What factors affect conformal coating adhesion when sticking to a printed 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.
Coating Cure
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 do I know if my conformal coating and the process is compatible with my printed circuit board and its components?
There are many different ways the conformal coating can interact with the circuit board in a bad way. However, they can split into two types of interaction; direct and indirect.
Direct interaction is when it occurs during or just after application of the conformal coating in the drying / curing stage.Again, this can be split down into two areas. Damage to the circuit board and parts is the first. Typical examples include solvent based coatings “melting” a plastic component sufficiently to cause integrity issues or the coating removing the inks marking components thereby rendering the component unidentifiable by inspection. The second is immediate deterioration of the performance of the coating or the masking materials themselves. An example include issues such as inhibition of the coating to cure or dry due to materials on the circuit board with certain coatings like heat cure conformal coatings or the circuit board creating conformal coating defects such as de-wetting 0r delamination. Another example is the coating attacking the masking materials which leads to harmful residues being let behind which then could lead to an indirect interaction.
Indirect interaction is where the coating or masking material has interacted with the circuit board materials and causes long term reliability issues. It could be stated that defect formation can also lead to this but what we are specifically talking about is a chemical reaction with the materials on the PCB that leads in the long term to field failures. A good example of this is lead free soldering materials which appear to be susceptible to interactions with coatings which lead to effects such as electrochemical migration and field failures.
So, how do you test to see if the coating is compatible?
Its fairly obvious to state but a series of tests is required to check the interactions. An examination of the component list and datasheets can quickly identify likely candidates which could be damaged by the coating and its solvents. You can also check to see if the cure schedule will damage the components. Will heating the PCB for 2 hours at 90C cause any failures? Other possible tests include over-exposure experiments of the individual components to the coating and its solvents to see if they are damaged or absorb the solvents and increase in weight (submersion in thinners for a few minutes is likely to be worse than the coating application.
The next stage is to apply the coating to the PCB and see how the coating dries and cures. If defects like lack of adhesion or de-wetting occur then you may need to look at either bringing in cleaning or improving the cleaning process. Also, you can now check to see if the coating fully cures on the board?
Finally, you have the reliability test stage of the process. Techniques like Surface Insulation Resistance testing, thermal cycling, and accelerated aging can shed light on the long-term reliability of the product. If the product corrodes or fails after testing then you know you have an issue and you can examine directly if the coating actually improved the reliability of the circuit board or whether it contributed to its downfall.
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Why do I get cracks in my conformal coating during processing?
Conformal Coating cracking is a failure mechanism in processing where the smooth surface fractures into sections with the cracks in the coating leaving the area below exposed to potential contaminants.
Causes of Cracking in Conformal Coating include:
- Cure temperature too high
- Conformal coating heat cured too quickly without allowing enough time for room temperature (RT) drying
- Film thickness too great causing coefficient of thermal expansion (CTE) mismatch and cracks occurring in the coating
- Operating temperature too high or too low causing the conformal coating to flex too much & crack.
To find out more click conformal coating cracking technical bulletin.
Using Conformal coatings to improve printed circuit board reliability
The use of conformal coating to improve printed circuit board reliability is a known process. Application by brush, dip, spray or selective method increases the long term performance of the printed circuit board.
Click http://www.conformalcoating.co.uk/technicalbulletins.php for further information.
Conformal Coating Printed Circuit Boards (PCBs)
Conformal coating printed circuit boards (PCBs) correctly using SCH Technologies Technical Libray. The reliability of printed circuit boards can be improved easily using conformal coatings applied using conformal coating equipment.
Click http://www.conformalcoating.co.uk/technicalbulletins.php for further information.
Conformal Coating 