Solder Mask Thickness Selection For PCB Builds
Solder mask thickness is rarely discussed until a board fails for a reason that traces back to it. The coating has to cover the copper, insulate adjacent features, protect the laminate during assembly and hold its shape around every pad and via. A layer that is too thin does none of those reliably, and one that is too thick creates its own set of process problems.
The thickness is not a single number. It varies with the method of application, with the height of the copper underneath and with the position on the board. Understanding that variation is what allows a sensible specification to be written and defended when a supplier asks what the tolerance should be.
What Determines The Coating Thickness
Screen printing deposits a relatively thick layer and is used where the mask must bridge over traces or fill a large area. Curtain coating and spray coating deposit a thinner, more uniform layer and follow the copper profile more closely. The choice is usually made by the fabricator around the resolution required and the surface finish. The behaviour of the ink itself is described under solder mask ink thixotropy.
The copper height underneath is the largest single influence. On a 35 micron copper layer the mask has to clear the track and still cover everything between the tracks, so the thickness over a track and the thickness on bare laminate differ by roughly the thickness of the copper itself.
The Practical Range
Most specifications call for 15 to 25 microns of solder mask thickness over the copper and 20 to 30 microns on bare laminate, measured after final cure. Fine pitch boards tend to sit at the lower end, because a thick layer limits the resolution of small openings and can leave residue inside the aperture.
High voltage boards and boards that will be handled repeatedly sit at the upper end. Thickness is measured on a cured coupon by cross section or by a non-destructive optical method, and the measurement position must be stated, because the value on a track and the value over laminate are not interchangeable.
Thickness Over Traces And On Laminate
Over a trace the mask follows the copper edge, and at a sharp corner the coating thins. This is where pinholes and skips are most likely, so a specification that quotes a single thickness without stating where it is measured is not useful. The minimum over the highest copper feature is the number that protects the circuit.
On bare laminate between traces the mask is thicker, which raises the dielectric strength of the layer and improves surface insulation resistance. The trade off is that the mask must be thick enough to resist handling yet thin enough to develop cleanly, and those limits meet somewhere around 30 microns.
Tenting And Via Coverage
Tenting means covering a via with mask instead of leaving the annulus exposed. It protects the via from contamination and from solder bridging, and it reduces the exposed copper area. To tent reliably the mask must be thick enough to bridge the hole without sagging into it or cracking across it.
Vias up to about 0.3 mm in diameter tent acceptably on a standard coating. Above that the mask tends to crack over the hole during thermal cycling, and the usual practice is to plug the via first and then tent over the plug, or to leave the via open with a defined annulus. The via structures these rules apply to are described under via in pad or plated through.
Dielectric Strength And High Voltage
Solder mask contributes to insulation, but it should not be treated as the primary insulation in a high voltage design. Its dielectric strength is high, typically above 50 volts per micron, yet a coating with a pinhole offers almost no protection at that point, and the layer is thin compared with the creepage distances a standard requires.
The correct approach is to design creepage and clearance into the copper pattern and to treat the mask as a supplementary layer. Where the mask does help is in reducing the risk of surface contamination bridging two adjacent conductors, which is why thickness and adhesion both matter on a high voltage board.
Curing, Measurement And Common Faults
Curing affects thickness indirectly. An under-cured mask shrinks further during assembly, and the thickness measured before final cure can fall by several microns afterwards. The cure schedule should be confirmed by a solvent resistance test rather than by time and temperature alone.
The common faults are mask on pads, mask lodged in a fine aperture, cracking over a tented via, and skips where the coating failed to wet the copper. Each of them appears in inspection, and each is easier to correct at the coating stage than after final cure. How the mask is called out with the rest of the fabrication data is described under PCB design and fabrication.
Process Control and Verification
On a design of this kind, curing is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.
Process Control and Verification
On a design of this kind, curing is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
Process Control and Verification
On a design of this kind, curing is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Is a thicker solder mask always better? No. Above about 30 microns the resolution of fine openings falls, residue can remain in the aperture and the coating becomes more prone to cracking. The right value balances coverage against resolution.
Why does mask thickness differ between suppliers? Application methods, cure schedules and measurement positions differ, so the same nominal specification can give different results. The measurement location and method should be agreed before the order is placed.
Can solder mask thickness be increased to fix a short? No. A mask is a supplementary insulator and a single pinhole defeats it entirely. The copper pattern and the process must provide the insulation, with the mask providing protection on top.
