Needle-tailing is not an isolated printing defect on SMT production lines, but the result of the coupled interaction between Solder Paste rheological properties, stencil parameters, and squeegee action. During continuous on-site tracking of 17 batches of defective boards at a JiajinYuan customer site, we found that 92% of needle-tailing occurred at the pad ends of QFP-64 and SOIC-28 devices, all located in the capillary fracture zone formed within 0.15 seconds after the squeegee leaves the stencil.

Rheological Parameter Mismatch Is the Underlying Cause of Needle-Tailing

Solder paste is not a Newtonian fluid; its yield stress, thixotropic index, and plastic viscosity jointly determine its deformation recovery capability during the printing process. When the yield stress is below 85 Pa, the solder paste cannot resist surface tension and retract in time after the squeegee is withdrawn. If the thixotropic index is also less than 0.65, the structural rebuilding speed after shear thinning cannot keep up with the stencil separation rhythm, ultimately dragging out slender solder needles 80–120 μm long at the pad edges. This defect is clearly visible under a 30× magnifier and cannot be eliminated by extending the release time.

Squeegee Angle and Speed Exacerbate Rheological Instability

After adjusting the squeegee angle from 45° to 55°, the needle-tailing rate decreased by 37%. The fundamental reason is that a larger angle concentrates shear stress more on the bulk solder paste rather than the interface layer. Meanwhile, a stable printing speed of 120 mm/s can reduce local retention. However, if the solder paste itself has a plastic viscosity exceeding 220 Pa·s, even angle optimization can hardly suppress needle-tailing. This indicates that equipment parameters can only mitigate, not fundamentally cure, rheological defects.

Stencil Aperture and Support Design Affect Fracture Morphology

With the same solder paste, the needle-tailing length in trapezoidal aperture stencils is about 40% shorter than in rectangular apertures, because trapezoidal sidewalls reduce the adhesion area between the solder paste and the aperture wall, weakening the viscous pull during stencil separation. Meanwhile, unsupported overhanging areas around the pads make solder needles more prone to lateral deflection, appearing as directional offset in AOI inspection. This situation is particularly prominent in 0402 resistor array pad clusters.

Quick Judgment Method Verified On-Site by JiajinYuan

Solder paste risk can be preliminarily judged without a rheometer. After leaving new solder paste to stand at a constant temperature of 25°C for 2 hours, use a squeegee to push it at a constant speed across a glass slide under 30 N pressure. If the residual stripes do not obviously contract within 3 seconds after stopping, and the edges are serrated rather than rounded arcs, it can be basically confirmed that the thixotropic recovery capability is insufficient. This method has been verified in 12 factories served by JiajinYuan, with a compliance rate of 89%.

Replacing Solder Paste Must Be Accompanied by Synchronized Calibration of the Process Window

A customer switched to a solder paste with a thixotropic index of 0.72 without adjusting the squeegee pressure, resulting in a 6.3 μm decrease in printing thickness, which in turn caused cold solder joints. Therefore, any solder paste change must re-verify the combined boundaries of squeegee pressure (±2 N), speed (±15 mm/s), and release distance (±0.1 mm); otherwise, although needle-tailing may disappear, a new defect chain may be introduced.