Conformal coating in electronics – what you need to know

Introduction

Electronic devices do not fail only because of design errors or manufacturing problems. Failures are also driven by the operating environment, which is often far more "creative" than many engineers ;-)

Sometimes a single daily temperature cycle is enough for moisture to condense on a PCB — a thin film of water that damages the electronics. Or perhaps fine conductive dust from the production hall settles on critical circuits and reduces insulation resistance by several orders of magnitude. In some applications, salt fog (the "sea breeze"), sulfur-bearing gases, and other odd phenomena come into play.. Sometimes an ordinary splash is all it takes to start electrochemical migration. These failure mechanisms require nothing more than contact between the electronics and the environment — plus a little time..

This is precisely why electronic assemblies are protected, among other methods, with conformal coatings, sometimes referred to informally as PCB lacquering or varnishing.

Enjoy the read :)

Conformal coating - function

A conformal coating is a thin, flexible polymer layer applied to an assembled electronic module (assembly). The functions a conformal coating performs are:

• Barrier against moisture ingress. The coating reduces the effect of moisture on the electronics. It is not a perfect barrier. It only slows moisture permeation, but it still significantly extends service life by pushing out in time such phenomena as electrochemical migration - dendrites, corrosion
• Barrier against contamination. A conformal coating shields the circuitry from direct contact with dust, debris and other foreign material. Most coatings also reduce the ingress of sulfur-bearing gases, which lowers the risk of silver creep corrosion, i.e. the conversion of the silver portions of components into unwanted silver sulfide.
• Reduced effect of vibration. To a certain extent, the coating reduces the effect of vibration on components and on their solder joints.
• Mitigation of tin whisker effects In ADHP applications (Aerospace, Defense, High Performance), conformal coatings are used as an additional mitigation method for tin whiskers. A properly selected and properly applied coating acts as a mechanical barrier that restrains whisker growth and prevents contact with adjacent leads - that is, violation of minimum electrical spacing.

What to keep in mind when using a conformal coating:

• Moisture is reduced, not eliminated. A conformal coating is semi-permeable. Every polymer has a non-zero water vapor transmission rate, which means that after a sufficiently long time moisture will reach the surface beneath the layer anyway. It is only a matter of time..
• The coating preserves whatever it finds. If flux residues, salts, fingerprints, moisture in the laminate, and so on are left under the coating, the result is not protection but something like a "micro-reactor" ;-) with its own electrolyte and an applied bias.. In effect, corrosion can develop - i.e. electrochemical migration (ECM), and sometimes, in combination with a poor-quality laminate, the CAF phenomenon. For high-reliability products, PCB cleanliness and a low moisture level in the PCB before coating are therefore not an option but a requirement.
• Coating thickness is not uniform. Surface tension and the rheology of the material cause the coating to flow off the vertical walls of components. This reduces coating thickness at sharp edges, package corners and lead edges. In these locations the conformal layer is much thinner than on the flat surface of the laminate. A very thin layer, particularly on metal edges, may not provide adequate protection, and electrochemical migration will develop within a short time. On the other hand, a layer that is too thick can impose stress on components because of the mismatch in the coefficients of thermal expansion "CTE" of the materials. This stress may crack solder joints or even fracture component bodies (e.g. glass MELFs).
• Effect on electrical insulation. Conformal coatings improve insulating properties because their dielectric strength is higher than that of air; however, a conformal coating should not be relied upon as a means of providing insulation. In other words, the coating improves dielectric withstand somewhat "along the way", but by itself it should not be used as the only form of protection. Please remember that adequate electrical insulation is to be ensured by the use of suitable insulating materials and by maintaining correct spacing between circuits (clearance and creepage) in accordance with the applicable UL or IEC standards.

Types of conformal coatings

Industry offers a range of materials, which helps in selecting the optimum coating type depending on operating conditions, application methods, reworkability and, of course, cost $$$.

The coating classification below is based on IPC-A-610 and J-STD-001.[1,2] The IPC standards define coating types by chemical composition. The breakdown of materials is therefore as follows:

• Acrylic (AR). Acrylic resins based on a single-component material that dries by solvent evaporation. Colorless, hard, usually with a fluorescent tracer added. Advantages: simple process, excellent reworkability (it dissolves in common solvents — a soldering iron passes through the layer with little resistance), good electrical properties, low cost. Disadvantages: poor resistance to solvents, acetone and fuels, moderate temperature resistance, mediocre moisture barrier. Thickness per IPC-A-610J: 25.4-127.0um [0.001-0.005 in].[1,2]
• Epoxy (ER). Most often two-component and thermally cured. Hard, chemically resistant, a good barrier. Advantages: very high resistance to abrasion, moisture and solvents, high operating temperature (up to 150C). Disadvantages: very difficult rework (the hard layer is hard to remove), cure shrinkage and high stress on components. Thickness per IPC-A-610J: 25.4-127.0um [0.001-0.005 in].[1,2]
• Urethane (UR). Urethane (polyurethane) is a one- or two-component material. Excellent chemical and mechanical resistance and a very good moisture barrier. Advantages: resistance to solvents and abrasion, good mechanical properties, low cost. Disadvantages: long cure time, very difficult rework, shrinkage during rapid cure and high stress on components. Thickness per IPC-A-610J: 25.4-127.0um [0.001-0.005 in].[1,2]
• Silicone (SR). An elastomer with the widest temperature window, typically from about -60C to +200C and beyond. Advantages: flexibility, wide operating temperature range, excellent performance under thermal cycling, good vibration damping, moisture resistance, very good dielectric properties. Disadvantages: soft surface and poor abrasion resistance, risk of siloxane migration (e.g. relay contacts), troublesome rework. Thickness per IPC-A-610J: 50.8-203.2um [0.002-0.008 in].[1,2]
• Paraxylylene (XY). Poly(p-xylylene), deposited from the vapor phase under vacuum (the Gorham process). Advantages: a thin and uniform film (fully conformal), covers edges and crevices evenly, an excellent barrier and biocompatible (implants, medicine, MEMS, avionics). Different variants offer a wide operating temperature range, up to as much as 350C. Disadvantages: costly process, requires dedicated equipment, masking is critical, no possibility of touch-up. Thickness per IPC-A-610J: 10.2-50.8um [0.0004-0.002 in].[1,2]
• Styrene copolymer (SC). Synthetic rubber copolymers are materials offering exceptional flexibility over a wide temperature range, up to 150C. Advantages: flexible, wide temperature range, high moisture resistance, good adhesion to the substrate, easy rework. Disadvantages: poor resistance to solvents and fuels (some grades). Thickness per IPC-A-610J: 25.4-76.2um [0.001-0.003 in].[1,2]

Application methods

Conformal coatings can be applied by a number of methods, from the simplest (with a brush..) through dipping and selective application (micro-spray, jetting), all the way to vacuum deposition (for paraxylylene).

More on application methods coming soon in the next article..

Summary

Choosing the right material for a conformal coating is an important decision that should first take into account the environmental requirements, then the application method and the cost of the material itself.

During PCB design, coating-related requirements shall be considered (e.g. providing adequate spacing between connectors and other, coated parts), a suitable solder mask should be selected to ensure good wetting and adhesion of the coating, and so on.

And finally, something important and often overlooked: a conformal coating does not fix a bad process, it locks it in. A coating applied over a dirty, damp PCB is a long-term investment in a customer complaint :(. Applied without thickness control, it gives a false sense of security. Applied too thick, it may damage certain components or their solder joints.

A coating type well matched to the application, together with an optimal application method, is one of the many key decisions made by an engineer designing reliable electronics.