The root cause of bridging in QFN soldering is not unreasonable pad design, but the uncontrolled rheological behavior of the Solder Paste from after printing to before reflow. After the fine pin pitch is compressed from 0.5mm to 0.3mm, even slight collapse or edge burrs of the solder paste are sufficient to form a metal bridge in the early heating stage. Especially when the ambient humidity exceeds 60%RH and the solder paste has been opened for more than 4 hours without nitrogen protection, the moisture absorption and softening of the rosin system will directly amplify this collapse trend.

We have verified that the viscosity attenuation rate of JJY JF-608 solder paste after 72 hours of storage at 25℃ is only 3.2%, far lower than the average level of 8.7% for similar products. This is due to the synergistic effect of branched rosin modified resin and 5# powder with a sphericity of ≥92%, which can maintain the linear range of shear thinning response during the scraping and printing process, ensuring smooth demolding while suppressing lateral spreading before reflow. In contrast, the edge contour of ordinary solder paste has become blurred within 30 minutes after printing under the same conditions, with obvious burr extension visible under a microscope.

The stencil aperture must be strictly designed with a 0.02mm negative tolerance matching the pin width. For example, a 0.3mm pin corresponds to a 0.28mm square aperture, and the electroforming process must be used instead of laser cutting. This is because the micro-molten slag formed on the hole wall by the latter will induce local tearing of the solder paste under the pressure of the squeegee, and such tearing can easily evolve into continuous solder bridges in fine pitch scenarios. We once used the same JJY solder paste to compare and test two types of stencils, and the bridging defect rate differed by 3.8 times. The problem lies in the solder paste fracture and re-adhesion caused by the laser hole wall roughness Ra > 0.8μm.

Setting the squeegee angle to 35° is the equilibrium point obtained from 27 production line measurements. When the angle is less than 33°, insufficient solder paste filling leads to an increase in cold solder joints; when it is greater than 37°, the vertical component force exerted by the squeegee on the stencil is too large, causing micro-spraying of the solder paste the moment it leaves the stencil. This spraying manifests as irregular solder bead groups under 0.3mm pitch, and these solder beads are pushed by the hot air flow during subsequent reflow to exactly get stuck in the gap between adjacent pads, forming bridging. Therefore, the squeegee pressure must be synchronously controlled within the range of 5.8±0.3kg to make the 35° angle truly effective.

The heating slope of the second temperature zone of the reflow oven must be limited to within 1.2℃/s, otherwise the QFN bottom solder ends near 150℃ will experience a micro-explosion effect due to violent vaporization of the Flux. This explosion disturbance, superimposed on the state where the solder paste has not yet been completely wetted, will completely break down the already fragile solder paste boundary. We once found at a customer site that bridging was concentrated on the 3rd to 5th columns of pins. After disassembly, it was confirmed that the deformation of the stencil support ribs in this area caused a locally high solder volume, and the excessive reflow slope further amplified this deviation. Eventually, all abnormalities were resolved after adjusting the slope and replacing the stencil.