Protecting a circuit board is as important as ensuring that it functions as it ought to. There are a wide range of options to doing this, including the use of soldermask and conformal coating. The coating process is one of the most-used and you will soon discover why it is so in this article.
What is Coating and Why Does It Matter?
Let us start with breaking down the basic term of coating. Wikipedia, the global free encyclopedia defines coating as “a covering that is applied to the surface of an object, usually referred to as the substrate.”
Wikipedia also highlighted some of the reasons or purposes of adding the coating as functional or decorative. In some cases, the application might be to serve the two purposes at the same time.
From the above submissions, you can deduce that the function of the coating is to help protect the surface it is applied. Now, we will look extensively into how the conformal coating for electronics works.
What is Conformal Coating?
Again, we refer to Wikipedia for this definition. It defines conformal coating as a “protective coating of thin polymeric film applied to Printed Circuit Boards (PCBs).”
It is so-called due to the “conformal” design that allows the coating to “conform,” align or adapt to the contours of the circuit board. The conformality of the coating also implies that it aligns to the shape of the Printed Circuit Boards (PCBs) and that of the components.
The other places that the coating conforms are:
- The leads of the electronic components
- The metallized areas of the PCB
- The solder joints and the;
- Exposed traces
Conformal Coating Materials
Several materials can be used to make the conformal coatings for circuit boards. These materials serve as the base and without them, the overall benefits of the coating, including the PCB protection, might not be actualized.
It is also important to note that these materials are also called the base chemistries or coating chemistries. They all have specific properties, operate better in different environments and improve the overall function of the circuit board, based on the application needs.
1. Acrylic PCB Conformal Coating
This is one of the most popular and using it makes all the differences in how easy the process turns out to be.
The acrylic conformal coating material or base chemistry provides all-around protection, fair elasticity and ease-of-rework.
The following are some of the attributes:
- Removal Process: it provides the facility for easy removal, whereby without the use of agitation, a wide range of solvents can be applied can be used for the faster removal.
- Acrylic conformal coating has a fair mix of abrasion and moisture resistance.
- It is also recognized for the higher dielectric property.
- The operating temperature range is wider.
- You can count on the excellent adhesion, which aids in the application to a wide range of substrates.
- The drying process is simplified.
Despite the huge benefits it offers, using the acrylic conformal coating process for your PCB might not make a lot of sense. It has been asserted that despite the good environmental protection and the affordability, it has a limited solvent resistance. This also translates to limiting the extent of the rework capabilities only for other applications that don’t require chemical resistance.
Below are some of the additional downsides of using acrylic conformal coating for your PCB:
- You may not want to consider Acrylic Resin (AR) conformal coating as the “best-in-class” because it is considered an entry-level protection mechanism for circuit boards.
- There is a lack of protection against solvent vapors and solvents.
2. Silicone Conformal Coating
One of the selling points of this circuit board coating chemistry is its wider operating temperature. Wikipedia puts it at between -40˚C and 200˚C. Still on the operating temperatures, Silicone conformal coating offers improved protection and this comes in handy for the high and low temperatures. It also makes the protection ideal for the electronics that tend to be exposed to temperature fluctuations.
The following are some of the advantages:
- Special Formulations: there are special formulations that come in handy for retaining the color and not changing the consistency of LED lights. That is, if the Silicone conformal coating is used to coat these lights.
- Salt Spray Resistance: while there is the argument on whether the absence of abrasion resistance wouldn’t reduce the coating’s performance, it is not entirely so. It doesn’t support abrasion resistance due to the rubbery nature. It replaces that with the salt spray resistance, which aids the circuit board’s resistance against the vibrational stresses.
- Flexible: the flexibility of the coating allows for the provision of dampening and impact protection for the PCB.
- Better wetting is actualized due to the low surface energy.
- Examples of the applications are outdoor signs and a plethora of LED-enabled electronics.
Here are some of the disadvantages of choosing to coat your PCB with the Silicone conformal coating:
- It has a lower resistance to corrosive gases and humidity than the other circuit board coating processes.
- Silicone conformal coating has a tougher removal process. It requires longer soaking time, the application of specialized solvents and subjecting the board to either an ultrasonic bath or agitation from a brush for the coating to be removed.
3. Epoxy Conformal Coating
From a surface meaning, you want to call this a circuit board coat made out of epoxy. In reality, it comprises a two-part compound, which helps to solidify the toughness of the coating.
In terms of the properties and benefits, the following make the list:
- Good dielectric properties
- The operating temperature can go as high as 150˚C.
- Chemical and abrasion resistance.
- Harder durometer
- This coating provides a very good humidity resistance.
- The Glass Transition (Tg) is higher
Here are some potential drawbacks to the use of epoxy conformal coating for protecting a Printed Circuit Board (PCB):
- The general permeability is not assured.
- Epoxy conformal coating is not as flexible as some of the other coatings, such as Silicone conformal coating.
- Removing the coating after the curing process is always difficult.
