Solder Paste is completely non-conductive at room temperature, as the tin powder in it is tightly encapsulated by rosin, organic acids and polymer resins in the Flux. These non-metallic media form an effective insulating barrier. Even when measured by the four-point probe method, its bulk resistivity remains stable above 10? Ω·cm, far exceeding the threshold for insulating materials. Therefore, simply applying un-reflowed solder paste will not form a current path between PCB traces.

However, the real risk on production lines never lies in its theoretical conductivity, but in situations where uncontrolled printing accuracy causes solder paste to be squeezed into IC pin gaps, overflow from QFN bottom pads, or flux enrichment caused by oversized BGA stencil openings. At this point, although no metal fusion occurs, chloride ions generated by thermal decomposition of rosin couple with moisture, inducing electrochemical migration during powered aging, which eventually grows conductive filaments between adjacent pads. This type of failure often cannot be detected during functional testing, and only manifests as a short circuit after 48 hours of high-temperature and high-humidity aging.

Last week, we encountered three similar failures on a medical motherboard production line. The root cause was not the wrong solder paste model, but the squeegee pressure being mistakenly adjusted from 0.35MPa to 0.42MPa, which caused the solder paste thickness outside the pads of 0.3mm-pitch QFP devices to exceed the standard by 0.08mm. Although no bridging occurred after reflow, residual halogen ions were driven by a 12V bias during subsequent ICT testing to continuously precipitate silver dendrites, eventually leading to intermittent short circuits at the 72nd hour. After switching to a no-clean solder paste with low halogen content (Cl? < 300ppm) and tightening the squeegee pressure tolerance to ±0.02MPa, the issue was completely resolved.

It must also be emphasized that if water-washable solder paste is not cleaned thoroughly, the flux film remaining at the bottom of 0201 components will absorb moisture in high-temperature environments and reduce surface insulation resistance. At this point, the resistance between pads measured with a 500VDC megohmmeter may drop sharply below 10?Ω. Although a DC short circuit has not yet formed, it is already sufficient to interfere with high-speed signal integrity, especially in DDR4 routing areas where it can easily cause false triggering or eye diagram closure. Such issues are completely invisible in AOI inspection, and can only be located through leakage current screening in flying probe testing.

Therefore, to determine whether solder paste causes short circuits, one cannot only look at its conductivity parameters, but must also focus on three hidden drivers: changes in stencil tension, fluctuations in the Z-axis downward pressure of the placement machine, and abnormal wind speed in the cooling zone of the reflow oven. These factors jointly determine the flux residue morphology and ion activity, and these variables are often classified as "secondary factors" in SPC monitoring charts, yet they are precisely what cause the most unreproducible sporadic short circuits in real production lines.