Protect system operation in corrosive environments
Humidity, condensation, salt-spray, corrosive gases or a combination of all of these elements, activate the start of the corrosion process, which can be accelerated by residues from soldering and other assembly processes prior to coating.
With the increasing adoption of electronics in developing countries, where atmospheric pollution levels are generally higher, there is a growing need for corrosion protection. Additionally, there is an increasing trend towards making smaller devices with greater functionality, resulting in higher density assemblies that are expected to operate trouble free for an extended lifetime, in ever-more aggressive operating environments.
Conformal coating types
There are many conformal coating chemistries and each have their own benefits and drawbacks. Conformal coatings are available in solvent-based, water-based and 100% active materials (nearly everything that is applied in liquid form is converted into solid protective coating), as well as vapour deposited coatings, in which monomeric gases are mixed together in a vacuum, where they are polymerized and deposited onto the surface of the PCB as a protective film.
Figure 1: Required conditions for corrosion of printed circuit board.
Most conformal coatings are either qualified to MIL-I-46058C or meet the requirements of the closely related IPC-CC-830B specifications. In addition they may be recognised by Underwriters Laboratories, either as a permanent coating, in which case the flammability of the coating is assessed to UL94V0, or as a conformal coating, where the electrical properties will be assessed as part of the UL746E standard.
These tests all require flat coupons to be coated with the conformal coating in question and are subjected to a variety of temperature and humidity conditions to assess the material's properties. Whilst perfectly acceptable to assess the potential performance of the material, the actual protective capability of the coating in the end use environment is of greater concern to the user. The rest of this article will be devoted to understanding the issues that relate to end use performance in potentially corrosive environments.
Corrosion is a complicated electro-chemical process with a variety of potential mechanisms and causes, well beyond the scope of this article. However, in the vast majority of cases, there are 3 requirements that must be fulfilled in order for corrosion to proceed.
Intrinsically electro-chemically dissimilar metals (e.g. Gold/Tin and Silver/Nickel), or the creation of an anode and cathode by application of applied voltage or potential difference.
The presence of an ionic species (usually Halides, Hydroxide etc).
The presence of mono-layers of condensed water, to dissolve the ionic species resulting in an electrolyte solution. In order to prevent the possibility of corrosion, it is necessary to remove one of the pre-requisite conditions.
The choice of metals is limited to those used in the solder and solder finish chemistries (which are often dissimilar) and there will always be areas of potential difference in an operating circuit. Cleaning can help remove ionic species, but cannot prevent the re-deposition of ionic species from the operating environment.
Figure 2: Printed circuit and solder corrosion.