Why Solder Mask Bridges Fail and How to Design Reliable PCBs

What is a solder‑mask bridge?

A solder‑mask bridge (also called a green‑oil bridge or mask dam) is the portion of solder‑mask ink that remains between adjacent surface‑mount device (SMD) pads. Its primary function is to prevent solder from flowing across the gap and creating a short circuit, especially on IC packages with very small pitch.

Two design options

Remove the bridge (open window) : The mask is opened over the entire pin area, eliminating the bridge. This may work for hand‑soldering, but in high‑volume SMT assembly the lack of a bridge often leads to solder‑bridge shorts that require exhaustive inspection.

Keep the bridge : The mask is left intact, preserving the bridge. The feasibility depends on the fab’s ability to produce a bridge of the required width. A bridge narrower than the fab’s minimum tolerance will delaminate or cause shorts.

Manufacturing capabilities

Different PCB manufacturers use different exposure technologies, which directly affect the minimum achievable bridge width.

Traditional CCD exposure (single‑ or double‑layer boards) : Alignment tolerances require a typical mask‑expansion of about 0.05 mm (≈2 mil) around each pad. Consequently, the bridge width is limited to roughly 0.05 mm.

Laser Direct Imaging (LDI) (multilayer boards, ≥4 layers) : LDI eliminates film‑based alignment errors, allowing mask‑to‑pad alignment of 1:1. Bridge widths as narrow as 0.10 mm or even 0 mm (no bridge) can be produced.

The relationship between mask‑opening width and bridge width is inverse: a larger opening reduces the bridge width. If the distance between adjacent pads is smaller than the fab’s minimum bridge width, the bridge may delaminate, leading to reliability problems.

Case study: STM32F103C8T6 (LQFP‑48)

The LQFP‑48 package has a centre‑to‑centre pin pitch of 0.5 mm and an edge‑to‑edge spacing of 0.2 mm. The datasheet specifies a default solder‑mask expansion of 0.05 mm on each side, leaving a bridge width of 0.10 mm. On a 1 oz copper board fabricated with LDI this width is at the practical limit; on a 2 oz board or with a fab that still uses CCD exposure, the bridge is likely to delaminate or cause solder‑bridge shorts.

Design files from JLC‑EDA show the same default expansion (0.05 mm) for this component, confirming the calculated bridge width.

Design recommendations for reliable PCB layout

Verify the fab’s process : Determine whether the board will be produced with CCD or LDI. For LDI‑based multilayer boards you can reduce the mask‑expansion for pins spaced under 0.2 mm to increase the bridge width and avoid delamination.

Adjust mask expansion : If the required bridge width falls below the fab’s minimum, either increase the mask‑opening (which reduces bridge width) or select a component with larger pad spacing.

Consider alternative packages or components : When the necessary bridge width cannot be achieved, choose a different package (e.g., larger‑pitch QFN) or a different part that meets the manufacturing constraints.

Early DFM review : Conduct a design‑for‑manufacturability review with the manufacturer before finalizing the layout to confirm that mask‑opening sizes are feasible.

By analysing solder‑mask bridge dimensions against the fab’s exposure technology and copper thickness early in the design phase, engineers can prevent costly redesigns and production failures.