As component densities increase and line spacings become smaller, printed wiring board (PWB) failures are more likely to occur. Those closely packed components and traces are now protected by less and less dielectric board material, increasing the likelihood of corrosion, shorts, and electromigration. One way to improve reliability of an assembly is through the use of conformal coating.

Conformal coating is defined as a thin polymeric layer which “conforms” to the topography of the PWB and components. It acts as an insulator, protecting the circuitry and components against shorts and contact with moisture and other contaminants. It also provides mechanical protection from vibration and thermal shock.

When high reliability is desired, a conformal coating should be applied. However, conformal coatings themselves can fail or cause the board to fail. This article summarizes the various sources of conformal coating failures and highlights some recommendations to avoid those potential failures.

Improper choice of coating
Selecting the appropriate coating based on the application will reduce the risk of failure. For instance, an acrylic coating would not be the ideal choice for an automotive application, because this coating type tends to soften (low glass transition temperature, Tg) with the high temperatures and exposure to moisture or petroleum residues. A better choice would be a silicone coating, which has a usable operating range of -55°C to +200°C and offers resistance to high humidity environments.(1) An ultraviolet (UV) cured coating may not be the best choice if the assembly in question has high-profile components. Shadowing can leave uncured coating which compromises the reliability of the PWB. Some coating manufacturers address this issue by adding catalysts which act as a secondary cure mechanism.

Shelf life and pot life
The manufacturer’s recommended shelf life and pot life should be reviewed and tracked closely. Careful monitoring of both unopened (storage) and opened (exposure to atmosphere) material for its useful life is key in preventing the use of expired material in the coating process. Specifically, coatings which cure through moisture can be difficult to work with as premature curing can occur in humid environments. Also, two-part systems require proper inventory controls and accurate mixing of the coating agents for best product performance.

Mismatches in coefficient of thermal expansion (CTE)
Understanding the material characteristics of the PWB and components is critical. Mismatches in CTE between the PWB and the coating can be a problem. Mismatching CTE can also result from applying too much coating, as CTE is volume-dependent, and the thicker the coating, the larger the change in volume with temperature changes.

In some instances, stresses due to thermal expansion can be compounded by a poor design. As shown in Figure 3-1, a PWB was designed such that a diode cracked under the stress of built-up conformal coating underneath it. Devices like this glass diode or other fragile materials are greatly affected by expansion effects (more specifically, modulus influences). A material like silicone can have a high CTE and low modulus (compressive or expansion forces). As a result, silicones in general do not cause significant stresses on sensitive components. On the other hand, urethanes and epoxies have been historically high modulus materials.

Moisture permeability
All coatings are porous, so water and other materials can and do permeate to varying degrees and cause failures (Figure 3-2). Some types of coatings are more permeable to moisture than others.

Compounds containing chloride are most vulnerable, and silicone was found to be permeable to volatile forms of the chloride compounds (i.e., hydrochloric acid [4 to 9 x10-9 cm2/sec at 121°C]) more so than the common non-volatile salt, sodium chloride [4 to 7 x10-11 cm2/sec])3. Silicone has also been found to increase absorption of hydrogen sulfide, a common by-product of burning fossil fuels.(4)

Permeability to moisture can also be an advantage, as a coating which allows a board to “breath” creates a low static moisture level. Trapped moisture can be a problem, and one standard recommends baking a PCB for a minimum of 4 hours at 93+/- 5.5 ºC before conformally coating.(5)

Thickness of coating
A coating’s effectiveness is a function of its end-use environment and thickness. The required thickness of each type of coating depends upon the application and the end use of the PWB. Table 3-1 provides thicknesses for common coating materials.(6)

Dielectric strength
The dielectric strength of a coating increases with coating thickness. This property is especially important in high voltage or high current applications. High voltage applications use the following equation to achieve the proper thickness1: T=V x 6.44 – where T is the calculated coating thickness in inches, and V is the maximum voltage of the circuit.

