COATING

A Conformal coating is a protective chemical coating or polymer film 25-75µm thick (50µm typical) that ‘conforms’ to the circuit board topology. Its purpose is to protect electronic circuits from harsh environments that may contain moisture and or chemical contaminants. By being electrically insulating, it maintains long-term surface insulation resistance (SIR) levels and thus ensures the operational integrity of the assembly. It also provides a barrier to air-borne contaminants from the operating environment, such as salt-spray, thus preventing corrosion.

SIL 1198-1 COATING

Product Specification

Density, per ASTM D1475

1.01 ± 0.03 g/cm³

Min. Solids Content, % by weight per Fed-Std-141, Meth. 4044

100%

Viscosity, per Fed-Std-141, Meth. 4287

270 ± 50 centipoise

VOC

0 grams/liter

Recommended Coating Thickness

50 – 200 microns

Drying Time to Handle per Fed-Std-141, Meth. 4061

5 min (100 μm, 50% RH at 21 ºC)
8 min (200 μm, 50% R.H. at 21 ºC)
1.5 min (100 μm, 8% R.H. at 60 ºC)
4 min (200 μm, 8% R.H. at 60 ºC)

Recommended Curing Conditions

24 hrs. @ RT (> 40% R.H.)

Time required to Reach Optimum Properties

7 days

Shelf Life at Room Temperature, DOM

12 months

Dielectric Strength per ASTM D149

18.5 KV/mm

Hardness, per ASTM D-2240-95

25 Shore A

Volume Resistivity per ASTM D-257

3.44 x 1014 ohms.cm (344TΩ.cm)

Surface Resistivity, per ASTM D-257

3.0 x 1014 ohms per sq.(300TΩ/sq.)

Insulation Resistance, per MIL-I-46058C

5.0 x 1013 ohms (50TΩ)

Moisture Insulation Resistance, per MIL-I-46058C

45.2 x 1010 ohms (452GΩ)


How do conformal coatings work?

A conformal coating is a protective barrier that shields sensitive electronic components against harsh environmental conditions such as moisture, chemicals and debris. They are not designed to be a total sealant. Conformal coatings are a breathable protective layer that will protect against the particular environment requirement but will also allow any moisture trapped in the circuit board to escape.

The particular advantages of conformal coatings can be summarized as follows:

·         Insulating properties allow a reduction in PCB conductor spacing of over 80%

·         Can help eliminate the need for complex, sophisticated enclosures

·         Light weight

·         Completely protect the assembly against chemical and corrosive attack

·         Eliminate potential performance degradation due to environmental hazards

·         Minimize environmental stress on a PCB assembly

Ideally, conformal coatings should exhibit the following characteristics:

·         Simple application

·         Easy removal, repair and replacement

·         High flexibility

·         Protection against thermal and mechanical shock

·         Protection against environmental hazards including: moisture, chemicals and other corrosive elements

How do you apply Conformal Coating?

To be effective, a conformal coating should completely cover the assembly and provide good coverage of sharp edges (components and component leads) and other contours (solder joints for example). It may even be essential to coat the edges of the board, as these will not even have any solder resist present.

The effectiveness of the coating is, to some degree, influenced by the efficiency of the application method, which also governs the type of coating used (some suppliers offer dedicated versions of the same coating product to suit different application methods).

There are essentially four main ways of applying a conformal coating:

1.     Dipping – limited to materials that do not cure quickly by moisture, oxidation or light

2.     Selective robotic coating – such as Asymtek, PVA or DIMA. All coating types can be used if the correct dispense head is selected

3.     Spraying – hand spray using a spray booth or aerosol can. All coatings can be applied in this way

4.     Brushing – requires extremely proficient and skilled operators in order to be suitable for production purposes

Finally you will have to consider the curing method determined by the coating selected, air dry, oven dry or UV light cure. The liquid coating should thoroughly wet all surfaces and cure without leaving surface defects. Epoxies are especially sensitive to surface defects. Epoxies can also shrink while setting and may lose adhesion as a result In addition; excessive shrinking during cure can place severe mechanical stresses upon circuit components.

The supplier, to create the special formulation required for a particular application, should be able to modify all of these properties. Other characteristics can also be enhanced. Accessory chemicals and solvents should only be blended with base polymer resins by the coating manufacturer. Plasticisers can also improve mechanical flexibility, and selected solvents in conventional coating formulations can improve adhesion of the cured film.

How much coating will I use and what will the cost be?

This is obviously related to how many boards you process but there is a simple calculation that can give you a reasonably accurate prediction. Solvent based coatings should be applied at 50µm dry thickness; UV coatings will be between 100µm and 200µm, depending on application method.

The calculation is therefore as follows:

Volume per PCB (mL) = [width (mm) X length (mm) X dry thickness (mm) X 100 / %solid] / 1000

Cost per PCB (£/PCB) = [cost (£/L) X Volume per PCB (mL)] / 1000