Solder Mask Thickness And Its Effects On Assembly

Solder mask is a liquid that is printed, dried, exposed, developed, and cured, and the film that results is thinner over a bare laminate than over a trace and thinner at the edge of a feature than in the middle. The nominal thickness quoted by a fabricator is measured over a conductor, because that is the surface that matters for most of the assembly process, and the variation around that figure is what causes many of the defects that are blamed on the mask itself.

This article explains how the thickness varies, how it affects printing and assembly, what it does to impedance, and how the film interacts with tenting and with the surface finish.

Why The Thickness Varies

The mask is applied over a surface that already carries the copper pattern, so the film over a trace is raised by the copper thickness and the film over the laminate is at the base level. A one ounce trace is about thirty five micrometres thick, which is comparable to the mask thickness itself, so the surface that the stencil or the component sees is a landscape of ridges and valleys. The ink also thins where it flows away from a feature during drying and curing, which leaves the edges of a mask opening lower than the field.

The nominal figure is therefore a maximum over copper rather than an average. A film quoted as twenty five micrometres over a trace may be less than ten over the laminate between two fine traces, and a heavy copper layer of two ounces produces ridges that are twice as high. The measurement is normally taken on a coupon or on a test area of the panel, with an optical or a magnetic gauge, and the acceptance criterion refers to the surface the measurement was taken on.

Effects On Printing And Component Seating

The stencil seals against the mask during printing, and the quality of that seal is what stops paste from bleeding under the stencil and bridging to a neighbour. A mask that is too thin, or that varies sharply from one pad to the next, gives a poor seal and produces the smear of paste between pads that appears as a bridge after reflow. A mask that stands proud of the board by an excessive amount can also hold the stencil away from the pads and reduce the deposit volume.

Component seating is the second consequence. A component rests on the mask beneath its body, which is intended, but a mask ridge that lifts the body away from its pads can leave one terminal in the air, and a component that is tilted produces an incomplete fillet and sometimes a tombstone. The effect is greatest for a small chip component with a thin termination and for a package whose body tolerance is tight, which is why the mask and the footprint are considered together rather than separately. The design rules that keep the components seated correctly are described under pad design standards.

Effects On Electrical Behaviour

The mask is a dielectric, so it changes the environment of the traces it covers. A microstrip that was designed with air above the conductor becomes a conductor covered by a polymer layer of a higher dielectric constant, which reduces the impedance by a small amount, usually between one and three percent for a fifty ohm line. The effect is small enough to ignore on a general board and large enough to matter on a tightly toleranced impedance specification, which is why the mask thickness and its dielectric constant appear in the field solution for such designs.

On a printed element the effect can be larger. A mask over a printed antenna or a printed inductor changes the electrical length and lowers the quality factor, and a mask layer of variable thickness changes those properties across the element. A mask that is applied over a matching network, or over a conductor that is meant to radiate, therefore has to be considered as part of the design rather than as a protective cover.

Tenting, Plugging And Vias

Tenting is the practice of covering a via with mask instead of leaving the annular ring exposed. It keeps solder and flux out of the hole during wave soldering, and it prevents a solder ball from forming on the opposite side of a via that is near a pad. The limitation is that a large via cannot be tented reliably, because the film is unsupported over the hole and can crack or sag, and the mask over a tented hole can blister if the hole contains air or moisture that expands during reflow.

Where a via must be covered and protected, a mask plug or a filled via is the more robust answer, because the hole is filled with material that supports the film. The choice between tenting, plugging, and filling is part of the via design, and it interacts with the pad geometry and with the assembly process rather than being a cosmetic decision made by the fabricator.

Defects That Come From Thickness

A mask that is too thin in one place shows up as a wetting defect, because the mask no longer separates two adjacent pads and the solder can flow across. A mask that stands too proud next to a pad forms a ledge that catches solder paste and creates a bridge, and a mask that is uneven across a fine pitch footprint produces deposits of different volumes. Blisters and bubbles indicate trapped solvent or moisture, and a mask that cracks at a corner indicates a film that is too thick or a cure that is incomplete.

A mask on a pad is the most damaging of the thickness related defects, because it prevents wetting and produces an open joint that looks like a component problem. A thin skimming of mask over a pad is difficult to see and difficult to detect before reflow, which is why the pad openings are inspected after development rather than only at the end of the process.

Specifying And Inspecting The Film

The drawing should state the mask type, the nominal thickness over copper and the tolerance, the measurement method, and the requirements for vias and for tenting. The mask ink itself has a rheology that sets how sharply the printed and developed edges form, which is described under solder mask ink thixotropy, and the geometry that makes the process easier is collected under manufacturable design guidelines.

Inspection covers the two things that matter: the thickness on a coupon, measured against the specified range, and the presence of mask where it should not be, checked by looking at the pad openings and at the contact areas. The film also has to survive the assembly process, which means the specification should be written for the thermal excursion that the finished board will see, not only for the state in which it leaves the fabricator.

FAQ

Where is solder mask thickness measured? Over a copper trace, because that is the surface that the stencil and the component see. The thickness over bare laminate is lower and is not the value quoted.

Does solder mask change trace impedance? Slightly. The mask adds a dielectric layer above the conductor, which lowers the impedance by a few percent on a typical microstrip. It matters on a tightly toleranced design and is negligible otherwise.

Why can a large via not be tented? Because the cured film has no support over the hole and can sag or crack, and because air or moisture trapped in the hole expands during reflow and blisters the mask.