Out-gassing
Some conformal coating failures involve out-gassing. This phenomenon can occur with heat, vacuum, or both, and results in a coating’s loss in weight from volatile emissions. These emissions can be low molecular weight compounds, uncured resin or hardeners from incorrect proportions of reactants, residue plasticizers, flame retardants, or solvents, as in the case of acrylics.(7) Out-gassing can also occur as a by-product of the curing step. Silicone coatings using an acetoxy-cure can release acetic acid. This failure mode is usually a concern for the aerospace industry, where such emissions can be flammable, irritating, and hazardous to breathe, or cause electrical failures because of the corrosive nature of acetic acid.(7)

Uncured coating
Inhibition or the inability for a coating to obtain the optimum properties at the manufacturer’s specified time and temperature can result in uncured coating. One example mentioned earlier was shadowing with UV cured coatings. Coatings can also be inhibited by contaminants such as organic acids from flux residues. In the case of natural latex (a common masking agent), the residue alkali can reduce curing of certain catalytic-type coatings.(3)

De-wetting
The critical parameter to the success of a conformal coating is its ability to adhere to the surface it is applied to. Since most coatings are liquids, the coating must be thick (viscous) enough that it remains on the PWB surface as a uniform film but not too thick or have too high a surface tension to cause voids. This wetting is influenced by changes in the composition of the coating and compatibility with the PWB surfaces (solder masking, board or component surfaces, potting compounds, etc.). Figures 3-3 and 3-4 show examples of conformal coatings which displayed de-wetting.

Surface contaminants
It is also important that surface contaminants (moisture, flux, oil, grease, fingerprints, etc.) be removed before coating. Rosin or water-soluble fluxes, because of their corrosive nature, must be cleaned off thoroughly, as such residues can cause failures. In addition, release agents, which are designed to free plastic parts from their molds, can inhibit the adhesion of conformal coatings.

The trend in no-clean or low-residue fluxes has reduced or eliminated cleaning steps; however, the residues left behind can be difficult to adhere to. Poor adhesion would be more likely with a smooth surface residue than one with a matte or dull finish. (1)

From a contamination standpoint, the ability to dedicate equipment to particular coating types is often important, as switching from one coating type to another can be an issue depending upon the materials.

In summary, to minimize failures, the following questions should be asked before utilizing a conformal coating:

1) What are the through-put or usage requirements?
This will dictate shelf-life and inventory requirements.

2) What is the end-use environment?
This is probably the most important question and will ultimately dictate the coating type and whether a conformal coating is needed at all.

3) What application capabilities are available?
Some methods of applying coatings provide a more consistent layer.

4) What is the end-use of the assembly (high power, RF, digital, etc.)?
This will dictate some of the requirements of the coating.

5) What are the surfaces composed of?
This will dictate adhesion limitations.

Summary
The EMPF has the capability to support analysis and processes associated with conformal coating. We have a number of partnerships, allowing the military and commercial markets access to technologies and a knowledge base which can address most conformal coating questions or process problems.

References
1. IPC-HDBK-830, “Guidelines for Design, Selection and Application of Conformal Coatings.”

2. “Workmanship Problems Pictorial Reference,”
http://www.workmanship.nasa.gov/wppr_comp_confcoat.jsp

3. “Chlorine contamination diffusion in silicones,” Alexander Teverovsky, Ph.D.,
http://nepp.nasa.gov/DocUploads/15BC4F7B-1305-4292-A4C1D2FDF5F82B7F/Chlorine%20contamination%20diffusion%20in%20silicones.doc

4. “Impact of gaseous sulphides on electronic reliability,” http://www.era.co.uk/news/rfa_feature_03.asp

5. NASA-STD-8739.1, “Workmanship Standard for Staking and Conformal Coating of Printed Wiring Boards and Electronic Assemblies,” August 6, 1999.

6. IPC-CC-830B, “Qualification and Performance of Electrical Insulating Compound for Printed Board Assemblies,” August 2002.

7. Coating Materials for Electronic Applications – Polymers, Processing, Reliability, Testing, J.J. Licari; 2003