Electronic Manufacturing and PCB Assembly by MIS Electronics

PCB Conformal Coating

Our conformal coatings protect your electronic printed circuit boards from moisture and contaminants, preventing short circuits and corrosion of conductors and solder joints.

They also minimize dendritic growth and the electro-migration of metal between conductors.

In addition, our 5 conformal coatings protect your circuits and components from abrasion and solvents. Stress relief is also provided, as well as protection of the insulation resistance of the circuit board.

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Conformal Coating Options

For conformal coatings, the most competitive options are materials based on: acrylics, epoxies, urethanes, parxylenes or silicones.

Acrylic
Acrylic coatings are typically solvent based and are easily repaired. Traditionally they were low cost but have become less competitive with rising regulation requirements and safety concerns with solvent usage. They were tough, hard, and transparent - but lacked stress relieving capability. Pot life was good since additional solvent could be added to keep the bath viscosity low and they exhibited low moisture absorption and had short drying times when volatile solvents were used.
Epoxy
Epoxy coatings are very hard, usually opaque and are good at resisting the effects of moisture. They possess excellent chemical and abrasion resistance, but can cause stress on components during thermal extremes. Cure chemistries can sometimes require extra precaution for safe handling. Fluctuating temperatures can cause considerable drift in viscosities and difficulties in controlling cure times and coating thicknesses.

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Urethane
Urethane conformal coatings are tough, hard and exhibit excellent resistance to solvents. Along with excellent abrasion resistance and low moisture permeability, they offer good low temperature flexibility. Their use is often prevented by their limited high-temperature capability and lack of repairability.
Paraxylylene
Paraxylylene coatings are very uniform and yield excellent pin coverage. Their limitations include high cost, sensitivity to contaminants and the need for vacuum application technique.
Silicone
Silicone coatings range from elastoplastic (tough, abrasion-resistant)to soft, elastomeric (stress-relieving) materials.
They possess the following characteristics:
- Heat cure or RTV cure

Usefulness over a very wide temperature range
Good moisture and humidity resistance
Processing versatility
Easy repairability
Low toxicity

How Silicones Protect Surfaces

Corrosion can occur when a continuous layer of water condenses on the electronic circuitry. The water needs only a few dissolved ions to become conductive and allow electrochemical reactions to start, resulting in corrosion of metals on the circuit or the creation of electrical leakage between conductors, especially with bias voltages that are present in the circuitry.

Protecting the circuit with silicone, however, has been found to be one of the most effective ways to extend the life of the electronics in harsh use environments. This has been documented over decades of use in the industry in many different applications and use situations.

One of the keys for providing protection to underlying circuitry and other metallic surfaces is attaining good wetting and adhesion of the cured silicone to all surfaces of the electronic device or module. Liquid water is easily excluded at the cured surface of silicone coatings, but water vapor, present as humidity in the air, can permeate to the interface. However, without a place to collect below the coating, it causes no ill effects. Good adhesion eliminates gaps where liquid water could condense. Besides humidity, conformal coatings provide protection from vibration and a wide variety of contaminants such as vapors, some solvents, and salt spray.

Key Conformal Coating Characteristics

Conformal Coating should ideally possess as many of the following characteristics as possible:

One-part product
Low viscosity
Spray, dip, select, or flow-coat application options
Long pot life
Rapid cure, no by-product
Sufficient under-component cure
Repairability
Wide range of operating temperatures
Non-toxic
Low cost
Robust adhesion

Conformal Coating Processing Options

Silicone conformal are easily applied by a variety of methods. A general overview of these processes is given below.

All of the coatings can be applied by the traditional methods of dipping, spraying or simple flow coating. For high volume, spraying or dipping is best. In low volume, simple flow coating can be used. Because of differences in curing mechanisms, special considerations must be made for removal of any solvents present or for curing higher thickness or curing under large area components.

Environmental compliance is globally creating some strong trends. There is a desire to avoid solvents that are frequently seen in non-silicone materials. This trend has generated a growing interest in the Elastomeric Heat Cure and Elastomeric Room Temperature Cure products. Similarly there has been a growing usage of no-clean fluxes, making the compatibility with the conformal coating and the underlying no-clean residues an issue to be considered.

Repairability and Component Replacement

Circuits coated with silicone conformal coatings may be repaired. This provides silicones with an advantage over many of the competitive coating options.

After removing the coating using solvent-swell or mechanical abrasion techniques, the defective device can be de-soldered and removed. Standard burn-through techniques can also be used. After removing the old solder, the area should be thoroughly cleaned with a solvent-soaked swab to ensure a good replacement joint. After component installation, re-coating can be accomplished.

	Dip Coating Dip coating offers complete encapsulation and represents the lowest process cost for high-volume applications. Boards are dipped into the liquid conformal coating, withdrawn, and excess material allowed to drip off before curing. With this method, masking is difficult. Board and tank temperature, withdrawal rate from the tank, cure temperature and amount of drip time affect applied thickness.
	Spray Coating Spray coating is usually used for medium-volume production; at medium- or high-volume production rates, application costs are low to moderate. Masking is easier but spray application makes it difficult to coat under components, and requires slightly more maintenance. The type of feed system, line speed, temperature of the material and atomization pressure used will affect the applied thickness.
	Select Coating Select coating can be used for moderate- to high-volume applications and often eliminates the need for masking because the material is dispensed only on selected areas of the circuit. It uses more expensive equipment for high volume processes and requires more maintenance than either dip or spray coating. Coating thickness is affected by material viscosity, temperature, pressures used and dispense head configuration.
	Flow Coating or Brush Coating Flow or brush coating is used for low-volume production or for making repairs. Equipment costs are very low. The coating is simply poured or brushed onto the boards and then cured.