The traditional liquid conformal coatings offer great protection for the majority of circuit boards requiring protection. Between the standard coatings like the acrylic, urethane and silicone materials most problems can be prevented.
However, there are times when they just can’t do the job.
There can be various reasons that these conformal coatings can’t be used.
For example, they may not protect the electronics effectively enough due to an extreme environmental exposure. A good example is where the circuit boards are required to be waterproofed or immersed in water. In this case, the liquid coatings may be inadequate.
Another reason may be cost. The value of the circuit board may be too low versus the cost of the coating production and material. Therefore, an alternative material and process may be required that is lower in price.
So what alternative coatings are there to the liquid conformal coatings?
There are many alternative coatings available to the traditional conformal coating materials that can provide extremely high protection to circuit boards.
These alternative coatings include:
Molecular Vapour Deposition (MVD) coatings
Atomic Layer Deposition (ALD) coatings
Some of these coatings offer a higher level of protection or create alternative surfaces. Others are lower in cost in production.
The defect was occurring just after the spray application of the coating. The system being used was a selective robot applying an acrylic solvent based conformal coating.
So far so good. Nothing complex in the process. But, the de-wetting was occurring all over the circuit.
I asked if the problem had always been there?
The answer was no, it had just started to occur in the last 2-3 weeks. Hence, my being there.
So, we dug a little deeper into the process. Without going into all the details it turns out that they had changed the solder resist and this was the root cause of the problem.
The original solder resist was fine. The conformal coating wet the surface perfectly.
However, the new solder resist had a lower surface energy and now the conformal coating was de-wetting all over the surface.
Well, the company had originally matched the material and process to the circuit board. They were compatible. They had applied a holistic approach to the process.
But, someone decided to change a material part of the circuit board. The failure came in not applying the holistic approach to check for compatibility of the three parts, the material, process and board.
This resulted in lots of scrap circuits, a defective conformal coating process and a company discussing whether to scrap a lot of bare laminates or whether they could salvage them.
How to avoid conformal coating problems?
If you are going to use a conformal coating check that all the parts are compatible. That is the material, process and board.
If you are going to change a part of the process, repeat the tests. Don’t assume the process is instantly drop in regardless of what you are told.
When I consider circuit board failures in the field, and they are due to the conformal coating, then I immediately think of two reasons for it occurring.
The first reason is that the wrong conformal coating material was selected.That is during the material specification the wrong material was chosen and it cannot protect the circuit regardless of how it is applied.
Molecular Vapour Deposition (MVD) is a vacuum deposition process that provides excellent barrier properties and surface energy control.
The MVD process produces a highly conformal thin film coating, typically less than 100nm.
Where is MVD used?
MVD technology is used to produce coatings such as:
Electrical insulation films
Liquid and vapor moisture barriers
Corrosion and oxidation barriers
Lubrication and anti-stiction films
Hydrophobic or hydrophilic surfaces
How does the MVD process actually work?
The process works by allowing small amounts of gas-phase chemicals introduced into the process chamber and reacted at the surface to form thin films.
Unlike traditional CVD and ALD flow processes, the MVD reaction takes place in a chamber under static pressure resulting in extremely low chemical use.
Samples are typically maintained at temperatures ranging from 30°C to 80°C during deposition.
Where is MVD used in technology applications?
Typical applications include:
Non-stick coatings for sophisticated microelectronics and parts found in smartphones, computers, displays, automobile sensors, and hard disks
Non-wetting coatings used on inkjet nozzles
Surface functionalization for biological assays
Anti-fouling and lubrication coatings for parts implanted in the human eye
Dielectric films used in virtual reality displays
Release layers for nano-imprint lithography
What are the advantages of MVD
The MVD process is designed to produce 100% coverage of all exposed surfaces on complex parts.
Conformal coating thickness control
The MVD process manages film thickness and thickness uniformity by dosing exact amounts of precursors and controlling reaction times.
Many other processes like Parylene are dependent upon amount of dimer and will continue to deposit successive polymer layers until it is completely used up causing thickness variation across the chamber.
Cost of process
MVD does appear to be a much faster process compared to Parylene to create like for like protection.
Also, it does not require silane pre-treatment and it only requires small amounts of chemicals. As a result, PCB processing cost could be very low compared to Parylene.
Multiple laminate layers are possible
MVD allows single component layers for basic barrier protection or customized laminate layering for complex requirements.
Most other films including Parylene are single component layers.
Water vapor transmission rate (WVTR) is lower than Parylene
The WVTR < 0.1 g/m2-day for a fast deposition time and < 0.00001 g/m2-day for a longer deposition time.
Parylene WVTR is typically 0.5 g/m2-day
MVD films are optically transparent and do not affect light transmission or reflection due to the relatively low coating thickness.
A component in the MVD coating is a flexible ceramic layer that acts to help preserve electrical isolation over time.
This can give a highly insulating coating finish.
MVD films are pinhole-free at a nanometer level thickness.
Parylene and some other materials are only pinhole-free at micron levels.
Coatings stable up to 450°C environment.
The MVD system is fully automated and requires only a push of a button to run a process recipe.