An Overview of the Conformal Coating Process
In the previous discussion (Part I), the history and current chemicals used in conformal coating were discussed and hopefully you now have an understanding of what chemistries are best suited for various applications. In this next section, we will discuss the issues around the physical process of applying these chemicals.
Conformally coating a board is achieved through a multitude of means from physical application to liquid or gas deposition. For the most common methods (spraying, brushing and dipping), a certain degree of manual labor is needed.
The most common is spraying. Think of paint spraying your board, and you get the idea. Most conformal coating houses have a room dedicated to spraying, where the room is vented, but air drafts are minimized and only the operator (who wears protective gear) is exposed to the sprayed material. This is identical to the rooms that auto body service shops use to spray paint your car. PCBAs are placed on some sort of rack and the operator then applies the material with a spray gun. Due to the physical properties of the material being sprayed and operator experience, a fairly controllable thickness can be applied.
Brushing is not only a means of applying the coating to a selective region (a controlled area, not the whole board), but is also used during the touch-up process. The operator literally uses a paintbrush to apply a (usually more viscous) coating of material. This does not require a special room, but due to the aromatic nature of most conformal coating materials, a vented or ducted work area is preferred.
Dipping is the third most common means of coating a PCBA. A fairly thinned out open-topped container of coating is used. PCBAs are mounted onto a dipping machine that controls the speed of insertion, duration of submersion and speed of withdrawal from the container. By controlling these three parameters and the viscosity of the coating material, the thickness of the coating may be controlled. If done improperly, ‘bookends’ may be created since the material flows to the bottom of the PCBA creating a thinner coating on the top and a much thicker coating on the bottom.
Printing is a newer approach to selective coating. Depending on the printer, one of two methods is incorporated: spraying or potting. Spraying is similar to the manual process but is more controlled as a machine is performing the task. This means that only a portion of the PCBA may accurately be coated as the quantity, dispersion and duration of the sprayed material are all finely controlled as well as the machine’s repeatability in controlling nozzle location.
The other form of printing is more like potting. This is akin to a 3D printer where a wall of material is built up from the surface layer by layer. A solid area may be created or just the walls left that are then filled with the same or another material. This is great for making connectors moisture or powder resistant that are not normally as robust.
The most complicated application method is vacuum deposition. The PCBAs are placed inside a chamber where the air is pumped out. The chamber is then filled with a very low-pressure gaseous form of the coating material. This means that the coating material can get virtually everywhere – behind pins, under chip bodies, and into every exposed cavity. Depending on the process, the chamber may be heated to also allow for chemical bonding to occur. The chamber is then refilled with air and the coated boards are removed.
Aside from epoxy and parylene, which are chemically cured, conformal coating materials need to be processed with solvents. Solvents are used to control viscosity and allow for easily facilitating the mixture of multiple types of material into one usable conglomerate.
Generally transparent to the process customer, solvents do affect a few items of interest. Environmental considerations may be one concern, but the solvability of components on the PCBA are likely more pertinent. Make certain that any plastic parts on the PCBA are identified according to chemistry so the coating house may pick solvents that are not going to wreck your nice new PCBAs. Also, the type of solvent and ratio of solvent to coating material determines curing time and sometimes the final material mechanical properties. The representative from the conformal coating house should be able to clarify these issues as they probably already have experience dealing with the trade-offs.
More often than not the liquid or gaseous nature of the coating material means that it can get into places that are not desirable like inside a switch, on the pins of a connector or over pads that need to be exposed. When this needs to be avoided, masking is used. Most commonly, masking is the application of masking tape over the areas to be protected by an operator who cuts and applies the tape according to instructions. Specific tape shapes may also be machine cut or purchased from a vendor.
When this is inadequate, more involved methods are used. A dissimilar material may be applied using brushing or printing. This material will have different physical or chemical properties so that it can be removed with either a knife/scraper or a solvent. Conversely, a mechanical plug or cap may be fabricated that is then placed over or inserted into the component or area that needs to be masked. A variant of the cap is a fixture, where a rigid or semi-rigid custom shaped device is pressed and held onto the PCBA to prevent material intrusion.
A third method is called bagging. This generally only works for situations where a wiring harness or other separate board feature needs to be completely isolated from the work area. A bag that is immune to the conformal coating process used is placed over the feature to be masked and then sealed somehow. Chemicals, tape or elastic bands are all candidates for this approach but depend on the process being used.
After a board is coated, it most likely is not yet ready. The coating material may still be fluid due to its chemical nature or the presence of solvents. Parylene is the one exception since it comes out of the chamber hot, but ready to go. Epoxy is a chemical reaction that takes time to harden, and so the cure time is the epoxy set time. This may be started by exposure to air or the chemical reaction of mixing two components. Other materials cure in the presence of moisture or exposure to UV light or a specific radio frequency.
In materials that use solvents, the solvents will need to evaporate. This can be accelerated by putting the PCBAs in an oven. The curing time in these cases is a combination of the ratio of solvent to material and the ambient temperature of boards being cured. Room temperature takes much longer to cure than 60’C. Additionally, some materials (like a few types of nanocoats) use the oven to also melt the coated material together to form a stronger coating.
Many PCBAs have things attached to them with wires; connectors, pots, even other boards. These wire connection most likely will need to be coated as well and the wire connections reinforced, so they do not break during handling. To accommodate this feature and still have a conformally coated board, wire staking is used. The most cost-effective way to address this is to have the board-wire hybrid fully assembled before being sent to the conformal coating house.
Staking is effectively using epoxy to glue the wires to the board. This serves two purposes: mechanical reinforcement and conformal coating the top wire/PCBA junction. Most assembly houses can stake the wires, but if they don’t, the conformal coating house will be able to do it. In the latter case, care must be taken to make certain the wires aren’t broken or detached during shipping/handling. Once staked, the PCBAs are then processed as normal, but the items attached to the wires will be bagged.
Now that the board is cured, the masking may need to be removed. This can be a very involved process, depending on the masking technique. Using a knife or solvents, the material is removed and then the work is inspected. The work may be fine but it could also require touch-up after it is completed. Touch-up usually involves brushing coating material on the areas in question and then curing the touch-up. Once complete, the actual conformal coating process itself is now done.
In Part III, we will discuss how to verify the quality of the work done and discuss considerations and procedures that should now make sense given that the chemistry and processes are understood.