Solder Mask Bridges and the LDI Process

A solder mask bridge is the narrow strip of mask that remains between two adjacent pads after imaging and development. Its purpose is to stop solder from bridging the pads during reflow, which makes it a functional feature rather than a cosmetic one. On a fine pitch device the bridge is only a few tens of micrometres wide, and whether it survives the process depends on the exposure method, the artwork and the copper geometry underneath.

The classic failure is a bridge that is printed in the artwork but missing on the finished board, or one that lifts after the first reflow. Both are common enough that the process capability of the fabricator, rather than the intention of the designer, decides whether a given pitch can have a mask dam at all.

Why Bridges Disappear

Mask is imaged through a film or drawn directly by a laser, then developed. During development the chemistry attacks the mask from both sides of the opening, so a narrow bridge is undercut from both edges at once. If the bridge is narrower than twice the undercut, it is removed completely. The undercut depends on the exposure energy, the development time and the chemistry, and it is the reason a fabricator quotes a minimum mask dam width.

Adhesion is the second mechanism. A narrow bridge sits on a small area of laminate and is bonded to it only by the cured mask. During reflow the mask expands faster than the copper and the laminate, and a bridge that has poor adhesion to the surface lifts at one end. Once lifted it may fold over the pad or fall away, and the protection it was supposed to provide is gone even though the board passed inspection.

Direct Imaging and the Bridge

Laser direct imaging removes the film and exposes the mask with a scanned beam. The resolution is limited by the beam diameter and by the scan strategy rather than by a photographic emulsion, and the practical result is a sharper edge and less undercut. That allows a narrower dam to survive for the same development process, which is why fine pitch boards moved to direct imaging as pitches fell below about 0.4 millimetres.

Direct imaging also removes the dimensional variation of the film, which is significant over a large panel. Because the image is generated from the digital data, the pattern can be scaled per panel to compensate for the shrinkage of the laminate during previous processes. The result is better registration between the mask openings and the copper pads, which in turn allows the dam to be placed more accurately between them. Both effects work in favour of the narrow bridge.

Designing the Copper Under the Mask

Copper geometry determines how much laminate is available for the mask to bond to. If the pads are wider than they need to be, the gap between them shrinks and the dam becomes too narrow to survive. Reducing the pad width, or using a solder mask defined pad where the mask overlaps the copper edge, changes the geometry in favour of the bridge and is often the simplest fix.

Solder mask defined pads bring their own considerations. Because the mask overlaps the copper, the exposed pad is smaller than the copper feature and the mask edge determines the solderable area. The overlap has to be large enough to be reliable but small enough to leave sufficient area for the joint, and the tolerance of the mask registration becomes part of the pad design. Where the pitch is very fine, this is often the only way to keep a dam at all.

Process Control and Verification

The variables that control the bridge are the exposure energy, the development speed and the chemistry concentration. All three drift, and a drift that removes a dam on one panel may produce a slightly wider dam on the next. Coupon features with narrow dams, imaged at the same time as the production panel and measured after development, give a direct indication of whether the process is inside its window.

Verification is normally visual under magnification, with a sample per panel and an inspection of the fine pitch areas on every panel for critical products. A bridge that is present but thinner than the specification will often survive inspection and fail during reflow, so the measurement should be of width rather than of presence. Some fabricators include the result in the process record so that a drift can be investigated before it becomes a yield loss.

Alternatives When the Dam Cannot Be Held

Where the pitch is too fine for a reliable dam, the options are to accept an open mask window over the whole device and control the solder volume instead, or to change the assembly process. An open window removes the bridging protection, so the stencil design, the paste volume and the reflow profile have to be tuned to prevent bridging by other means. Stencil apertures with a reduced area ratio, and a stepped stencil, are the usual tools.

Another option is to increase the spacing between pads by changing the component package or the fanout. A device with a slightly larger pitch costs nothing more and removes the problem entirely, and it is worth considering before committing to a process that has no margin. Where the package cannot change, the design should at least be reviewed against the manufacturing tolerances the fabricator can actually hold, so that the assembly house is not asked to compensate for a geometric impossibility.

Questions to Ask the Fabricator

The useful questions are specific. What is the minimum mask dam width that can be held reliably on this copper thickness, and is that figure quoted before or after development? Is direct imaging used, and what is the registration tolerance to the copper? What coupon features are imaged to verify the dam, and how often are they measured? The answers determine whether a fine pitch design is straightforward or a gamble, and they are worth obtaining before the layout is frozen rather than after the first batch is rejected.

It is also worth asking whether the fabricator has seen the specific component footprint before. A pitch and a pad geometry that are common in the industry will have a known process window, while an unusual combination may need a trial. A short conversation at the quotation stage is far cheaper than a redesign after the panels have been produced and the assembly line has found the bridges missing.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Can a mask dam be added after the mask is imaged? No. The dam is created by the imaging and development process. It either survives the process or it does not.

Is a solder mask defined pad always better on fine pitch? It allows a dam where a non solder mask defined pad cannot, but it makes the mask registration part of the pad tolerance. The trade should be evaluated for each device.

Why does a bridge that looks fine fail in reflow? Because it is thinner than the specification and lifts as the mask expands. Measuring the width, not merely confirming that a bridge exists, is what prevents that surprise.