The solder spike issue in fine-pitch pin printing is not an isolated defect, but a result of the coupled effects of Solder Paste rheological properties, squeegee pressure distribution, wall roughness of stencil apertures, and PCB pad coplanarity. We observed on the JJY SMT line that for 0.35mm pitch QFN devices, once consecutive 3 boards have solder spikes longer than 0.12mm, the post-placement offset rate will rise above 17%.

Solder spike morphology directly reveals process mismatch points

Slender filament-like solder spikes indicate low Solder Paste Viscosity or excessively high squeegee speed, while short, conical solder spikes are mostly accompanied by residual solder paste on the stencil bottom that has not been cleaned in time. We found when using the JJY 12μm electropolished stencil that if the cleaning interval exceeds 8 boards without performing the dry wipe + wet wipe process, the solder spike occurrence rate will surge from 2.3% to 9.6%, which is most prominent especially at the root area of 0.4mm pitch SOIC pins.

First article inspection must include quantitative solder spike determination under a microscope

First article inspection cannot only measure height and area; it must use a 20x stereomicroscope to measure the maximum extension length of solder spikes and record their directionality. The JJY standard requires that for 0.5mm pitch devices, solder spikes must be ≤0.1mm and must not extend toward adjacent pads; otherwise, the squeegee angle must be immediately adjusted to 58°±2° and the printing results retested three times, because every 1° deviation in angle will cause the solder spike length to fluctuate by around 0.03mm.

Stencil tension attenuation is a hidden cause of solder spikes

After 100,000 printing cycles on the line, a 12% drop in stencil tension will intensify local deformation when the squeegee presses down, which in turn causes solder paste to be forcibly dragged in the narrow gaps between pins to form solder spikes. We tracked the JJY 302 stainless steel stencil and found that when the tension is below 38N/cm², the occurrence frequency of solder spikes at the edge of 0.3mm pitch BGA pads increases by 4.2 times, at which point the machine must be stopped to recalibrate or replace the stencil.

Uncontrolled temperature recovery environment also induces solder spike variation

If solder paste taken out of the refrigerator is not temperature-recovered for 4 hours in a 22℃±3℃ environment per JJY specifications, its insufficient thixotropic recovery will exhibit abnormal ductility during printing shearing, causing the dispersion of solder spike length to expand to ±0.08mm, far exceeding the normal range of ±0.03mm. This variation is especially severe when humidity is >65%RH.