As modern electronics assemblies continue to use ever greater packaging densities and are subjected to increasingly hostile operating conditions, the use of high performance conformal coating materials will become increasingly commonplace for many manufacturers.

When high reliability is desired, a conformal coating should be applied. Conformal coating is 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 as well as other contaminants. It also provides mechanical protection from such things as vibration and thermal shock.

Choosing the right conformal coating for the specific PWB design application is critical as there are a variety of different materials available. PWB cleanliness and optimal application methods are also very important to ensure good adhesion and reliability.

Inspection

Any conformal coating production process can be visually inspected in production using a UV microscope (Figure 6-1).
All approved coatings contain a UV trace that glows with a bright blue luminescence under UV light to make coated and uncoated areas easily detectable. With experience, operators can use the degree of luminescence as a measure of both presence and volume of coating at different locations across a board’s surface.

The necessity for inspection becomes evident when a single void or bubble provides a path for moisture to reach the substrate and reduce the coating effectiveness. With automatic coating, a random or pre-selected interval test may be adequate, but 100% inspection is always recommended and is mandatory in high reliability and safety critical applications.

It is also important to inspect boards after rework to ensure any reapplication of coating material doesn’t end up on the inside or underside of devices that should not be coated. The IPC-CC-830 and IPC-A-610 specifications demand inspection be carried out and define conformal coating defects as:
Ref: IPC-A-610D Section 10.5 - “Non conforming defects... Voids, bubbles, adhesion loss, de-wetting, ripples, fisheyes, orange peel, or foreign material that expose circuitry, or adjacent conductive surfaces.” It is generally recommended that inspection be done visually with UV and at 4X magnification.

Conclusion

Choosing the right conformal coating and application process is not easy. It is advantageous to work with a vendor that has the experience of solving hundreds of specific coating problems and can give advice on relevant reliability standards and specifications. The correct approach will result in the reduction of end product failures in the field. These can be the most expensive problems a company will ever experience, and in safety critical applications, life threatening

When inspecting conformally coated PWBs, understanding the materials and processes used in production, and the acceptability criteria, will assure a high degree of reliability.