Solder Paste slump is not a slight flow that disappears a few seconds after printing, but a phenomenon that occurs at room temperature or during the pre-reflow holding stage, where insufficient viscosity, slow thixotropic recovery, or overly fast stencil separation causes the paste on pads to visibly overflow at its edges, drop sharply in height, and intrude into adjacent pad areas. Its occurrence is often accompanied by a simultaneous rise in defects such as post-reflow bridging, tombstoning, and solder beads. We have found in production lines that when solder paste is stored at temperatures above 25°C and left unsealed for more than 72 hours, the probability of slump increases by more than three times.

Preparatory work must strictly control ambient temperature, humidity, and material condition. Workshop temperature must be stabilized at 22±2°C, and relative humidity maintained at 45%±5%. Before opening, solder paste must be allowed to thermally equilibrate in a 22°C environment for at least 4 hours; after equilibration, mixing time must be no less than 90 seconds, with the mixer speed set to 600rpm. Stencil tension must be greater than 40N/cm. Before first use, new stencils must be wiped three times with 1000-grit silicon carbide paper combined with IPA, and blown dry with Class 100 clean air after each wipe; otherwise, residual solvent will exacerbate solder paste wetting and spreading.

Among core parameters, stencil aperture ratio is the primary variable determining slump tendency. For 0201 components, it must reach 1.5 or above; for 0402 components, at least 1.35. If the aperture ratio of QFP-100-pin devices falls below 1.25, the slump rate will exceed 18%. Solder Paste Viscosity should be controlled at 650±50Pa·s (25°C / shear rate 10s?1), with a thixotropic index required to fall between 0.62 and 0.68. Too high an index makes demolding difficult, while too low an index eliminates slump resistance. Our measurements show that when the thixotropic index falls below 0.6, the center height of pads decays by 22% within 15 minutes after printing, far exceeding the 15% upper limit allowed by IPC-7525B.

In terms of operating steps, the squeegee angle must be locked at 55±3°, pressure controlled at 5.5±0.3kgf, and printing speed set to 25±2mm/s. The separation rate between stencil and PCB must strictly not exceed 2mm/s; otherwise, solder paste will be pulled and deformed at the moment of peeling. Each printing interval must be at least 45 seconds to allow a sufficiently strong gel film to form on the solder paste surface; otherwise, the slump rate will spike to 12.7% by the third consecutive board printed. We once used a Keyence VHX-7000 microscopy system to measure a lead-free solder paste in a 30°C environment, and found that after printing the fifth board, the pad edge collapse angle dropped from an initial 68° to 39°, already triggering the SMT failure judgment threshold.

Abnormality troubleshooting should start from three points simultaneously. First, check whether the equilibrated solder paste shows stratification or surface oil bleeding. Second, confirm whether there are micro-scratches or clogged apertures on the stencil bottom (especially in fine-pitch areas below 0.3mm). Third, verify whether the Z-axis pressing depth of the placement head exceeds 0.15mm, as excessive pressing will squeeze the uncured solder paste outward in all directions. In one mass production run, a mis-set placement pressure of 0.22mm caused the slump defect rate in the BGA area to soar to 9.4%, which fell back to within 0.3% after shutdown and recalibration.

An empirical technique is to establish a dynamic monitoring mechanism. Every 2 hours, use an optical profilometer to sample and inspect the solder paste height distribution on 3 boards, with a focus on the height difference between the pad center and its four corners; a difference greater than 8μm triggers an alert. At the same time, the first printed board after stencil cleaning must undergo X-ray scanning to confirm no hidden bridging. We consistently print 5 test boards with the same batch of solder paste during each material change, store them in a constant-temperature chamber, measure the highest point height every 30 minutes, and plot a decay curve. As long as decay exceeds 13% within 60 minutes, that batch of solder paste is immediately taken out of use. This practice has reduced customer complaints caused by slump to zero at one factory over a six-month period.