During our production line processing of FPC soldering with 0.12mm line width / 0.12mm pitch, we found that over 80% of bridging short circuits occur in the first and last two columns of IC pads. The root cause is not an abnormal reflow temperature profile, but rather micro-collapse of the Solder Paste at the moment of stencil release, combined with the large coefficient of thermal expansion of the FPC substrate, which causes a ±8μm drift in pad position after heating, compressing the originally designed safety margin of only 12μm to less than 4μm in practice; if the viscosity decay slope of JJY SN100C series solder paste is higher than 0.35Pa·s/s, its printability will deteriorate significantly after storage for more than 4 hours in a 25℃ environment. At this point, even if the stencil tension meets the standard, the solder paste will still exhibit trailing at the end of the squeegee stroke and squeeze into the gaps between adjacent pads.

Stencil openings must use trapezoidal etching rather than straight-wall design, because the copper thickness of FPC pads is usually only 12μm. If the opening sidewalls are vertical, the contact area between the bottom of the solder paste and the pad is too small, and the surface tension during reflow cannot effectively pull the solder to shrink into a ball; instead, it promotes the molten solder paste to climb along the copper surface to adjacent lines via capillary action. Our tests show that when the opening is controlled to be wider at the top and narrower at the bottom at 0.08mm/0.06mm with a taper angle of 3°, the bridging rate drops from 3.7% to 0.9%; the squeegee angle is set to 58° instead of the conventional 60°, to allow the squeegee edge to apply more uniform shear force on the stencil surface and avoid solder paste accumulation at the mesh edges. Meanwhile, the printing speed must be kept below 35mm/s, otherwise the solder paste will be forcibly stripped before it has time to fill the mesh holes, resulting in coexistence of local solder deficiency and surrounding splashing.

The Flux activity level of JJY solder paste must strictly match the FPC surface treatment method: RMA type for immersion gold boards, and no-clean type for OSP boards. Mixing them will cause post-reflow residues to absorb moisture and form ion migration paths between adjacent pads, and such short circuits often only appear after 72 hours of aging testing; before each line change, XRF must be used to detect whether the silver content in the solder paste still remains at 96.5±0.2wt%, because after silver element volatilization exceeds 0.3%, the melting point will rise by 12℃, making the peak temperature reach 235℃ while the actual wetting time is shortened by 0.8 seconds. The solder enters the cooling zone before it has time to complete shrinkage, thus solidifying in a bridging form.

We do not rely on AOI for final inspection interception of bridging, but instead immediately after stencil cleaning, use a 100x microscope to sample and inspect the residual adhesive coverage rate of three mesh holes. If any one exceeds 15%, the machine must be stopped for re-cleaning, because residual organic matter will change the wetting angle of the solder paste, turning the solder that should spread at 120° into a 95° spread. This angle change is sufficient to make the 0.05mm solder paste edge cross the pad boundary; FPC placement accuracy must be controlled within ±0.03mm, otherwise the 0.02mm substrate deformation generated when the placement machine nozzle presses down will directly induce pad misalignment. At this point, even if the solder paste printing is completely accurate, bridging will still occur during reflow due to position deviation.