During mass production of 0201 package resistors, we found that conventional Solder Paste tends to suffer from poor release and edge stringing when the stencil aperture width is too small. High-tack solder paste, with its higher yield strength and superior shear-thinning properties, can stably maintain its profile on the pads. Combined with a 30° squeegee angle and 80mm/s printing speed, it reduced the component tombstoning rate during the solder paste placement step from 1200ppm to below 300ppm.

When the production line switches to a double-sided reflow process, the solder joints after the first side soldering must withstand the squeegee downward pressure during the second side printing and the Z-axis impact of the placement head. At this point, if conventional solder paste is used, micro-displacement or even solder joint cracking often occurs due to incomplete curing of the joints. High-tack solder paste still maintains high adhesion below 150℃. Combined with adjusting the stencil thickness from 0.12mm to 0.10mm and increasing the aperture ratio to 92%, it ensures that the first-side solder joints are not disturbed during the secondary printing process.

For QFN devices with thermal pads, we once encountered the problem of excessive bottom voidage after reflow. Later, we replaced the solder paste with high-tack solder paste and simultaneously changed the stencil aperture from a full-open design to a grid-like design. By utilizing its high cohesive strength to suppress the lateral flow of the solder paste during reflow, the void area ratio was reduced from 38% to 19%, and the device position offset after placement was stably controlled within ±0.03mm.

In step stencil operations, solder paste accumulation or trailing easily occurs at the boundary between thick and thin areas, causing components to be lifted or laterally shifted during placement. After switching to high-tack solder paste, due to its more linear shear stress response and faster rebound recovery, combined with reducing the squeegee pressure from 5.5kg to 4.2kg, it not only avoids excess solder volume in the thick areas but also ensures sufficient filling in the thin areas. The first-pass yield of solder paste placement increased from 89% to 97.6%.

In old factories with large ambient temperature fluctuations, conventional solder paste begins to collapse significantly within 15 minutes after printing, while high-tack solder paste can maintain effective shape retention for up to 32 minutes in an environment of 25±3℃. This makes non-continuous operation periods such as placement machine changeover debugging and Feeder replenishment no longer quality risk points. Especially when multi-variety, small-batch orders are produced in interleaved fashion, it reduces the number of PCBs that have to be scrapped due to exceeding the waiting time limit.

We tested high-tack solder pastes from five different brands. Under the same stencil parameters and equipment conditions, their printing height coefficient of variation (CV value) is generally 22% to 35% lower than that of conventional solder paste. The key differences lie in the coating stability of the Flux system on the metal powder surface, as well as the synergistic effect between rosin resin and thixotropic agents. This consistency in microstructure is directly reflected in the coplanarity pass rate after solder paste placement, which is particularly evident for TQFP-type devices with more than 100 pins.