When considering the long term reliability of Printed circuit boards (PCBs), it is important to understand that they can fail due to being exposed to harsh or extreme environments such as airborne contamination, salt spray, moisture, fungus, dust and corrosion.
By definition conformal coating is a protective non conductive dielectric layer that is applied onto the printed circuit board assembly to protect the electronic assembly from this damage. The coating when applied well gives added protection. So, if the conformal coating is poorly applied to the PCB then the circuit board is more likely to fail in the long-term.
Looking specifically at the application of the coating there are several defects that can occur during processing that can fundamentally impair the performance of the conformal coating. This can lead to reduced long-term reliability of the PCB. These defects can include Capillary Flow, De-wetting, De-lamination, Cracking, Orange Peel, Pin holes, Bubbles & Foam.
Logically, it is wise to avoid these effects in the process which can potentially improve the lifetime of the circuit board and limit problems in the field.
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Waterproofing a Printed circuit assembly (PCA) is a very difficult process when using conformal coatings or similar thin-film technologies. What you are trying to achieve is to have either flowing or stationary water on the surface of the board or even immersion in water or a similar liquid without causing any of the electronics to fail in the short or long-term.
Unfortunately, preventing problems from water on a circuit board is one of the most difficult problems in coating application. This is because any conductive area on the circuit board or components not adequately covered by the conformal coating is a potential shorting point for the electricity.
So how can it be done?
The answer is yes it can. It is possible to waterproof a circuit board with conformal coating. However, the cost in terms of processing may be prohibitive.
First of all consider what you are trying to achieve?
You want to cover every conductive surface of the components and the circuit board with conformal coating to an adequate thickness. Any area not covered will conduct and possibly cause problems. To achieve this you have to use a coating method that covers all surfaces, back, front and sides. This means the coating has to go underneath components, up the sides of walls and into all the minute gaps. To do this with most processes is almost impossible and only two coating methods lend themselves to this (unless the circuit board is naturally very flat and has few components). They are dipping and parylene coating.
So lets consider the dip coating process. Dipping will coat everywhere you allow the coating to go. But, you have to deal with the concept of slumping and thin tip coverage, where the wet coating pulls back from sharp edges and leaves them exposed to conduction. To overcome this problem it may take multiple layers of coating successively applied to eventually to cover all of the sharp tips. However, it has been proven to be successful and is used in industry as a method for “waterproofing” a circuit board. So, if your PCB is suitable for dip processing then this gives you the first option.
Then there is the parylene process. First consider that Parylene is a vapour deposition process differing significantly to all of the other liquid conformal coatings available. Parylene coatings are completely conformal when applied to circuit boards, have a uniform thickness and are pinhole free. As a result, component configurations with sharp edges, points, flat surfaces, crevices or exposed internal surfaces are coated uniformly without voids. Therefore, at the right thickness of parylene, it is again possible to waterproof the circuit board. so, if you can balance the cost of the process and the possible processing requirements for masking etc then you have a second successive process for waterproofing a circuit board.
There are several factors that can affect adhesion conformal coating. These include the cleanliness of the PCB, the coating material / PCB and component surface compatibility and how well the coating is cured.
Cleanliness of the Circuit Board
Of the three factors considered here, the cleanliness of the PCB appears to be the most important for local area adhesion problems where the coating either does not wet or is poorly adhered. The cleanliness of the board can be affected by many factors like the laminate and component cleanliness , process contaminants added such as flux and cleaning residues and particulates from the surrounding production area. This can be very critical specifically in a no clean process and care has to be taken here.
The Conformal Coating Material / Laminate Surface Compatibility
This could be considered as the single most important factor if wide spread de-lamination of the coating is seen. If the coating comes off like a sheet of plastic then it is probably due to the surface energy of the laminate being incompatible with the coating applied even though technically the laminate is clean. Measuring the surface energy of the circuit board with dyne pens and consulting with the material supplier can quickly identify this issue.
This can be important when multiple layers are being applied and the degree of cure can affect the adhesion between the layers. The best advice is follow the manufacturers recommendations and this should not be an issue.
So why is adhesion important for the conformal coating?
Well first of all no one wants to have a conformal coated PCB that looks bad because the coating is de-laminating or blistering. But, ultimately the critical reason is that the coating is put on to protect the circuit board and if it isn’t adhered then it could affect the functionality and reliability of the board. Therefore, it is crucial to ensure that the coating has good enough adhesion to provide an adequate level of protection to the circuit whilst in operation. Also, a conformal coating seals in contamination as well as keeping it out. If there is ionic contamination below the surface then this can quickly lead to long term reliability issues with the conformal coating failing due to cleanliness issues which will need to be addressed.
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How does corrosion occur even when I am using a conformal coating to protect my printed circuit board?
In conformal coating, there are several mechanisms that cause failure of printed circuit boards (PCBs). One of these failure mechanisms is corrosion. This is defined as the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.
To learn more click Conformal Coating failure mechanisms Corrosion to find out more.