Non Conformal Coating on Printed Circuit Boards
You’ve probably heard the phrase “it fits like a glove” before, which brings to mind a soft pair of tight fitting gloves that keeps your hands warm while giving you a better grip. While gloves like this are great for gardening, driving, or any number of similar tasks, they wouldn’t, however, be much help to a knight fighting an opponent armed with a sword or a mace. In that case, you would want a very stiff metallic pair of gloves to give you the maximum amount of protection.
To protect a printed circuit board, it is common to apply a conformal coating. Like a glove, this is a thin film of material that is brushed or sprayed on that protects the circuit board from moisture, dust, and other contaminants. While a conformal coating will work great in many circumstances, there are some environments so harsh that the board will need more protection. Boards in these situations need the knight’s metallic gloves of heavy armor, which for a PCB is known as a non conformal coating. Let’s take a closer look at what this is, and how it is used.
What is a Non Conformal Coating and How is it Different than a Conformal Coating?
A conformal coating is named because it “conforms” to the contours of the object being coated. On a printed circuit board this means that a non-conductive material has been applied in a thin coating over all of the areas of the board; the substrate, exposed metal, solder joints, and the components. Conformal coating will offer protection to the board while granting relatively easy access to the board for repairs.
A non conformal coating is much thicker than a regular conformal coat, and its application is referred to as potting or encapsulating. A non conformal coating will completely cover the device or board within it giving it the maximum amount of protection. This means that access to the protected device or board is more difficult than with a conformal coating, but the tradeoff in protection is worth it. Consider a motocross bike where the electronics are subjected to extreme vibration, heat, dirt, and moisture. This is a prime example for the need of a thick non conformal coating for maximum electronics protection.
There are three main types of compounds used for non conformal coatings, and each has its pro’s and con’s:
Epoxies: This compound provides a very high level of resistance to temperatures and chemicals, as well as providing a lot of strength to the components on the PCB. On the other hand though, the compound will shrink during curing which can be a problem for PCB components, and they can become brittle at lower temperatures.
Urethanes: This compound performs better than epoxies where temperatures are lower or are cycling, and urethanes put less stress on PCB components because they are more flexible. The downside is that it is less resistant to higher temperatures and chemicals than epoxies.
Silicones: This compound is much more versatile, and is resistant to moisture, chemicals, and ultraviolet light. It also is rated for the widest temperature range and is very flexible as well. The detracting part of silicones is that they are the most expensive of the compounds, and they are not well suited for situations where strength and rigidity are required.
Now that we’ve seen what non conformal coatings are made up from, let’s take a look at their application.
Non conformal coating can help protect components on your circuit board
The Difference Between Potting and Encapsulation
The first non formal coating option is “potting,” which is also known as embedment. Instead of adding a coating of protection, potting is where the board or device is placed into a shell, case, or pot where it will be treated. Once in this pot, one of the compounds is poured in covering the device or board completely. After the liquid compound hardens, the board or device is completely encased by the compound, and the pot will remain with the hardened compound to become part of the final product. Potting compounds are usually very thick due to the need to completely encase the device or board.
There are many benefits to using potting for protection which include:
The compound will create a hermetic seal completely around the device or board that has been encased.
The pot and compound can withstand extreme environmental conditions.
Potting also provides very good electrical insulation for the device or board within it.
The shells, casings, or pots can be low in cost.
In scenarios where potting isn’t the best process for the board though, encapsulation may be a better choice. Like potting, encapsulation may use a shell or mold around the area that is to be treated. Where it differs from potting however is that the shell is removed after the compound has hardened. This gives a hardened coating of protection to the area on the device or board that has been treated.
Like potting, encapsulation also provides resistance to vibration and shock, and it creates a barrier of protection against contaminants. It also has these benefits as well:
It creates a seal to keep out moisture and corrosive agents.
It helps with flame retardancy and heat dissipation.
It acts as a good electrical insulator.
The reusable shells and molds have a low cost.
Now that you know more about a non conformal coating, the next question will be how will you design for it?
To design complex PCBs that need non conformal coating, you need powerful PCB design tools
How Non Conformal Coatings will Affect the PCB Designer
As a PCB designer, you need to know up front what the protective requirements of your board will be so that you can choose what kind of coating that you will use. You can find a lot more specific information on non conformal coating by taking a look at IPC-HDBK-850. This standard details the guidelines for the design, selection, and application of potting materials and encapsulation processes for printed circuit board assemblies. You will also want to work together with your manufacturer to understand what they can offer you, and what the best process will be for your board.
You will also want to work with a set of PCB design tools that will have the capabilities to not only capture, simulate, and lay out your design, but will also produce accurate manufacturing instructions. Usually, this will mean creating drawings that will explicitly show where any coatings should be applied, or not applied as well as detailed notes to specify the coating materials.
Fortunately, the PCB design system that you need to accomplish all of this is already available from Cadence. OrCAD PCB Designer has the different features and capabilities within it to take a board from schematic conception all the way through PCB layout. It also has within it powerful drawing tools that will allow you to create precision manufacturing drawings so that there isn’t any confusion as to what you need for non conformal coating.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.