Solder Mask Thickness and Its Measurement

Solder mask is applied to a specified thickness, and the thickness decides the dam height, the electrical isolation and the cure. Measuring it is a routine check that is often replaced by a visual judgement, which is the point at which the specification stops being enforced.

Why the Thickness Is Specified

The mask has to cover the copper and the laminate, and it has to be thick enough to isolate at the voltage the design uses. It also has to be thin enough to cure fully and to resolve the finest dam.

The three requirements give a range rather than a minimum. A mask that is too thick is not better, because it cures incompletely and it flows at the dam. Our fabrication notes describe how the range is stated.

Where the Thickness Is Measured

The thickness is measured over a copper feature and over bare laminate, because the two differ. The mask over a trace is thicker than the mask over the laminate beside it, and a single reading describes only one of them.

The measurement is usually taken with a magnetic or an eddy current gauge on a copper area, and the reference surface has to be the copper rather than the table. A measurement taken with the wrong reference reports the dielectric as well. Our board quality notes describe how the finished surface is judged.

An incompletely cured mask behaves like a thin one under a solvent, and a fully cured mask that is too thick cracks at the edges. The two checks are therefore reported together rather than separately.

The cure is measured by a solvent rub, a hardness test or a spectroscopic method, and the result is recorded against the thickness. A thickness inside the range with an incomplete cure is a defect that a thickness check alone does not find.

The dam height is set by the mask thickness, so a mask that is thinner than specified gives a shorter dam and a lower resistance to bridging. The paste volume that the dam has to hold is set by the stencil rather than by the mask, which is why the two are reviewed together.

The mask also has to fill the space between fine features without leaving a void. A mask that is too thick bridges that space at the application rather than at the cure, which is a defect that appears as an incomplete fill between two pads. Our coating notes describe the same measurement problem in a coating process.

The thickness is controlled by the application, and the parameters are the screen or the spray settings, the viscosity and the number of passes. Each of those moves the thickness and each is recorded.

Where the thickness is drifting, the cause is usually the viscosity rather than the setting, because the viscosity changes as the mask is used. That is the reason the viscosity is checked rather than assumed.

A single reading on a coupon describes the process and not the board. The board measurement is taken at several places, including the densest area and the sparsest, because the mask flows differently in each.

A specification that is enforced at one point is a specification that is met at that point. Our paste volume notes describe the same reasoning about where a measurement is taken.

Acceptance and Its Evidence

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

The sequence of operations is part of the specification, because a different order produces a different result from the same steps. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts.

Checks Before Release

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Is a thicker mask better for isolation? Up to the point where the cure and the dam resolution suffer, so it is a range rather than a maximum.

Can the thickness be measured after assembly? It can where the surface is accessible, and the reading is affected by the coating and the components around it.

What does gopcb provide for mask thickness? We provide a stated range from the electrical and resolution requirements, measurements over copper and over laminate with the reference defined, cure measured and reported with the thickness, several measurement positions including the densest area, and records per batch.

Verification and Records

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.