Solder Bridging on PCBs: Causes, Detection and Prevention

A solder bridge is an unintended solder connection between two adjacent pads, leads or terminations that should be electrically isolated, which creates a short circuit on the assembled board. It is most common on fine-pitch parts such as QFPs, SOICs, QFNs and connectors, where the gap between neighboring pads is small enough for excess or slumped solder paste to join during reflow.

What causes solder bridging?

Bridging comes down to too much solder in the wrong place before reflow, or molten solder free to flow where it should not:

• Solder paste: paste that has warmed up, absorbed moisture or passed its stencil life loses viscosity and slumps between pads before it melts.
• Stencil: apertures too large for the pad, a stencil too thick for the finest pitch on the board, poor gasketing against the PCB, or a dirty underside that smears paste between pads. A misaligned stencil offsets every deposit toward its neighbor.
• Print settings: excessive squeegee pressure, worn blades and infrequent under-stencil wiping.
• Reflow profile: a fast preheat ramp causes hot slump before the paste melts, and a long time above liquidus gives molten solder more time to wet across a narrow gap.
• Placement: high placement force squeezes paste out from under leads, and an offset part pushes its leads toward the adjacent pad.
• Component and pad design: no solder mask web between fine-pitch pads, oversized pads, and bent or non-coplanar leads.

The same slump and excess paste that bridge fine-pitch leads also throw off stray spheres of solder, so lines fighting bridges often see solder balls as well.

How is solder bridging detected?

Manual inspection works on coarse pitch but becomes unreliable at fine pitch as operators tire, and bridges under a component body cannot be seen at all.

SPI (solder paste inspection) cannot see a bridge, because bridges form in reflow. It does see the precursors: bridged paste, excess volume and offset deposits, and stopping those boards before placement is the cheapest fix.

2D AOI is the standard post-reflow check for visible bridges between gull-wing leads and chip terminations, where a bridge shows as continuous solder across the gap. 3D AOI adds height, which helps separate a real solder bridge from flux residue or a reflection that only looks like one in a 2D image. AXI (automated X-ray inspection) is needed for hidden joints: BGAs, QFN center pads and other bottom-terminated components.

Solder bridging is item 08 in DaoAI's inspection coverage: unintended solder connections between adjacent pads or leads, including on QFP packages. Bridges between adjacent through-hole pads after wave soldering are a separate check, item 13, which runs in post-wave inspection on the A3 and in DIP / through-hole inspection on the P3 and P5. When the machine writes the inspection program from one golden board, AI auto-programming generates the pad, pin and bridge regions itself, and you choose per package type whether bridge regions are inspected, generated only, or skipped.

False calls are the usual reason bridge checks get switched off. On DaoAI, operator feedback teaches the model which variations are acceptable, while solder bridges remain inspection targets.

How to prevent solder bridging

Fix bridging at the printer first, because most causes show up in SPI data before placement. Check values against your paste datasheet and stencil supplier.

If bridges are slipping past your current inspection, or bridge checks raise so many false calls that they get turned off, send us one of your own boards and we will test it for you, free of charge.

FAQ

What is the difference between a solder bridge and a short circuit?

A solder bridge is one physical cause of a short: solder joining two conductors that should be isolated. Shorts can also come from debris, bare-board defects or misplaced parts.

Can SPI prevent solder bridging?

SPI cannot see a bridge, which forms during reflow, but it catches the causes: bridged paste after printing, excess volume and offset deposits. Holding those boards before placement prevents most print-related bridges.

Can AOI detect solder bridges under a QFN or BGA?

No. Optical inspection only sees what is visible from above, so bridges under a component body need X-ray inspection (AXI), while AOI covers bridges between visible leads and terminations.

How do you repair a solder bridge?

Apply flux and remove the excess with a soldering iron or solder wick, then re-inspect. If the same location bridges repeatedly, correct the stencil or print process instead.