Solder Paste Bridging: Why Adjacent Deposits Join

Solder paste bridging is contact between two adjacent deposits of paste, either as printed or after the paste has spread on the board. It becomes a solder short at reflow because the two deposits melt into one continuous volume of alloy.

The condition is created before the board reaches the oven, which is why inspecting only after reflow produces corrective action that is too late. A deposit that has already touched its neighbour cannot be separated by any profile change.

What Paste Bridging Is

The two deposits may touch across the whole gap, or they may be joined by a thin filament of paste that follows a scratch or a smear. The second case is harder to see, and it is often produced by the wiping action or by a stencil that has been dragged across the board.

Bridging differs from the solder bridge that forms during reflow from a single over sized deposit. The single deposit problem is a volume issue, while bridging is a shape and spacing issue, and the two need different remedies. A deposit joined to its neighbour by a thin filament is often the result of a wipe that dragged paste across the mask, so the mask surface between the pads deserves inspection as well as the deposits themselves.

Slump and Deposit Geometry

Slump is the loss of printed shape as the paste spreads under its own weight. A paste that slumps will move toward the lowest energy shape, and where two deposits are close together the lowest energy state is a single merged volume.

Slump is worse when the paste is warm or when its viscosity is low, and it is worse on a surface with high surface energy such as bare copper. Print quality therefore depends on the temperature of the paste, the temperature of the board and the surface it is printed onto. Slump is also time dependent, so a board that waits before reflow gives the paste longer to spread, and the interval between print and reflow should be limited and recorded rather than left to the production schedule.

Stencil Aperture and Area Ratio

An aperture that is too large for the pad leaves paste overhanging the solder mask, where it is free to spread sideways. Reducing the aperture to a value the pad can contain, and matching the stencil thickness to the paste volume required, removes much of the risk.

Where the aperture must stay large to deliver volume, the paste can also be segmented into several smaller openings within the same pad. That approach is common on thermal pads and on large connector pads, and it limits the size of any single volume of paste that can spread.

Paste Volume and Metal Content

Deposits that are too tall for the gap between pads have more material available to spread, and a tall deposit also slumps further because it has further to fall. Volume should be set from the joint requirement rather than from the aperture area alone.

Metal content interacts with the same behaviour. A paste with a higher metal load holds its shape better after printing, while one with more flux is more fluid. Two pastes with the same nominal viscosity can therefore behave differently on the board. Metal load also changes the thermal behaviour, since a paste with more alloy and less flux melts to a smaller volume and pulls back from the mask more strongly.

Print Parameters and Separation

Squeegee speed and pressure affect how much paste is pushed sideways on the foil. Excessive pressure drives paste under the stencil and onto the mask, where it can connect two deposits that were printed separately.

Separation speed is equally important. If the board is pulled away while the paste is still tacky and stringing, the paste can be drawn sideways and touch a neighbouring deposit. Verification is a print inspection, not a machine readout, because the effect is on the board rather than in the settings.

Solder Mask Dams and Pad Design

Solder mask between the pads is what physically contains the paste. A dam that is too low, too narrow or absent altogether allows paste from one pad to move into the next, and the problem shows up as bridging on the parts of the board where the mask is thinnest.

The mask dam also has to survive the printing process. If the dam is under cured it softens and deforms under the squeegee, and if it is contaminated it may not repel the paste. Mask height and cure state belong in the discussion of bridging, not only in the discussion of mask quality. Where the dam is marginal, the practical remedy is to reduce the deposit volume on the pads it separates.

Reflow Behaviour and Wetting

At reflow the paste melts, wets the pads and pulls toward the metal it can wet. Where the mask properly separates the pads, surface tension pulls the alloy back from the dam and the joints form separately; where the mask is missing or the paste was already joined, the alloy stays connected.

Wetting speed matters in marginal cases. A deposit that melts early and spreads before the flux has activated can reach across a gap that the same deposit would not cross with a slower, better controlled profile. Time above liquidus and the ramp into the peak should be part of any investigation of intermittent short circuits.

Detection Before and After Reflow

Paste inspection before reflow is the only way to catch bridging early, because the deposits are still separate volumes at that point and can be measured individually. A system that reports deposit area as well as height will flag a deposit that has spread beyond its aperture.

After reflow, detection moves to optical or X-ray inspection, and the finding is a short rather than a bridging deposit. The solder dam and pad geometry then have to be re-examined, because by that stage the printing evidence has been destroyed by the heat.

Prevention and Control

Prevention combines three things: a paste that resists slump and is used within its working life, a stencil and aperture design that is matched to the pad, and a mask dam that is tall enough and fully cured. Each of them is a process control rather than a material choice alone.

The verification routine should include a first article check with the deposits examined edge to edge, a temperature and humidity log for the print area, and a defined interval for paste replacement on the stencil. The measurement methods described for paste inspection give the numbers that show whether those controls are holding.

FAQ

What causes solder paste bridging? Most cases come from a deposit that is too large for its pad, a paste that slumps, a solder mask dam that is too low or missing, and print parameters that push paste sideways.

Can bridging be detected before reflow? Yes, and that is the best time to find it. Paste inspection measures each deposit separately, so a deposit that has spread into the gap is visible before the alloy melts and connects the pads.

Does changing the reflow profile fix bridging? Not if the deposits have already merged. The profile can influence marginal cases by changing when and how fast the paste spreads, but the geometry and paste behaviour have to be corrected first.

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