Is molecular vapour deposition a possible replacement for Parylene and conformal coatings?

There are many new coatings that could potentially be alternatives to the traditional liquid and Parylene materials that are currently used around the world.

Molecular Vapour Deposition (MVD) is one of these new coatings.

Invented in 2004 this novel coating has been used in many ways including liquid and vapour moisture barrier films and hydrophobic or hydrophilic surfaces.

Also, it has found mainstream uses in many diverse areas such as computers, smart phones, automotive sensors and hard disk drives.

Molecular vapour deposition coatings have been applied to smartphones
Molecular vapour deposition coatings have been applied to technologies like smartphones

Now, it is being considered as an alternative coating to Parylene.

Nexus decided to investigate this coating and see just how it works, what are its advantages and what are the issues in using it as an alternative conformal coating.

What is Molecular Vapour Deposition?

MVD is a vacuum deposition process that was invented and patented by Applied Microstructures (AMST) in 2004.

The MVD process produces a highly conformal thin film coating, typically less than 100nm.

The coating provides novel barrier properties and surface energy control.

What type of coatings does the molecular vapour deposition process produce?

MVD technology is used to produce thin film coatings such as:

  • Electrical insulation films
  • Liquid and vapor moisture barriers
  • Corrosion and oxidation barriers
  • Lubrication and anti-stiction films
  • Hydrophobic or hydrophilic surfaces
  • Biocompatible surfaces
  • Reactive coatings

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

How does MVD actually work?

In the MVD process, small amounts of gas-phase chemicals are introduced into the process chamber and react at the surfaces to form films.

Unlike traditional CVD and ALD flow systems, 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.

What family is MVD from?

MVD belongs to both the families of chemical vapor deposition (CVD) and atomic layer deposition (ALD) methods.

 Advantages and Disadvantages of MVD

 Advantages

  • Quality of finish. The advantage of the MVD process over a comparable liquid phase process is the control and minimization of particulates on the treated surface.
  • 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.
  • Complete Coverage. 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.
  • Multiple laminate layers are possible. MVD allows single component layers for basic barrier protection or customized laminate layering for complex requirements (see figure 1). 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
  • Light transmission. MVD films are optically transparent and do not affect light transmission or reflection due to the relatively low coating thickness.
  • Electrical insulation. 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.
  • Pinhole-free. MVD films are pinhole-free at a nanometer level thickness. Parylene and some other materials are only pinhole-free at micron levels.
  • Coating stability. Coatings stable up to 450°C environment.
  • Ease-of-Use. The MVD system is fully automated and requires only a push of a button to run a process recipe.

Disadvantages

  • Substrate Cleanliness. This is important to the quality of finish similar to many other vapour deposition processes. The MVD substrate cleanliness requirement is similar to that of Parylene.
  • MVD is a batch process. This can have a limiting factor on throughput although scaling the system is easier than a Parylene system due to the conformal nature of the coating throughout the chamber.
  • Process time. Standard coating deposition time is on the order of 2 to 3 hours per batch process. Deposition time depends on number of coating layers required. Process time is limited by chamber size.
  • Masking Requirements. The need for masking is dependent on the barrier requirements. For ultra thin coatings masking may not be required. in other cases it may be essential.
  • Cost of Process. Ultimately, MVD may not be cost effective for certain levels of electronics.

So is MVD a possible alternative to Parylene?

Lets look at the advantages.

The process times look lower and the film performance looks improved in many ways versus Parylene. Also, the chemical costs look lower in comparison. Finally, there may be no need to mask since the coating may be so thin.

In terms of disadvantages to Parylene there don’t appear to be too many problems since the machine is similar in cost.

The limitations may come when comparing with other liquid chemistries and the usual issues occur similar to Parylene versus liquid conformal coatings. However, there are has been no direct testing of the performance of MVD coatings versus Parylene or the other conformal coatings.

So, in reality yes it could be possible that it is an alternative to Parylene and other coatings, especially in specialist areas.

But, we need to know more.

Where now for MVD?

Working with Nexus, the MVD coating is going to be directly compared with a range of conformal coatings including traditional liquid materials, nano coatings and Parylene in an initial selection of key IPC CC 830 tests.

The results should benchmark MVD and how it compares to the other materials both technically and commercially.

From these results we can then potentially assess the impact of MVD on the conformal coating world.

Co-Authors

Dr Lee Hitchens, Nexus

Mike Khosla, VP Engineering and Operations, Applied Microstructures, Inc.