The surfaces of Printed Circuit Boards (PCBs) are protected with parylene coating because of the excellent protective features of the coating. As a conformal coating process, it is one of the ways you can keep the circuit board components safer.
In this article, we let you into some of the protective capabilities of the parylene c coating process.
What is PCB Coating?
Let us start by understanding the meaning of coating. When a PCB is said to be “coated,” it means that it has had some thin, protective materials applied to the surface. These materials are responsible for preventing a direct contact between the PCB components and the environment. To this end, important parts like the exposed metal, component leads and solder joints are protected with the help of the coating.
What is Parylene Coating?
It is one of the different types of coating options you can explore to protect the circuit board’s components.
Parylene coating is defined as a conformal coating comprising of a thin film coating technology, which is used to improve the capabilities of devices. You can also use parylene coating to integrate excellent moisture and chemical protection on these devices.
Step-by-Step Parylene Coating Process
The steps to coating a device or a Printed Circuit Board (PCB) with the parylene coating is divided into four processes.
Each of these processes are important and when followed to the latter; helps to bolster the device or PCB component’s protection.
1. Adhesion Process
This is the initial process and it entails the solidification of the bond between the underlying substrate and the parylene.
2. The Masking Process
The creation of a mask is for two purposes. The first is to demarcate or create a space between the protected areas on the PCB and the other areas that were not coated. The second purpose is to prevent the extension of the parylene to the areas or surfaces that needed to be kept free of the coating.
3. The Coating Process
The main coating process starts after the mask has been applied. It is preceded by the placement of the PCB components or parts on a fixture. This fixture is then loaded into the vacuum deposition chamber.
The following are some of the techniques used in the parylene coating process:
- The Chemical Vapor Deposition (CVD) process is used for the coating. The deposition method includes the vaporization of the parts up to 150-degree Celsius, followed by the pyrolyzed process of up to 650-degree Celsius. It is to be followed by the use of up to 22-degree Celsius of vapor-deposition into the parts.
- Room temperature must be used during the coating process.
- A specialized vacuum deposition equipment should be used too.
4. Post-Coating Processes
This refers to the different implementations to be made after the PCB components have been coated.
The post-coating processes include, but are not limited to:
- Parylene Coating Thickness Measurement: the thickness of the parylene films is to be measured. The measurement process requires the direct placement of spectral reflectance on the PCB parts.
- Remove the Mask: when the parylene coating thickness’ measurement is completed, you can then remove the mask. Take care not to fracture the coating’s fixture while at it.
- Inspection: after the parylene coating removal, the PCB parts would then be inspected. The inspection is carried out for a wide range of reasons, such as validating the presence of the quality attributes and the meeting up with the client’s requirements. Some of the quality attributes specify the adhesion-testing, coating thickness and area of coverage.
- Product Labeling: the completion of the microscopic-inspection of the components ushers-in the packaging of the parts. Proper labeling, as per the client’s requirements must be followed.
- Shipping: we can help ship your parylene-coated PCBs in real-time. Depending on the volume or quantity, you should expect to have the parts in a couple of days.
Parylene Deposition Method
There are a couple of things you need to know about how the deposition of parylene conformal coating is done.
1. Temperature Consideration
The deposition process is done at ambient temperature. For this purpose, a specialized vacuum deposition equipment or specialized vacuum system is used.
2. Molecular Level Basis
The deposition process takes place at a molecular level, which signifies the deposition or placement of molecules on each of the parts placed in the vacuum chamber.
The molecular deposition process is also significant for the following reasons:
- It leads to the creation of an even conformal coating that covers the sharp points, grooves, gaps and crevices on the parts.
- The molecular deposition process also aids the excellent control of the thickness.
- It also bolsters the growth of the films, a molecule at a time.
3. The Parylene Deposition Process
The steps involved in placing the molecules via the deposition include:
- Parts’ Fixture: the parts are fixed into a vacuum coating chamber.
- Dimer Heating: “dimer” refers to the solid, granular/powdery material, which is used for the process. It is placed inside the vaporizer, and heated at a higher temperature. The process leads to the losing of the “double-molecule structure,” and the outcome is the reduction to a “single molecule vapor.” The heating process leads to the turning of the dimer from the solid state to a dimeric gas.
- Vapor Polymerization: the gas is then pyrolized to cleave the dimer to its “monomeric form.” It is important to mention that the process typically requires the flow of the dimer into the pyrolysis furnace, where additional heat is added to the dimer. This is what leads to the conversion to monomer vapor.
- Gas Deposition: it is then time to deposit the gas or monomer gas onto the PCB parts’ surface. It is first moved into a room-temperature deposition chamber, where the vapor polymerizes onto the substrate. The deposition is in the form of thin, transparent and uniform polymer film.
Types of Parylene Coating
Up to four (4) different types are available for parylene coating PCB. They include:
1. Parylene C
It is a type of parylene coating comprising of a combination of carbon and hydrogen. This is why it features both the dielectric and barrier properties.
The composition of an atom of chlorine in place of one hydrogen atom per molecule is the reason why it features low permeability. Some of the characteristics are:
- Higher moisture resistance.
- Support for thicker layer application.
- It offers better protection from corrosive gases and chemicals.
- Parylene C has cost and processing advantages, especially due to the less machine time.
2. Parylene D
Although it is much like Parylene C, the Parylene D coating type use two atoms of chlorine in place of two hydrogen atoms. It also features a slightly higher temperature tolerance and up to 100-degree Celsius of assembly protection.
The benefits of choosing it include:
- It holds high when exposed to higher temperatures.
- It features improved electrical properties and physical strength characteristics.
3. Parylene F
This type of parylene coating comes in handy for the applications where the Parylene D don’t hold up. For example, it is more resistant to Ultraviolet (UV) light exposure than the Parylene D.
The properties include but are not limited to:
- The formation of four (4) fluorine atoms on the aromatic carbons.
- It has good crevice penetration.
- It is capable of operating at higher temperatures.
Note: it is better to use the Parylene F coating for the medical device applications, especially those requiring higher temperatures. On the flipside, the raw materials are quite expensive and it has a slower deposition time.
4. Parylene N
As one of the most popular parylene coating types, Parylene N has a linear structure and has a carbon-hydrogen combination on each of the molecules. It is often considered to be the basic structure of the Parylene coating types, due to the following properties:
- Low dielectric constant that doesn’t change due to frequency discrepancies or changes.
- During the deposition process, it becomes more molecular-active.
- Low dissipation factor.
- The increased crevice penetration prepares the way for the coating’s penetration into smaller openings and tubes.
- It is typically used with applications or devices requiring higher frequencies.
- The melting point is up to 788˚F or 420-degree Celsius.
- It has a higher dielectric strength.
Parylene Coating vs. Non-Conformal Coating
There are two (2) major coating processes for Printed Circuit Boards (PCBs). These are the conformal and the non-conformal processes.
For the conformal, which is what the parylene coating process is all about, it entails the application of a non-conductive material on a thin coating over the PCB’s surface. Some of the covered surfaces are the components, substrate, solder joints and exposed metal.
On the other hand, the non-conformal coating process requires a thicker coating procedure that covers the entirety of the board.
Thus, the difference between the two is that the parylene coating process covers only specific surfaces, while the non-conformal coating procedure covers most of the board’s surfaces.
Conclusion: Work with Parylene Coating Companies
The process of applying a parylene coating on the board’s surface requires precision and the technical know-how. This is why working with RayPCB is a step in the right direction. We are one of the few companies you can trust on to apply the thin conductive material all over the required surfaces on your PCB, while making sure that the material conforms to the board area’s contours.
Contact us today with your requirements to get started.