4. Polyurethane Conformal Coating
This is one of the closest rivals to the acrylic conformal coating, in terms of the superior protection (when used in harsh environments), and the improved chemical resistance.
Commonly used within the aerospace applications, it allows for the very good abrasion resistance and good flexibility to improve these applications’ performance, especially at lower operating temperatures.
Additional attributes of the polyurethane (urethane) conformal coating include:
- Less reversion potentials
- Good dielectric properties
- Solvent resistance
A major downside to the polyurethane conformal coating is the solvent resistance, which is due to the combination of the abrasion and chemical resistances.
That also leads to difficulties in removing the coating. Typically, you need CCRF (specialized products/solvents) to remove it. The removal process also includes agitation with a brush, and a longer soaking time.
5. Parylene Conformal Coating Chemistry
This is also known by other names, such as Fluorinated or Non-Fluorinated Poly-Para-Xylylene coating. It is popular for the excellent uniformity it offers, regardless of the part geometry.
The application process is via vapor phase deposition and some of the attributes include:
- A low dielectric constant
- A thin application of the coating still assures of excellent PCB protection.
- The higher dielectric property and superior resistance make it ideal for use in extreme temperatures.
- It also offers excellent resistance to solvents and moisture.
- Chemical inertness and;
- Low environmental impact process
The downsides to parylene conformal coating include reduced chances of recoating when the vapor phase deposition equipment is not used; and difficulty in reworking. This is due to the dependence on abrasion techniques.
6. Water-Based Conformal Coating
As you may have guessed, this coating process has to do with the use of water. The water needs to be evaporated or removed from the system.
The benefits of the water-based conformal coating base chemistry include:
- It uses a flexible application method, which includes either the spray or dipping processes.
- Improved environmental safety
- The chemical resistance is impressive.
7. Nano Coatings
This refers to the type of PCB conformal coating that dissolves the coating in a “fluorocarbon-based carrier solvent.” Also called thin film conformal coating, its application process is either via spraying or dipping.
The following attributes define the thin film conformal coating:
- It supports dual application methods – dipping and spraying methods.
- Thin film conformal coating chemistries provide a minimal amount of hydrophobicity. This comes in handy for the prevention of losses tenable when the PCB is exposed to humid environments.
A major turnoff to the use of the thin film (nano) conformal coating is that the surface protection for a PCB is not as powerful as what the other base chemistries offer.
PCB Conformal Coating Methods
The conformal coating types closely relate to the methods used for the processes. Thus, if you can be able to pick the right coating method, you might have just solved one crucial part of the puzzle.
We present to you, some of the common methods of ways that the PCB coating types can be applied to the surface of the Printed Circuit Board (PCB).
1. Brush Coating
This is a manual process requiring the use of a brush to apply the coat to the PCB’s surface. The following are needed:
- A brush for applying the coating.
- A container for storing the conformal coat.
All you need do is to dip the brush in the container to get some coat and use the same on the surface or defined area of the circuit board.
Consider using the brush conformal coating method for the following reasons:
- If you are reworking the PCB.
- If you are working on a limited number of Printed Circuit Boards (PCBs).
2. Dipping Method
This is a high-speed and repeatable process and thus, it requires the highest levels of precision. Factors like the withdrawal speed, speed of immersion and the coating’s dwell time influence the outcome of the dipping process.
The following are worth considering before using the dipping method for PCB coating:
- Masking: applying the soldermask before the dipping helps to prevent leakage of the coat into different parts of the PCB, including seeping beneath the devices.
- For the best results, consider using the dipping method when coating both side of the PCB.
3. Spraying Method
As the name suggests, this has to do with using sprays to protect or for coating PCBs. There are different variations as to how this can be actualized.
The following are some of the common conformal coating spray methods to use:
4. Selective Coating
This does not apply the conformal coating at every part of the PCB, but instead focuses on specific parts. Most times, and for the best results, a conformal coating machine is used for this process.
The benefits of using the selective coating process, especially if the machine is used, are:
- It is faster.
- Selective coating with the machine, helps to process coating of multiple PCBs in the shortest time possible.
- The accuracy of the conformal coating machine is assured, as it aids the application of the coating where it is needed.
5. Aerosols
This is a solvent-based conformal coating that requires the use of a spray gun. Typically, the spray gun is used to apply the coating where it is needed on the PCB.
The following are some of the attributes of the aerosol spraying method:
- It is a cost-effective conformal coating application method.
- Depending on the number of PCBs being handled, you can make the spray application either on the bench top (for rework and small-volume productions) or spray booths for medium-scale productions.
6. Batch Spraying Method
Consider this method if you are working on high-volume PCB productions and you need to facilitate the process in good time. By making a batch-by-batch spraying, it helps you to get the best quality possible.
