Recently, fluoropolymer coatings are being used more often to protect printed circuit board assemblies. This is because they have very specialized properties that are very different to traditional conformal coatings and these properties are being utilized highly effectively.
To understand these properties you have to understand what a fluoropolymer coating is made of.
Typically, the coating is comprised of fluorocarbons and characterized by carbon-fluorine bonds.
The coating itself is not susceptible to Van der Waals forces (interfacial electrostatic bonds). Therefore, the surface energy of the fluoropolymer coating is extremely low.
This means that there is no adsorption of another coating or liquid on the surface of the fluoropolymer and the coating shows the familiar, hydrophobic, non-wetting characteristics with water and oil.
This non-wetting is one of many of the key properties making them so popular.
The surface energy of the fluoropolymer coating is extremely low. There is no adsorption of water on the surface of the fluoropolymer and the coating shows the familiar, hydrophobic, non-wetting characteristics.
So, what other properties do fluoropolymer coatings have that help protect circuit boards?
Generally, fluoropolymer materials have very specialized properties.
For electronics the five key properties that are driving the interest in fluoropolymer technology are:
Being highly hydrophobic (water repellent)
Being extremely thin whilst still providing protection
Having a high moisture barrier and corrosion resistance
Having a high chemical resistance
Having high dielectric properties
To really understand the benefits you have to look at the key difference compared to a conventional conformal coating.
The key reason for using a fluoropolymer coating in electronics: No masking required
First, consider a conformal coating applied at normal thicknesses of 25um or more.
The coating is a high insulation material. Therefore, it must not be applied to a connector or part that can be damaged or needs electrical conductivity.
Further, the conformal coating naturally has good mar resistance and it cannot be easily abraded. It is tough to the touch and not easily removed.
So, if it was applied to the wrong component it would ruin the connection.
On the reverse side, a fluoropolymer coating can be applied at 1-2um in thickness and can still show the same performance characteristics.
Further, it is also extremely soft and shows almost no mar resistance when rubbed or abraded.
This allows the fluoropolymer coating to be applied to all connectors without fear of damage as the coating is easily removed or scratched away and the electrical circuit is easily made when the mating parts are connected together.
This key parameter of not requiring masking combined with the hydrophobic nature of the material and its other key characteristics makes the fluoropolymer coatings highly desirable in protecting electronic circuit boards.
Parylene is a conformal coating film that is applied using a specialised vapour deposition application process.
This means it is very different to all of the other liquid conformal coatings available on the market.
Three reasons why Parylene is better than traditional liquid conformal coatings
The fact that the Parylene film is deposited onto circuit boards in a vacuum leads to many unique advantages.
Here are three key reasons to use Parylene:
The Parylene coating is completely conformal to the surface of the Printed Circuit Board (PCB) or product. The coating has a uniform thickness and is pinhole free. Therefore, components with sharp edges, points, flat surfaces, crevices or exposed internal surfaces are coated uniformly without voids.
Parylene coating provides an excellent barrier that exhibits a very low permeability to moisture and gases compared to traditional liquid conformal coatings. This means that circuit boards coated in Parylene generally are more “waterproof” than the same circuits coated in a liquid conformal coating.
Parylene has excellent electrical properties compared to normal conformal coatings. These include low dielectric constant and loss with good high-frequency properties, good dielectric strength, and high bulk and surface resistance.
So, you are considering outsourcing your conformal coating or Parylene process to a subcontract supplier.
What’s the next step?
Assuming you have decided this is the right choice then the next task is to choose the right service provider.
This can be as difficult as selecting the right PCB manufacturer or laminate provider. There are good contractors and there are others. There are small providers and there are large-scale turnkey solutions.
The obvious answer is “the right one for me” but how do you ensure that you make the right decision?
Here are a few of the questions you should ask before signing up to the wrong coating solution provider and stop you wishing you had kept the coating process in house.
The three key points to consider when choosing a subcontractor
When considering the supplier look at the three main areas.
The order you look at is up to you but ultimately you need to be happy in all three to keep sending the work out.
These three key areas are:
If you get these three key areas correct then you have succeeded. Everyone will be happy and there are no more problems with conformal coating.
However, within each of these areas, lie a lot of questions to be asked and if one of them isn’t working properly then who knows what the consequences are around the corner.
So, let’s consider each of these areas in turn in the order they are listed.
Everyone wants good quality. But what does that mean?
Whose quality are we judging against and how do we reach agreement?
One of the most important factors in subcontract conformal coating services is to agree what the PCB coating finish should look like.
It can be if you define exactly what you want as a customer. The problem comes when you don’t know!
The reality is most customers have a preconceived idea of what a conformal coated printed circuit board should look like and the key for the customer is to communicate this to the provider.
Let me give you an example.
Consider a simple connector on a circuit board like the one below.
Then decide on what statement you agree with below:
Only the pins must not have coating on but the rest of the connector body does not matter.
The whole of the connector must have no coating on it all but there can be a gap of 1-2mm around it free of coating.
The whole of the connector must have no coating on it all and there can be no area around the connector free of coating.
