Selective conformal coating — get the coating plan and layout right.

What selective coating delivers

A conformal coating is a thin protective film on the assembled board that shields the electronics from moisture, dust, dirt and condensation. "Selective" means the lacquer is applied only to defined areas and deliberately keeps others free — such as connectors, heat-sink surfaces, test points, controls or components that must not be coated over.

SMTEC runs selective conformal coating reliably and double-sided. The lacquer used is the Peters Elpeguard SL 1307 family, dried inline in a conveyor oven after application. This combination of dosed application and defined drying produces an even, reproducible protective film — even for smaller quantities and prototypes.

So the machine knows where to coat and where not to, it needs one clear input: the coating plan.

The coating plan: three zones

A clean coating plan clearly marks three zone types on the assembly. Only when all three are defined can the machine program the coating reproducibly:

// Zones in the coating plan — source: SMTEC Design Guidelines

Because lacquer is a liquid medium, it cannot be stopped at a millimetre-precise edge. So distinguishing only "coated" and "non-coated" is not enough. You need a flexible coating plan with run-out zones — which is exactly what the third, optionally coatable zone provides.

Why run-out zones are needed

When liquid lacquer is meant to stop at a border, in practice it always runs slightly past the ideal line, or pulls back a little through surface tension. Without a buffer the lacquer either ends up on a surface that must stay free, or you get an unclean, incomplete edge.

The free zones absorb exactly this tolerance. They sit between the area that must be coated and the area that must stay free, and may be coated without harming function. This lets the lacquer run out in a controlled way without reaching the critical "do not coat" surfaces. Anyone who omits these run-out zones in the design forces production into the only alternative — a lacquer dam.

Layout requirements

To make selective coating cheap and reliable, a few rules should already be observed in the layout:

// Layout rules for selective coating — source: SMTEC Design Guidelines

The 3 mm clearance is the key figure: it is the spatial implementation of the run-out zone. Cleanly separating and grouping coated and non-coated zones reduces the number of critical transitions — and with it the programming effort and the risk. And using only closed vias instead of open ones prevents lacquer from wicking through the board to the opposite side into an area that must stay free.

Avoiding lacquer dams

If very small clearances between coated and non-coated areas still have to be kept, the machine can apply a dam. This lacquer dam is a more viscous bead of material that holds the liquid coating back at the border and forces a clean edge.

That works technically — but it is cost-intensive: the dam material is expensive and the extra application step lengthens the process time. Lacquer dams are therefore not recommended. The far cheaper solution lies in the layout: sufficient clearances of at least 3 mm, grouped zones and generous run-out zones. Planning this from the start saves dam material, process time and back-and-forth during release.