Surface Modifiers Are they a viable solution in conformal coating Processing?

Article by Dr Lee Hitchens

In recent times, I have come across more and more coating materials that I personally think fall into the category of surface modifiers. Although there are definitions of conformal coatings throughout the industry, and depending on your interpretation, these coatings are or aren’t truly conformal coatings in the strictest sense, they are becoming more and more accepted. Therefore, it’s important to understand them.

What are Surface Modifiers?

When we talk of these coatings we are talking of ultra thin coatings that are applied at a few microns in thickness. Typically, liquid conformal coatings are applied in the range of 25-75um so they are considerably thinner in nature.

What they do is effectively change the surface energy of the substrate they are applied to. This change in energy makes it harder for liquids like water and oils to wet the surface and cause corrosion etc. When we consider the actual value of surface energy the coating creates we are in the range of Polytetrafluoroethylene or PTFE as its better known. The best-known brand name of PTFE-based formulas is Teflon by DuPont Co., which discovered the compound. As we all know Teflon helps avoid “things” sticking or wetting to it. So, these coatings work on the same principle.

How are Surface Modifiers applied?

There are several variations out in the market now but two of the most popular methods are (a) immersing the product in a liquid which dries onto the substrate and (b) Partial vacuum deposition where the coating is deposited onto the surface in a gaseous state. We can take a look at both methods in a little more detail.

Liquid immersion

Typically, for liquid application, these fluorochemical polymer coatings are suspended in an organic solvent such as a Hydrofluoroether (HFE). They are non-flammable, generally low toxicity and not VOCs (Volatile Organic Compounds). Their concentration is in the region of 1-2% solids.

The surface modifier can be spray, dipped or brushed. But, the preferred method is dipping since one of the most important advantages highlighted by the manufacturers of these coatings is that in general you don’t need to mask the components. So, the material is held in a dip tank of some description, the substrate is immersed quickly into the liquid, withdrawn from the tank, and the coating dries extremely quickly, leaving the board coated. It is a very simple process.

Partial Vacuum Deposition

For the alternative method, the technique is slightly more complex. Again, different methods apply here but in general they follow similar lines of process. That is:

The substrates are loaded into a chamber

The pressure is lowered on the system

the surface of the substrate is plasma cleaned to create free radical sites (very clean areas) on the surface

The monomer (coating before it joins together) is released as a gas into the chamber

The monomer forms covalent bonds with the free radical sites as the coating loves to stick to clean sites

The coating polymerises (the small monomers join together to make very long molecules called polymers) and forms the hydrophobic layer

The chamber is brought back to room temperature and the substrate is coated.

There seems to be more steps here but since the process is automated it is a very straightforward process.

 

So, what are the advantages of Surface Modifiers?

 

Well, by reviewing the applications above, it quickly becomes clear that their are certain advantages with these coating processes compared to liquid and parylene coatings. Here are a few highlighted points from the manufacturers literature:

 

Excellent water repellency

 

The material reduces the surface energy of the surface to typically 11-12 dynes / cm and is similar to PTFE. So, most liquids will ball up on the surface and just run off the board.

 

Does not require masking

 

The coating is so thin that generally it does not interfere with the electrical resistance of the components on the surface. Therefore, no masking is required.

 

Fast Drying

 

The boards can be dry in seconds, if not a few minutes, so this is a high-speed process.

 

Complete protection inside and outside of the unit

 

The coatings are applied either as a low surface energy liquid or gas. They penetrate everywhere. So, they should coat all surfaces offering complete coverage.

 

High Temperature Resistance

 

Many of these coatings have high temperature resistance in the range of 175-200C so compete very well with traditional coatings.

 

Good Solvent Resistance

 

Will survive chemical attack from materials like toluene very well.

 

Good for the environment and the operator

 

They are non-flammable, generally low toxicity and not VOCs (Volatile Organic Compounds). So, great for everyone.

 

So, what are the disadvantages of the surface modifier coatings?

 

So, lots of advantages and traditional conformal coatings are now dead and not required. Well, clearly this isn’t true yet since these coatings have been around for a few years and we are still using other materials. So, why aren’t they completely replacing the liquids and parylenes of the world?

 

Well, lets review a few key points that could be holding up this revolution.

 

First, lets consider the claim that that no masking is needed. After all, these materials are very thin so should provide little resistance to connections being made. However, if you do dig deeper into some of the data sheets they do warn that this statement cannot be guaranteed and that testing must be completed. Okay so we test it and it works. No problems and we satisfy ourselves that we won’t see an issue in the future.

 

However, consider the fact that the coating has almost no mar resistance. Mar characterises the ability of the coating to resist light damage. Of course we want it to have none. Otherwise, we need masking. But, that means the coating structure is very fragile. So, if you plan on handling the circuit board then extreme care has to be taken.

 

This leads on to another major obstacle. How do you know the coating is there or damaged?

 

The coatings are optically transparent. Being microns thick they cannot be seen very easily at all. Nor do they have a UV trace like traditional coatings. So, clearly there is going to be a confidence issue on whether the coating is actually present and in a good form to protect the PCB. It should be pointed out this point is being addressed by a few suppliers and UV dye is being added to a few formulations and may help on this issue.

 

So, what other issues could be of concern for surface modifiers?

 

Another point is cost. Whichever way you look at this the process is not low cost unless you are coating lots of boards. First, the material is extremely expensive to buy. A few hundred dollars per litre is the minimum but there are minimum order quantities too so you have to spend a significant amount of money first on the coating itself. However, cost per PCB is pretty competitive so if you coat a lot of boards this problem goes away.

 

Next, consider the application process.

 

For the liquid process you have a very fast drying time. That also equates to a fast evaporation rate. Therefore, a dip tank full of coating will soon start to literally boil off material (losing lots of money) unless you control the losses. So, you have to invest in a refridgerated coating system similar to the old CFC vapour degreasers. Except, the old CFC machines were not efficient and these systems start at $50-80K as they have to be very good at holding the material and preventing expensive losses.

 

For the deposition process its a similar story for costs. You need specialist equipment, which is likely to be owned by the proprietary coating provider, so again costs are not low for entry into the process. Also, this tends to be a batch process so scaling up for high volume is not simple.

 

So, start up can be expensive but if its high volume then this will be amortised out.

 

Are there any other reasons to not consider the coatings?

 

Well, there are a few minor points but they are dependent on the coating supplier. Some surface modifiers or processes may be more sensitive to less than perfectly clean substrates. However, that’s the same for all coatings. Also, a few coatings may need some post bake but not all. Again, depends on the process.

 

However, the single most important barrier to change is the fact that this is still a relatively new process. Inertia to change due to the fact these coatings are radically different to traditional coatings can be a huge barrier. Factor in the point that you can’t easily see the coating and it is fairly fragile and you can imagine the reluctance in many industries to change to this process where failure in reliability can lead to death, injury and huge costs in liability.

 

That said, many industries are suited to the coating and are adapting to it. Consider the mobile phone and tablet industry for one and you can see a process and product marriage that suits each other very well. High volume electronics that are packaged and not handled at all, coupled with the fact that reliability is unlikely to kill someone makes the surface modifiers an interesting prospect and one which is being tested right now, live, on products in the public domain.

 

The question remains though. Will these coatings become as popular and dominant as the traditional materials? We will see over the next 5-10 years as commercial products grow in the electronics sector.

 

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