Conformal coatings protect 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 electromigration of metal between conductors. In addition, the use of conformal coatings protects circuits and components from abrasion and solvents. Stress relief is also provided, as well as protection of the insulation resistance of the circuit board.
Conformal Coatings Options
For conformal coatings, the most competitive options are materials based on: acrylics, epoxies, urethanes, parxylenes or silicones.
How Silicones Protect Surfaces
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 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.
Urethane 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 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 coatings range from elastoplastic (tough, abrasion-resistant) to soft, elastomeric (stress-relieving) materials.
They possess the following characteristics:
- Usefulness over a very wide temperature range
- Good moisture and humidity resistance
- Processing versatility
- Easy repairability
- Low toxicity
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.
Key Conformal Coating Characteristics
Besides humidity, conformal coatings provide protection from vibration and a wide variety of contaminants such as vapors, some solvents, and salt spray.
Conformal Coatings 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
- Wide range of operating temperatures
- Low cost
- Robust adhesion
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.