temperature of the stencil, blasting the liquid solder into fine particles that fly onto the back of the PCB or onto component bodies, forming typical splatter. At the same time, the local viscosity of the solder paste drops sharply, causing stringing and breakage during printing release, which solidifies into isolated solder balls after cooling.

If the solder paste is forced into use without sufficient temperature equilibration time, there is a large internal temperature gradient: the center remains cold while the surface layer has already softened. During stirring, shear force causes some solder particles to detach from the flux coating, so they cannot coalesce during reflow and directly burst into balls. More problematic still, the surface of the solder paste has already slightly oxidized during cold storage at 5℃~10℃; if temperature equilibration is too rapid, the oxide film cannot be reduced by the activator in time, leading to poor wetting during the reflow stage, and the solder agglomerates into balls instead of spreading.

The entire can must be allowed to equilibrate to room temperature for a full 4 hours before opening, with the ambient temperature controlled at 22℃±3℃. It must not be placed in direct airflow from air conditioners or near ovens. Before opening, gently shake the can for 10 seconds to loosen the paste, then use a spatula to slowly press down along the can wall and stir for 90 seconds; high-speed electric stirrers are prohibited. After stirring, let the paste stand for 15 minutes to defoam before loading it onto the printer for stencil printing.

Once opened, the solder paste must be used up within 24 hours; any excess after this time must be discarded and must never be mixed with new paste. After each use, immediately seal both the inner and outer lids tightly to prevent flux volatilization. The same can of solder paste must not undergo multiple cycles of cold storage and temperature equilibration, and leftover paste must not be returned to the refrigerator. The stencil cleaning interval must not exceed 2 hours, to prevent dried residual paste debris from mixing into new paste and causing splatter.

If the equilibrated solder paste shows obvious stratification, excessively long stringing, or resistance to spatula spreading, it indicates the paste has failed and must be taken out of service immediately. During printing, check the solder volume on the first board; if there is burr-like overflow at the edges of the stencil apertures, it is a sign of abnormal viscosity. If the ramp-up slope of the reflow oven exceeds 2℃/second, it will aggravate splatter and must be adjusted to within 1.2℃/second.