The primary function of conformal coating is to protect electronic circuit boards to ensure the longest possible life and highest device reliability

Conformal coatings protect boards from moisture, water, harmful gases, and other environmental factors that can cause damage and corrosion.

They should also prevent ionic migration as it may cause short circuits. This is especially important in high humidity, salt-laden environments.

Traditional conformal coatings such as epoxy, polyurethane, acrylic and silicone have thickness recommendations of 50 to 75 microns. These 3 types of coating are generally hydrophilic and more susceptible to moisture penetration so an application even thicker than 75 microns may be desirable. However, because of their relatively high Tg (~ 30-90°C), thickness beyond 75 microns will induce excessive stress on components mounted on the PWB and may dramatically shorten the life of the device.

CC7130-PRTC and SC7130-CC are effective from 12.5-50 micron thickness. Thicker coating is not necessary but will not have any negative effect.

AIT conformal coatings SC7130-CC and CC7130-PRTC are super hydrophobic. When the conformal coatings are hydrophobic the thickness requirements are not as important. In addition, they block moisture from penetrating that may otherwise carry with it corrosive elements such as sodium, chloride, sulfur dioxide, etc.

Silicone coatings are generally hydrophobic, but they tend to have a large free space volume allowing for fast moisture penetration. Any defects in coating will results in “pot-holes”: water retention areas that may cause corrosion over time.

Silicone is known to allow 10 times or greater moisture penetration than typical acrylic conformal coatings. Corrosive elements such as sulfur dioxide, sodium and chloride ions can be carried along with moisture onto the surfaces that the conformal coating supposed to protect. High moisture permeability is a significant issue for silicone conformal coatings.

Silicone conformal coatings have a low surface energy nature that contributes to their hydrophobic nature. However, this same low surface energy nature, along with the molecular form of the monomers in film format, can cause migration to undesirable areas. This can lead to bonding and soldering problems if rework or repairs are needed.

Conformal coatings are not intended as physical protection media.

General mechanical strength is not required. As long as the conformal coating remains intact at low and high temperature, low and high humidity, and resists water and common chemicals and solvents, they will be able to perform.

Typically, high strength or hardness means high modulus of elasticity. These stronger coatings, such as epoxy and acrylic, may impart excessive interface stresses. These stresses can cause component malfunctions or coating delamination from the circuit board, rendering them ineffective.

Dielectric constant is a measurement of response to an electric field. A dielectric constant higher than 3.0 means molecular response to an alternating field and current. In conformal coating, this may affect the circuit speed. It is not a desirable characteristic for high frequency devices.

Dielectric loss is the loss of energy that goes into heating a dielectric material in an alternating field. This represents the internal molecular friction of the conformal coating. Dielectric loss greater than 0.1 will generally indicate some heat inside the molecules; this is not a good property to have in a conformal coating.

Dielectric strength is a measurement of insulation strength. In conformal coating, generally, the higher the better.

However, circuit boards are designed to provide electrical insulation under normal operation. As long as the conformal coatings do not negatively affect the dielectric strength in the air, they generally will be adequate.

Generally, the greater the hydrophobicity and the lower the extractible ionic impurities the better: this helps prevent attracting water that may enable ionic impurities to become mobile and potentially negatively affect the dielectric strength.

Brush coating, spray coating, and dip coating are industrial standards.

Brush coating is used for smaller volume production. It has the advantage of precision application to  the intended areas. The cost of labor is higher.

Spray coating is used for larger volume production and usually faster. Like in painting, masking is required to prevent applying the coating onto surfaces where it isn’t wanted.  The masking process costs time and money.

Dip coating is used for even larger volume production and is generally the fastest method. Again, masking will be required.

Vacuum deposition is used to apply coatings such as Parylene and ultra-thin oxide coatings. The process is complicated, time consuming, and expensive and yields a small batch size. As such, this method is a limitation in commercial electronics and usually confined to a small volume, high-cost production.

SC7130-CC and CC7130-PRTC are optimized for the industrial standard spray-dip-brush methods but with Parylene performance at 12.5 to 50 microns thickness.

Applying masking tape prior to coating is a common method. Often the tape is in the shape of dots and stripes as these are more suitable for flat areas. Masking tapes for conformal coating are generally polyester or polyimide carriers with a pressure sensitive adhesive. Even if the adhesive layer is anti-static, the tape as a whole is usually not totally anti-static.

In some cases “rubber boots” may be put on the connectors and pins to protect them from being coated. As the boots must fit the connectors precisely, they are all specially customized. The high cost of the mold and molding for these boots makes limits their suitability to high volume applications.

AIT provides masking tapes (such as MT-100-S) that use proprietary, intrinsically anti-static flexible carriers. They have the ability to conform to small curvatures and are a superior masking solution for most applications.

AIT liquid masking gel MG-150-SG can be applied to pins and connectors. The material self-forms into a boot, bypassing the need for expensive molding. It can be removed by simple peeling and reused.

In all cases, whenever possible, the masking should be removed before the conformal coating cures. This will prevent tearing the coating when the masking materials are removed and also allow easier cleanup (and reuse, in case the case of “boots”).

Most circuit board edges are cut and are not protected with a coating.

Moisture and water can easily migrate between the board solder mask and the board itself.

Whenever possible, conformal coating should be applied to the edges of circuit boards to block this migration and ensure the longest life and highest reliability for the board.

Consequences of induced stress from conformal coating, which can be 1000s of psi in stress, can range from hairline conformal coating cracks, to large conformal coating cracks, to coating delamination or even to damage of electronic components.

Flexibility is an important factor. Conformal coatings must be able to accommodate differential expansion of the coating and the board.

• Circuit boards are typically made from fiber-glass reinforced epoxy with a planar CTE in the range of 16 to 20 ppm/ºC.

• As conformal coatings cannot match this low CTE, flexibility and the ability to stretch are key to preventing induced stress from CTE mismatch

• Typical conformal coating CTE’s are 55 to 65 ppm/ºC for acrylic and ~35 for Parylene.

Glass transition temperature (Tg) is an important factor. A flexible, low Tg coating will reduce stress at low temperatures commonly experienced in outdoor use.

• Traditional acrylic and epoxy conformal coatings often have glass transition temperatures around 10 to 20°C. Low temperature cycles will induce excessive stress and reduce the reliability of the device (http://www.slideshare.net/CherylTulkoff/how-conformal-coating-can-kill).