All three options provide a connector free of coating. All three options work. All three options could be considered fine by various different customers.
However, the order they are stated is also lowest difficulty (aka cost) to highest difficulty to actually complete the work in a coating production line.
So, defining how you want to coat the board intimately impacts on the price of the project.
A key issue highlighted
Unfortunately, this highlights a key problem in conformal coating processing.
There are no standards that state what is the best solution and only guidelines. Ultimately, it comes down to a decision made by the customer as to how the PCB should be coated.
This means it is a crucial factor for both the customer and the supplier to define the level of quality. Get this right and most of your problems are sorted.
If the coating house is good they will help you define this from the beginning. They will not assume any level of quality but ask you what you need.
If they don’t help you then hope that they can guess what they need to provide you with.
So what’s the next stage in quality?
So, you have agreed what quality of finish you want. Assuming that you know which conformal coating material you want then the rest should be easy.
Well, that statement is right as long as:
The supplier knows how to use your coating material correctly
Has the right equipment to apply the conformal coating
Has selected the right process for your PCB
Knows what to do when things go wrong
So, what you really need to do is find a subcontract conformal coating supplier that knows a lot about conformal coatings or Parylene.
So, you are considering outsourcing the coating work to an outside contractor but want the PCB coated when you need it.
Unfortunately, conformal coating is normally one of the last processes in a long line of operations so any delays in the manufacture of the PCB is normally being compounded by sending it out to a coating house.
Therefore, you need a fast turnaround option and your coating service should be flexible on this, allowing you choices on getting the PCBs coated.
However, you may want to consider the speed you require the PCB to be coated since it can lead to:
Potential problems with the process
Mistakes due to staff and machine availability
Availability of material
Local or global supply
Low cost offshore facilities
These factors can influence the price significantly.
So, we have examined quality and turnaround time. The critical factor that ties these two areas together is price. Let’s take a look at this area.
“I want this PCB coated for 45 cents”, says the customer.
“Okay can we see the board”, says the coating service provider.
The customer produces a 12”x6” PCB with 25 surface mount connectors with via’s everywhere and asks for coating both sides and wants all of the connectors not to be coated.
Okay, we have a mismatch in perception and this sounds ridiculous. But, it happens more regularly than it should.
Some customers have no idea of what it costs to coat a PCB. After all it’s just coating.
However, it’s up to the coating house to educate them so that they can get what they need and reach the happy point of all three areas satisfied.
So, what price should it be?
Well factors to be considered by the subcontractor are:
Material specified by the customer
Process to be used on the PCB
Volume of PCBs to be supplied
Amount of masking / keep out areas on the PCB
Amount of coating to be used per PCB
Is there cleaning involved before coating?
Yes cleaning is required, extra cost added in
No cleaning is not required, may be extra cost for finishing process if a lot of contamination
Speed of turnaround required (impact on resources, drying, curing)
Quality required (how much time finishing, inspecting, how close to look?)
So, the last two factors, turnaround and quality, tie directly back to the price and in reality have a huge impact.
Therefore, we find unsurprisingly all free factors should be considered as a whole and not separately.
Subcontracting out your conformal coating process isn’t difficult if you look carefully at the three areas of quality, turnaround time and price.
If you can achieve all three with your subcontract coating house then you will be happy.
If you ignore one of the factors then it may be a less pleasant position.
Silicone (SR) conformal coatings are inorganic materials. They are polymers in which atoms of silicon and oxygen alternate in a chain. They differ to the organic polymers like the acrylics and urethanes.
The silicone coatings normally have a very wide temperature range of operation compared to the other conformal coatings. Typical range can be -55°C to +200°C (-67°F to +392°F). Like the organic coatings (acrylics and urethanes) they have good moisture protection. They also have good chemical resistance to polar solvents.
Curing of silicones occurs through several different mechanisms, depending on the conformal coating, including RTV (Room Temperature Vulcanisation), Heat, UV, Moisture / Condensation and Catalysed Cure.
In production they can be difficult materials to use due to the cure mechanism being difficult to control. Good housekeeping can minimise these effects. SR coatings can also require different coating equipment or options compared to the organic coatings. This should be considered as part of the holistic approach to conformal coating selection.
The different conformal coating material properties
Conformal coatings can be considered in many different ways.
This includes the different families of materials, their individual properties and the chemistries of these coatings.
The different ways we can examine conformal coatings include:
There is a vast range of options and considerations available.
Polyurethane (UR) conformal coatings generally provide good humidity & moisture protection although not always as good as the acrylics. However, it normally is enough to protect the circuit board.
Normally, urethanes are selected for their excellent chemical resistance. This is because the coatings cure rather than dry. That is they cross-link by one of many different methods including heat, UV, moisture and catalysed cure.
Typically they have higher dielectric properties compared to the acrylic conformal coatings.
Their chemical resistance, however, can be a limitation since rework and repair generally is more difficult than the acrylic coatings.
UR coatings are normally available as either single or two-component formulations. Pot life is dependent on the cure mechanism but can be more limited than the acrylic coatings.