Semblant, a nanomaterial company based in Melbourn, Cambridgeshire, United Kingdom, has developed a plasma polymerization method of applying a thin, protective coating to electronics that requires only one step, bypassing curing stages and often, they say, eliminating the need for masking.
Conformal coatings—which are used to protect electronic assemblies from exposure to the environment and to extend the life and reliability of the devices—are specially formulated lacquers that are generally applied as a liquid using manual techniques such as brush coating, spray coating or dip coating.
Traditional coating processes can also be automated using robots to apply the conformal coating in a more controlled manner. But it is still time- and labor-intensive, and generally requires deposition of multiple materials, masking of sensitive areas and a curing stage that emits solvents.
To make electronics protection easier and more accessible, a new class of conformal coat has been developed using a low-power plasma chamber.
A key property of plasma polymerization, according to Semblant, is that the coatings tend to deposit in a pinhole-free, uniform, conformal nature on all surfaces in the plasma chamber that are exposed to the plasma gas. This means it is possible to easily coat around corners and edges of components, which can be problematic with traditional liquid coating methods, as the liquids tend to run off of sharp edges. Plasma polymers also tend to be very adherent, forming good bonds with the substrates being coated. This can eliminate problems such as coating delamination during high- or low-temperature exposure.
Here’s how it works: Parts are placed into a plasma chamber, racked so that all surfaces will be exposed to the plasma gas. The plasma chamber is then pumped down to a vacuum and a precursor compound is introduced to the chamber as a gas.
Once the flow of the precursor into the chamber has stabilized, the generator is switched on and the precursor compound vapor is ionized into a gas plasma. The precursor then deposits a coating; its thickness is controlled by adjusting the time that the process is allowed to run.
After the desired thickness is achieved, the generator is switched off, and the precursor gas is purged from the plasma chamber. The chamber is then brought back to atmospheric pressure, and the coated parts are removed with no further drying or curing required.
This process can be used to deposit a wide range of materials. These include simple hydrocarbons, more complex hydrocarbons such as acrylates and vinyl monomers, fluoropolymers and other halo-hydrocarbons, as well as silicones and other silicon-containing materials.
Electronic assemblies have been coated using the Semblant plasma polymerization process and then analyzed to inspect the conformal nature of the coating. It has been found to easily cover the complex contours of the interface between solder pads and soldermasks, and to replicate the bumps and curves in the surface of both materials. It was even able to fill in small divots in the solder.
Plasma-deposited fluoropolymers can be used as board-level protective coatings for PCBs, Semblant says, adding that these coatings have been shown to be highly effective at preventing oxidation and corrosion on PCBs exposed to harsh environments. The company says its plasma-deposited fluoropolymer coatings have been shown to be particularly effective at preventing creep corrosion from high-sulphur or high-moisture environments.
Check out a video at semblant.com/nanomaterial-solutions/conformal-coatings.
Information provided by Andy Brooks; Siobhan Woollard; Gareth Hennighan; and Tim von Werne, Ph.D.; Semblant Ltd.
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