Parylene Thickness – When a Little Goes a Long Way

When a client chooses a parylene coating for their product, one of the first questions they ask is “what thickness should I use?” and, as with most things, context matters. More isn’t always better. There’s almost always a trade off, because the thicker you go, the longer it takes to coat. At some point you’re going to have diminishing returns. There’s no universal thickness that works best for all applications, so today we’re going to discuss how parylene coating works and how to determine what thickness is best suited to your specific needs.

How Does a Parylene Coating Grow?

The parylene coating process starts with a powdered chemical, called dimer. The thickness of the coating is controlled by the amount of dimer in the first chamber. The dimer is heated and becomes a vapor, which is pulled into a connected chamber by a vacuum where it’s heated even more, further breaking down the dimer, until it’s pulled into a final chamber where it coats a device on every surface and becomes a polymer. A diagram of this process is shown in Figure 1 below.

Figure 1. Overview of the parylene chemical vapor deposition polymerization (CVDP) process.

What Parylene Coating Thickness Should I Choose?

Here’s where context matters. Every project has its own priorities.  Do you need the best level or protection at any price or the best protection within a specific budget? Are you looking to meet specific industry-standard guidelines?

Keep in mind that by going thin, like 0.1 to 2 μm (microns), you may want to evaluate if there are pinholes or breaches in your coating. Products with relatively flat structures or simple geometry, such as silicon wafers, will likely be pinhole-free. Products that have more complex geometries and large differences in component heights can produce shadowing. The shadowing can affect the coating distribution and leave pinholes in the film. If there are any breaches in the film, there are pathways for corrosive compounds to get in. A thinner coating may give a level of protection that is sufficient for some products that aren’t expected to be exposed to corrosive materials, but still desire a quality barrier defending against minimal contamination. The advantage to going thin is that you’re decreasing the cost and time to coat.

Coating Thickness Standards

When product reliability is a matter of life and death, you may want to look at the MIL-I-46058 and IPC-CC-830 coating standards. Each calls for thickness ranges of 12.5 to 18 and 12.5 to 25 μm, respectively, and are thick enough to provide robust protection against harsh environments. Those thicknesses are pinhole-free on a wide variety of products and have served for decades of reliability in industries requiring the best protection available, such as the aerospace, defense, medical device, telecommunications, and more.

If your product will be venturing into the vastness of outer space, the NASA-STD 8739.1 standard calls for a range of thicknesses from 13 to 51 μm. Especially towards the higher number, there is definitely more than a sufficient coating material to form a coherent film. This gives a level of protection that’s more than adequate for aerospace, defense, and industrial products that are exposed to incredibly harsh environments and temperatures. Depending on which parylene type you’ve selected, especially with a thickness around 50 μm, one coating run may take well over a day.

Differences in Deposition Rates

How long is it going to take to coat at the thickness you chose?

Well, that depends on a number of factors with one of the most important being the type of parylene you selected. Growing the same thickness coatings for different parylene types will take different lengths of time because they each have different deposition rates and unique properties. That deposition rate depends partly upon its sticking coefficient, which is the ratio of the number of monomers that stick to a surface to the total number of monomers that hit the surface, including those that bounce off.

A higher sticking coefficient means that monomers in the vapor phase stay where they land. One of the issues caused by a higher sticking coefficient is an increase in shadowing, which is an issue wherein the parylene coating builds up around taller elements leading to a thinner layer of coating on areas hidden behind the blocked-off or “shadowed” area. However, a higher sticking coefficient usually means a faster deposition rate as well.

A lower sticking coefficient means that monomers effectively bounce and land again to eventually stick. An issue with a lower sticking coefficient is that you’re going to have a longer deposition time due to a slower deposition rate.

Parylene D deposits faster than Parylene C, but D doesn’t usually deposit evenly and can cause shadowing, unless you modify the coating process, such as:

• Moving the entrance and exit locations of the monomer vapors into and out of the deposition chamber to control the flow of the vapor around the products to help make a more even coating
• Rotating the product coating holder at an optimized speed of rotation, again to help make a more even coating
• Heating the deposition chamber above room temperature to help lower D’s sticking coefficient

Parylene C coatings deposit much faster than Parylenes N or F(VT-4) and its deposition chamber is at room temperature, so it doesn’t require heating or cooling. Parylenes N and F deposit at roughly the same rate which can be improved by cooling the deposition chamber. Parylene AF-4 has limited availability, costs significantly more than the rest, deposits even slower, and requires a cooled deposition chamber to get a decent coating deposition rate.

Besides deposition rate, many other properties vary between the parylene types and a few of these general relationships are shown in Table 1 below.

Table 1. Relative properties of common parylene types

 

Coating Fast and Slow

Forcing a faster deposition rate can increase the risk of an uneven coating. For all of the parylenes, you can increase the deposition rate by decreasing the deposition chamber temperature, as well as increasing the monomer pressure.

There are multiple issues to balance when trying to speed up your coating run, such as:

• Increasing the deposition rate
• Decreasing the coating time
• Ensuring coating thickness uniformity throughout the chamber
• Ensuring coating quality throughout the chamber

Coating time can be shortened by simply choosing a thinner coating, but, selecting the coating thickness depends on how you expect your customers to use your products.

It Matters How You Use A Coating

You should consider the coating cost and a coating process against your product’s end use environment and what the cost of failure would be. Table 2 below gives general information about what thicknesses of parylene coatings are appropriate for certain applications.

Table 2. Parylene thickness levels and ideal applications

* Relative to Parylene C and including the same pump down time for evacuating the deposition chamber with the same number and type of products to be coated.
** UT Type describes a general class of ultrathin coatings that range from 0.1 to 12.5 microns thick.
*** The protection level as related to IEC 60529 – Degrees of protection provided by enclosures (IP Code), but is also dependent on product design.

Wrapping It Up

Selecting a thickness for your parylene coating should be based upon a number of factors, including how your customers expect to use your products and the environmental conditions to which your products will be exposed. Your budget and timeframe are also important considerations. In the end, there is no “right” coating thickness that works universally for all applications, but there is a thickness that’s “right” for your specific application. Parylene coating companies provide services that can help guide you to the best solution.