Printed circuit boards (PCBs) are a vital part of any electronics product. They are also one of the most vulnerable—highly susceptible to the environment where they are being used. Any number of factors can affect the PCB including heat or abrupt fluctuations in temperature, humidity, water, dust, salt, solvents, and levels of UV, leading to corrosion or a breakdown.
No matter where your PCB is eventually going to live, it’s beneficial to protect it with a conformal coating. The coating conforms to the contours of the board which allows the board to function while extending its life and not adding to the size. As a protective, insulating layer, the conformal coating will need to withstand the required operating conditions of where the PCB is located. For example, if your PCB will be operating in a location subject to condensation, it’s important to use a conformal coating that is known to resist water.
The conformal coating process is one of the last processes on the PCB assembly line. This is due to the fact that once a PCB is coated it is very difficult to repair or modify.
Conformal coatings are typically classified by polymer type. There are five major categories:
- Acrylic – typically solvent-based formulations, which are easy to apply and repair, while providing good protection against moisture and abrasion. However, acrylic provides poor solvent resistivity, poor temperature resistance and fair mechanical strength.
- Polyurethane – provides excellent humidity resistance, better mechanical strength, better abrasion resistance and better resistance to solvents, but can be difficult to remove.
- Epoxy – offers excellent mechanical strength and abrasion resistance, and good humidity resistance, but they have poor temperature resistance, poor reparability and poor flexibility.
- Silicone – affords excellent temperature resistance and good resistance to moisture. Silicones are available in moisture cure (RTV), heat cure (with or without moisture secondary cure) or UV cure (moisture secondary cure). Silicone provides poor abrasion resistivity and poor mechanical strength.
- Paralyene – often the protective coating of choice for challenging electronics, aerospace and medical applications, this type resists the effects of organic solvents, inorganic reagents and acids.
But you can also find hybrids and special types: acrylated urethanes, acrylic-modified silicones, or polyamides.
There are components of a PCB that cannot or should not be coated. When designing the board and picking a coating application method, the areas which will not be coated should be taken into consideration in the layout.
Conformal Coating Application Variations
Just as important as which type of conformal coating is used, is how it is applied. As with most processes in the electronics industry, there are several ways to apply conformal coatings to the product. Some of the methods are typically performed manually while others are automated.
- Dip coating: This traditional process can be done manually or automatically and can easily adapt to dipping only the components that should be protected. This is typically a low-cost and simple method but it can be hard to manage the consistency, viscosity and there is a high potential for contamination.
- Brush application: Only done manually, an operator dips a brush into the coating material and paints it on the PCB where it is needed. It’s inexpensive, simple, and effective, but not the best solution if mass production will be needed.
- Atomized air spray: A method of painting, this method is also manual but with less control. Spraying involves overspray so you must mask any components that shouldn’t be exposed to coating. Again, a low-cost method but difficult to manage issues such as consistency and cleanliness.
- Needle dispense: This method can be manual by hand or automated by a robot that places beads of coating in the appropriate areas. There can be inconsistencies, but masking is not needed and is typically precise and inexpensive.
- Automated selective coating: Conformal coating can be selectively applied by a dispense mounted to a robot that is programmed to move and distribute the material to designated locations on the PCB. While a more expensive option, the disadvantages of this process are easier to mitigate.
Many times a single board requires more than one dispense method for proper coating.
Conformal Coating Cure Options
Once coated, the last important variable to consider is how to cure it. There are several options for curing the conformal coating and the variations are very important when considering the long-term reliability of the product and circuit board.
- Moisture curing: Moisture curing requires exposure to humidity at a specified level for a given duration of time so the coat holds. The source of humidity is critical to prevent contamination.
- Heat cure: Heat for curing is usually a combination of infrared and convection. Heat is used in two different ways depending on the chemistry of the material—either evaporating the solvent present in the coating material or by triggering a chemical reaction.
- Ultraviolet cure: UV cures reactively through a short period of UV light exposure, eliminating the need for long ovens but raises the temperature quickly and sometimes so extremely that components break off.
- Reactive curing: The reactive chemical method of curing is usually only seen in systems with plural components where a fast cure is required.
While the cure mechanism isn’t a primary criterion when selecting a coating, it does have a direct impact on the type of application method that will be feasible so it’s important to understand the different methods.
Other Considerations for Conformal Coating
Conformal coating means the equipment is only as flexible as the material allows it to be, so consider what the end-product requirements are when choosing a PCB coating product. Also, material viscosity changes with temperature so if your PCB will be housed in an environment with drastic temperature fluctuations, you will want to find a conformal coat that will maintain the viscosity throughout the day to avoid a film build up.
Because conformal coating requires a great deal of process knowledge, including an understanding of material characteristics, curing and in-line integration issues, it’s important to work with a manufacturer who provides a comprehensive service and understands the advantages and disadvantages of each variety.
ACDi has the resources and experience with conformal coating services to provide a single source of support throughout your product lifecycle. We are familiar with each of the types, methods and uses, and can help you determine the best one for your application.