You also need a combination of the following for the setup and best results:
- Air compressor
- Spray gun and;
- Spray booth
Step-by-Step Process to Conformally Coating a PCB
The production processes for conformal coating application on PCBs are to be sequentially-followed. Here is how to start and finish the process:
1. Clean the PCB
The surface of the circuit board must be cleaned for the entire process to go as it ought to go. First of all, validate that the surface is clean or unclean using any of the following testing methods:
- Surface insulation resistance
- Ionic contamination testing and;
- Ion chromatography
When you have confirmed the need to have the board cleaned, any of these cleaning procedures will help you make an excellent job out of it:
- Plasma cleaning
- Aqueous cleaning and;
- Solvent cleaning
2. Prime the Board
This involves a preparatory method of getting the stage set for the main coating to be applied. In priming, the preparatory coating will be applied to the PCB. After this comes the conformal coating.
You can use a wide range of processes to actualize this. It all depends on the type of base chemistry or material you are working with. While the gas phase deposition serves as priming for parylene coatings, a combination of wetting and adhesion works best for the polyurethane and acrylic base chemistries. You can use primers for the silicone conformal coating.
No matter the type of resin, material or base chemistry used, the essences of priming a PCB include:
- Boosting the coating retention via the formation of a binding layer.
- It also ensures better adhesion.
3. Mask the Board
Whether soldermask or any other masking process; the goal of masking the PCB is to prevent some of the components from getting exposed to the conformal coating, when it is applied.
Examples of the masking materials are masking boots, masking shapes, liquid latex, masking dots and masking tapes.
4. Apply the Coating
With the specific areas of the PCB now coated/masked, you can now proceed to apply the conformal coating. You can use any of the methods we discussed earlier.
Remove the Mask and Finish the Surface
You are now free to remove any of the masking materials used to protect the surface of the circuit board.
You can then use any of the PCB surface finishing methods to preventing coating defects, ensure the quality of the conformal coating and ensure the compliance with the standards.
Conformal Coating Drying and Curing
There is a difference between drying and curing the conformal coating applied to the surface of a Printed Circuit Board (PCB). Drying has to do with the confirmation of the circuit board’s excellent coating. Curing has to do with the confirmation of the PCB’s attainment of the desired mechanical and electrical properties. These properties help bolster the board’s protection while in use.
With that being said, let us look at some of the options for curing your conformally-coated PCB:
1. Air Dying
Also called film forming, it is a curing process used to cure the standard, solvent-based acrylic conformal coating.
Note that it is the ideal process for applications that don’t require speed. If the latter is to be the case, consider using the heat curing method.
You can also use air drying to cure the water-based conformal coating on PCBs. However, the curing process, especially during heating, should be well-handled, to prevent issues arising to the longer drying duration.
2. UV Curing
This involves the use of ultraviolet (UV) light to cure the conformal coating. The speedy process makes it ideal for the high-volume PCB designs. Thus, it is ideal for coating circuit boards used in high-end industries, such as consumer electronics and automotive industries.
3. Heat Curing
You can consider using the heat cure process if the other methods, such as UV curing and moisture curing don’t make the process faster.
The benefits of heat-curing a PCB’s conformal coating include:
- It can cause the system to polymerize, i.e., accelerate the curing process.
- Using the heat cure process also make curing of both the single and multicomponent systems a breeze.
4. Evaporative Curing
The evaporative method curing conformal coating entails sipping out or evaporating the liquid in the coating. For better results, you can combine with other curing methods, especially the heat cure method. That leads to having only the coating resin behind.
Choosing the Right Conformal Coating for a PCB
Now, you have seen that there are several options to choose from. With up to four (4) different PB coating types, which should you go for?
We will help you make a better decision, by using two parameters. These parameters or considerations are the type of material to be used and the feasibility, in terms of how it is to be used on the PCB.
Conformal Coating Chemistry Selection Criteria
Here are some of the common criteria that help you decide if the conformal coating is the best for your PCB project:
- Check the mechanical, electrical and chemical compatibility of the base chemistry with the PCB components to be coated.
- Find out the extent of the protection. Does the conformal coating protect against chemicals, moisture or both?
- Check to be sure that the chemical, physical and electrical requirements of the conformal coating material match with the target applications.
- What is the maximum operating temperature of the electrical device?
Feasibility Concerns
These have to do with the considerations for the commercial, process and production pathway for the conformally-coated PCB. The following are some of such concerns:
- The quality of the base chemistry/material is important.
- Rework Process: while some conformal coating materials can be easily overhauled and fixed, others may not present a similar less-difficult process. So, find out how easy the material’s rework process might be.
- Process: this refers to the pattern that can be used to achieve the best results. Examples are repeatability and uniformity.
- How much does the base material go for? It is best to work within your budget. You can always consult us at Hillman Curtis for expert tips on choosing the best conformal coating material that aligns with the budget you set aside.
- Curing Process & Duration: what is the best method or way to cure the material and how long does it take?
- Throughput Time: this refers to the speed of applying and drying the base chemistry for conformal coating on a PCB.
Wrapping Up
Using conformal coating on your PCB helps to protect the circuit board, make it adaptable to different applications and environments, as well as improve the functions of the integral components.
