Solder Bridging Control on Fine Pitch Assemblies

A solder bridge is a connection between two joints that should be separate. On a fine pitch assembly the joints are close together, the paste volume is small and the margin between a good joint and a bridge is narrow, so bridging is the defect that defines the process capability. It can be created by the print, by the placement or by the reflow, and the three have to be separated before the correct fix is obvious.

How a Bridge Forms

Molten alloy moves by surface tension, and it moves toward the region where the surface energy is lowest. If there is a continuous path of alloy between two pads when the joint melts, the alloy will remain connected unless it separates into two fillets, and separation requires enough surface tension force and enough time.

A bridge therefore needs both a path and a reason not to separate. Understanding which of the two is present is the difference between reducing the paste volume and extending the profile, and the two corrections have opposite effects on the joint. Because solder bridging is a balance between the material present and the forces that pull it apart, both halves of the mechanism have to be considered before a change is made. The path comes from excess paste, from a smear on the mask or from displaced material, and the failure to separate comes from too little time above liquidus, an oxidised surface or a geometry that holds the alloy in place.

Paste Volume and Stencil Design

Excess paste is the most common cause. A deposit that is taller than the gap between the pad and the lead, or one that spreads laterally onto the mask, provides the material that forms the bridge. The volume should be the minimum that produces an acceptable fillet, and it should be verified by measurement rather than by inspection after reflow.

The stencil design determines how the volume is delivered. A reduced aperture area, a thinner foil and a shape that releases cleanly all reduce the amount of paste that can spread. Where the pitch is very fine, an aperture that is divided into two smaller openings can deliver the required volume with a lower peak height, which reduces the tendency to bridge. The relationships are covered in solder paste volume and stencil design.

Solder Mask Dams and Pad Geometry

The mask dam between two pads is the physical barrier that keeps the alloy apart. A dam that is missing, narrow or lifted provides no separation, and the bridge forms across the gap. The dam width is therefore a capability parameter, and a design that requires a dam narrower than the process can hold reliably will bridge at some rate.

The pad geometry contributes as well. Pads that are wider than the lead, or that extend further toward each other than the lead spacing requires, give the alloy more area to connect. Reducing the pad width and increasing the gap between pads are design changes that improve the margin, and they are usually cheaper than tightening the process. The registration relationship between the mask and the pads is described in PCB quality assessment.

Placement and Component Seating

A component that is placed with too much force displaces paste sideways, and the displaced material is the bridge. A component that is placed off centre sits closer to one pad, which reduces the gap on that side. Both effects are larger on a fine pitch part because the distances are smaller.

The placement force should therefore be set per package type rather than globally, and the placement accuracy should be verified for the specific component. Where the component has a high lead count, the coplanarity of the leads also matters, because a lead that does not touch its pad leaves its paste to spread toward the neighbour. Lead coplanarity and placement are covered in lead coplanarity and placement.

Reflow Profile and Separation

The profile determines whether a marginal bridge separates. A longer time above liquidus gives the surface tension more time to pull the alloy into two fillets, and a slower cooling allows the separation to complete before the alloy freezes. A short, hot profile freezes the bridge in place.

The soak also matters because it allows the flux to clean the mask surface between the pads. A mask surface that is clean and has a low surface energy repels the alloy, which helps the separation. Where the process is marginal, the profile adjustment is often the change that removes the last few bridges without reducing the paste volume further.

Inspection and Repair

Bridging is visible and is normally caught by inspection, either visually or by automated optical inspection. The important point is to record the position, because a bridge that appears at the same place on every board is a design or stencil problem while a random distribution points to the paste volume or the print.

Repair of a bridge requires the excess alloy to be removed rather than simply reheated, because reheating usually reproduces the same geometry. The joints should be separated with a suitable tool and the area cleaned, and the result inspected against the same criteria as a new joint. The repair limits and the acceptance criteria are described in solder joint acceptance criteria.

Cleaning, Mask Condition and Surface Energy

The condition of the mask between the pads affects how the alloy behaves. A clean mask with a fully cured surface repels molten alloy, while a mask that is contaminated with flux residue or that is under cured allows the alloy to wet it. Solder bridging across a mask dam is therefore often a mask condition problem rather than a paste volume problem.

Cleaning practice interacts with this. A stencil that is not wiped often enough deposits paste on the mask between the pads, and the paste becomes the path that the bridge follows. The wipe interval and the mask condition should be reviewed together when a bridging rate rises, because both produce the same symptom and both are corrected by different actions.

Capability and Design Rules

The bridging rate is the measure of the process capability for a given pitch. A design that runs at a low rate has margin, and one that runs at a high rate is at or beyond the capability of the line. The rate should be tracked with the pitch and the paste volume so that the limit is known before a new product is released.

The gopcb engineering team uses that record to advise on pad and mask geometry during the design review, because the cheapest way to control bridging is to design a pad and dam geometry that the process can hold. Where the design cannot be changed, the process settings are optimised and the capability is documented so that the expected yield is known rather than assumed.

FAQ

Does less paste always reduce bridging? It reduces the material available to bridge, but too little paste produces a poor fillet. The volume should be the minimum that still meets the joint criteria.

Why do bridges appear on one side of a component? Usually because the component is placed off centre or because the paste print is uneven in that direction. Both should be measured.

Can a bridge be removed by reheating? Rarely. The alloy has to be physically separated, and the remaining joints should be inspected afterwards.