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Conformal Coating Materials

Selective conformal coating systems supported by Nordson ASYMTEK's world-leading process knowledge provides advantages at every level of production

Conformal Coating Material Types

Nordson ASYMTEK's selective conformal coating systems provide advantages for electronics manufacturers at every level of production from batch to high-volume, in-line production. Flexible, integrated coating and curing systems with choice of applicators provide precise coating with process controls and traceability. The result is cost savings, increased throughput and productivity.

Conformal coating materials generally have base formulations of acrylic, polyurethane, copolymer (acrylate polyurethane), or silicone. Conformal coating materials are selected by considering the environment in which the electronics will be operating. Among other factors, one should consider aspects of moisture, dust, conductive particles and chemicals. A further inspection of the environment often reveals an extensive list of elements that can shorten the lifetime of non-coated electronics.

While the selection of coating material is often the first step to define a conformal coating process, the flexibility of Nordson ASYMTEK's conformal coating equipment support a change in preferred material type as needs evolve in your production. Conformal coating material base formulations all have benefits over one another.

Conformal Coating - Acrylic and Polyurethane

Acrylic is typically solvent based, but also exists as water-based solutions. The viscosity is typically low, and can be adjusted by adding additional solvents. They cure by drying - accelerated by heat. However, the temperature profile is critical to ensure fast solvents evaporate before they reach boiling temperatures. Repair can be done by dissolving the coating, and after repair new coating can be applied without creating any stratified seams.

Polyurethane materials are mostly solvent based, but also exist as water-based solutions. The viscosity is typically low, and like acrylics, can be adjusted by adding solvents. They cure by a combination of drying and cross-linking, accelerated by heat. Due to the cross-linking they offer additional chemical resistance. Repair can be done by combination of dissolving the coating and mechanically removing it. After repair new coating can be applied, however, the inability to dissolve the material can cause stratified seams.

For both acrylic and polyurethane materials, they are typically applied with the film coater applicator technology, offering unmatched throughput, edge definition, wet dispense resolution and accuracy. Alternatively they can also be applied using swirl technology.

When curing the above two material families, the temperature profile is critical to ensure fast solvents evaporate before they reach boiling temperatures. In addition, it is important that the drying occur from the bottom up to avoid entrapment of outgassing solvents, hence IR heat source is most suitable. Various solvents, often considering how fast they dry, are available in ensure the best match for your production.

Conformal Coating - Copolymer

Copolymer conformal coating is a mix of acrylic and urethane that might contain very small (often insignificant) amounts of solvent that lower the viscosity, but still much less solvent than acrylic or polyurethane formulations. Therefore, copolymers are a viable solution for significant reduction of Volatile Organic Compound (VOC). This base formulation is often referenced as "UV materials" because they have an added photo initiator. They cure by exposure to UV light (typically A+C) that activates the photo initiator. The decomposition of the photo initiator results in the cross-linking. These materials are typically not repairable, but in extreme cases can be repaired by dissolving the coating and mechanically removing it. After repair, new coating can be applied, however, the inability to dissolve the material can cause stratified seams.

Copolymer materials are typically applied using a swirl applicator. However, certain versions can also be applied using the film coater technology.

The above three base formulations (acrylic, polyurethane and copolymer) vaporize when exposed to excessive heat. Symptoms include a discoloring, followed by a reduction in mass.

Conformal Coating - Silicone

Silicone, the last of the general base formulations, offers a viable solution for VOC-free production, although it has high viscosity. Silicone has high temperature tolerances and chemical resistance. There are generally three cross-linking reactions for reactive silicone polymers: free-radical reaction, which is activated by heat; condensation reaction, which is often accelerated by heat or moisture; and addition (catalyst) reaction, which is activated by heat and/or specific light wavelength (e.g. UV). Silicone formulations are in general difficult to rework partly due to their chemical resistance and non-vaporizing by heat.

Conformal Coating - Summary

When conformal coating started in the 1960's, the formulations were two-component. In the 1980's, when electronics manufacturers wanted to automate the process, they moved toward single-component formulations. Today, electronics manufacturers are struggling with some of the inherent consequences of these formulations, such as the short shelf life, time-to-final properties, adhesion, thermal cycles, and in some cases, peak temperature. Generally speaking, two-component formulations offer superior performance. All of the above conformal coating material base formulations can be the foundation of two-component versions.

See the chart below for comparisons of the different types of conformal coating materials and how to apply them.

Table: Conformal Coating Type Comparisons

 

Acrylic

Urethane

Copolymer

Silicone

Viscosity

Low

Low

Mid

High

Typical applicator technology

Film Coating

Film Coating

Film Coating / Swirl

Swirl

Curing

Drying

Crosslinking

Crosslinking

Crosslinking

Volatile Organic Compound (VOC)

Yes

Yes

No

No

Chemical resistance

Low

High

Very High

High

Thermal span

Low

Low

Moderate

High

Glass Transition Temp

Low

Low

Mid

NA

Repairable

Solvents

Solvents & mechanical.

May result in stratified coating after repair

Solvents & mechanical.

May result in stratified coating after repair

Solvents & mechanical.

May result in stratified coating after repair

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