Solder Bridging Causes And Process Fixes

A bridge is a connection of solder between two conductors that should be separate. On a fine pitch package it is a single deposit that has spread across the gap; between a lead and a neighbouring pad it is a thread formed as the solder solidified; and under a component it can be a hidden connection that only a test will find. It is one of the most common defects in surface mount assembly and one of the most diagnosable, because the cause usually leaves a signature.

This article explains where bridges come from, what each cause looks like, how the process is corrected, and how the design reduces the risk.

What A Bridge Is

Two mechanisms produce the same result. The first is excess solder: more paste than the joint can absorb, so the molten alloy spreads beyond the pad and meets the neighbouring one. The second is a mechanical transfer, where a thread of solder is drawn between two points as the board leaves the wave or as a component is placed, and it solidifies before it can retract.

The distinction matters because the remedies are different. Excess solder is corrected by reducing the deposit, while a thread is corrected by changing the thermal profile or the direction of travel. A board that shows bridges in one place is telling you which of the two mechanisms is at work, and a board that shows them everywhere is telling you about the paste or the stencil.

Paste, Stencil And Print

Paste that has absorbed moisture slumps and spreads after printing, and the slumping is the beginning of a bridge. Paste that is warm behaves the same way, and paste that has been left on the stencil for hours has usually done both. The remedy is a working life, a room at controlled temperature and a print that is checked rather than assumed.

The stencil contributes through its thickness, its apertures and its condition. A stencil that is too thick deposits too much; an aperture that is oversized or badly shaped places paste outside the pad; and a stencil that is not wiped between prints carries paste on its underside, which smears across the mask and into the gaps. The relationship between the aperture and the deposit is described under stencil aperture design and paste release.

Placement, Reflow And The Board

Placement causes bridges in two ways. A component placed with too much force squeezes the paste sideways from under the terminations, and a component placed off centre leaves more paste under one side than the other. On a fine pitch package the placement force is the largest single variable, because the paste must be compressed without being displaced.

Reflow then decides whether the bridge survives. The surface tension of molten solder pulls a joint towards its pad, and a bridge that is only just formed will often separate if the profile gives it time at temperature. A profile that heats too quickly freezes the bridge in place, and a profile with a peak that is too low leaves the deposit only partly melted and therefore unable to retract. The board itself contributes through its mask, its finish and its pad geometry, and the dimensions that the joint needs are described under PCB pad design standards.

Correcting The Process

The correction is applied from the most likely cause and in order of effort. The paste is checked for age and temperature and is replaced if it is out of its window. The stencil is inspected, cleaned and measured. The print is measured, and the volume reduced where it is high. The placement force and offset are checked. The profile is reviewed for the time above liquidus, and only then are the design rules revisited.

The order matters because a design change is expensive and a process change is not. Where the same design runs on two lines and only one produces bridges, the cause is in the line, and the design should not be touched until the line has been excluded. The reverse is also true: where every line and every operator produces the same bridge, the design is the common factor. The general rules that determine which design is printable are described under manufacturable design guidelines.

Inspection And Escape

Inspection for bridges is visual, with a low angle of view along the rows of leads so that a thread is seen against the background rather than end on. Automatic optical inspection finds most of the visible bridges, and X-ray finds the ones under a package, but neither can find a bridge that is internal to a joint. In circuit test is the method that finds those, which is a reason to keep access to the nets even on a board that is otherwise fully inspected.

The escape rate is what matters, not the detection rate. A bridge that is found by inspection has cost the line time; a bridge that escapes has cost the customer a failure. When a bridge escapes, the question to ask is not why the inspection missed it but why the process produced it, since the same cause will produce more of them. The mechanisms that move a component and produce similar symptoms are described under SMT component shift causes.

Preventing It In The Design

The design can do three things. It can leave enough gap between pads for the process to work, which is a matter of following the capability of the shop rather than the density ambition of the project. It can keep the paste volume appropriate to the joint by using an aperture that is 1:1 with the pad and reducing it only where the area ratio requires. And it can avoid unnecessary density, such as a pad that is wider than the component needs or a mask dam that is narrower than the process can hold.

Where the design cannot be changed, the remaining measures are in the stencil and the mask. A stencil with a slightly reduced aperture separates the deposits and is often enough, and a mask dam between the pads is the last barrier, because solder that has been printed onto the mask will not wet it. Neither measure is a substitute for a correct pad geometry, but both have rescued a design that was already released. The sequence as a whole is described under PCBA development process.

Additional Considerations for This Build

Practical attention to solder bridging pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating solder bridging explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

Why do bridges appear only on one side of a package? Usually because the placement is off centre in that direction or because the board is not level in the machine, both of which concentrate the paste on one side. The pad geometry is the same on both sides, so the difference is in the process.

Does a second reflow remove a bridge? It can, if the bridge is thin enough to separate under surface tension when it melts again, but relying on that is not a process. A bridge that survives the first reflow will usually survive the second as well.

Can flux cause bridging? Flux alone does not bridge conductors, but a flux that is not fully activated leaves oxide that prevents the solder from retracting, and a residue that is conductive can look like a bridge to a test. Both are corrected by the paste and the profile rather than by cleaning.