Plasma Polymerization: A Versatile and Attractive Process for Conformal Coating

Estimated reading time: 13 minutes

Editor’s Note: This article was originally published in the Proceedings of SMTA International, Orlando, Florida, October 14-18, 2012, and appeared in the February 2013 issue of SMT Magazine.

Abstract

Conformal coatings are designed to protect electronic assemblies and products from damage caused by exposure to the environment and to extend the working life and reliability of the device. These coatings are specially formulated lacquers and are applied by brushing, spraying, dipping, or selective coating in a time-consuming and expensive process that often requires deposition of multiple materials and masking of areas where coating is not allowed. The lack of a simple, inexpensive and effective method of conformally coating electronics has prevented many manufacturers from implementing any protection for their products.

A new class of conformal coat has been developed using a low-power plasma chamber and depositing an ultra-thin polymer coating. The types of materials that can be deposited using this method range from acrylics to silicones and fluoropolymers. Plasma deposition is a simple one-step process that can be used to apply a thin, uniform film as a true conformal coating which requires no curing or the use of any solvents. In some cases this can also remove the requirement for masking contacts and connectors, eliminating a labour and time-intensive step from the conformal coating process.

Introduction 

The function of a conformal coating is to protect electronic assemblies from damage caused by exposure to the environment. These coatings are generally applied as a liquid, using manual techniques such as brush coating, spray coating, or dip coating. Spray coating, needle dispense, and dip coating can also be automated using robots to apply the conformal coating in a more controlled manner. All of these techniques can be time and labour intensive, and generally require that sensitive areas of the assembly, such as connectors and RF components, be masked off prior to coating to prevent the liquid conformal coating materials from coming into contact with these areas. After application of the coating, these materials generally need to be cured to harden, typically using either UV exposure or heat, or some combination of the two. This step adds more time to the coating process, and can emit unpleasant and potentially harmful solvents as the coatings dry. A new method of applying a thin protective coating to electronics would be highly desirable if it could deliver the required level of protection from the environment while eliminating the unwanted process steps. Plasma polymerization offers such a solution.

Plasma Polymerization Plasma polymerization is defined as the formation of polymeric materials under the influence of plasma conditions [1]. The deposition of solid coatings under plasma conditions has been well studied since the 1960s, with a very wide range of materials now accessible [2]. The solid materials deposited under plasma conditions are generally referred to as plasma polymers, but they are unique and distinct from traditional polymers in that they lack the repeat structure that typically defines a polymer chain. Additionally the materials tend to be highly cross linked, and not soluble in any chemical solvents. One of the advantages of plasma polymers is the fact that they tend to deposit as thin, pin-hole free films in a relatively simple one-step process. This property is key in the application of plasma polymers as conformal coatings for electronics. Figure 1 shows the chemical structures of traditional straight chain PTFE polymer and a plasma deposited fluoropolymer. The plasma polymer is composed of a mixture of C-F and C-C bonds, and is highly cross linked while the PTFE material consists solely of CF2 repeat units.

Figure 1: Chemical structures of a traditional linear PTFE polymer and a plasma deposited fluoropolymer [3].

Another key property of plasma polymerization is that the coatings tend to deposit in a very uniform, conformal nature on all surfaces in the plasma system which are exposed to the active plasma gas. This means that it is possible to easily coat around corners and edges of components, which can be problematic with traditional liquid coating methods as the liquids can tend to run off of these sharp edges. Plasma polymers also tend to be very adherent and form good bonds with the substrates being coated. This can eliminate problems such as coating delamination during high or low temperature exposure.

Plasma Polymerization Mechanism & Equipment

The mechanism of plasma polymerization is very complex. The high-energy ionization which takes place in the plasma system breaks the precursor gas into ions, free electrons, radicals, and neutral fragments, all of which can be involved in the recombination of these fragments into plasma polymers on the surface of a substrate. The exact chemistry of the resulting coating is dependent on the chemistry of the precursor along with a host of parameters specific to the plasma deposition system such as chamber design, electrode configuration, RF frequency and power, pressure and flow rate of the precursor.

There are a range of plasma systems in the market which are suitable for use as plasma polymerization coaters. Figure 2 shows three such systems manufactured by Nordson MARCH, which vary in size and can be useful for anything from small-volume R&D work to high-volume manufacturing. These systems have been specifically engineered to run optimized coating processes and to convert both gaseous and liquid precursors into plasma polymerized coatings. The small-volume system has an internal volume of 4.5 cubic feet, the medium volume system has an internal volume of 15.5 cubic feet, and the high volume manufacturing system has an internal volume of more than 30 cubic feet.

Figure 2: Examples of typical plasma deposition systems suitable for conformal coating. Left to right: A small-volume/R&D system, a medium-volume system, and a high-volume manufacturing plasma system.

Example Coating Process The coating process used in plasma polymerization is typically a simple, one-step process. Samples are placed into a plasma chamber using appropriate racking such that all important surfaces will be exposed to the active plasma gas. The plasma chamber is then pumped down to a vacuum level on the order of 10s to 100s of mTorr depending the specific process. The precursor material is then introduced to the plasma chamber as a gas. This precursor may be a gas at normal atmospheric conditions, such as some hydrocarbon, fluorocarbon, and amine gases, or it may be a liquid that has been converted to gas. This conversion can be achieved using methods as simple as pulling a vacuum on the headspace of a high vapour pressure liquid, passing a carrier gas through the liquid and then introducing the precursor-saturated carrier gas to the chamber, or as complex as direct liquid injection via an atomization process for liquids which have a very low vapour pressure.

Once the flow of the precursor into the chamber has stabilized, the RF generator is switched on and the precursor vapour is ionized into a gas plasma. When run at the appropriate conditions, the precursor will then deposit a coating on the surface of the samples and the thickness of the coating can be controlled by adjusting the length of time that the process is allowed to run. After the desired coating thickness has been achieved, the RF generator is switched off and the precursor gas purged from the plasma chamber. The chamber is then brought back to atmospheric pressure and the coated samples are removed from the system. No further drying or curing of the samples is required. This process is illustrated in Figure 3.

Table 1: Properties of typical plasma polymers.

Figure 3: Samples are placed in the plasma chamber. The plasma chamber is evacuated. The precursor gas is introduced and the plasma activated. After the desired thickness has been deposited, the chamber is brought to atmosphere and the coated samples can be removed.

Plasma Deposited Conformal Coatings

As described above, the plasma polymerization process can be used to deposit a wide range of materials. These include simple hydrocarbons, more complex hydrocarbons, such as acrylates and vinyl monomers, fluoropolymers and other halo-hydrocarbons, as well as silicones and other silicon containing materials. Table 1 lists the types of materials which have been coated and characterized and highlights their key properties.

The plasma polymer coatings can range in thickness from a few tens of nanometers to several micrometers depending on the application requirements. The thickness of the coating can be measured using physical techniques, such as surface profilometry or atomic force microscopy, or it can be measured optically using ellipsometry or reflectometry. Figure 4 shows the thickness of two coatings as measured by a Dektak surface profilometer. In this case, the coating thicknesses are approximately 300nm and 600nm.

Figure 4: Dektak surface profilometer measurements of two plasma-deposited conformal coatings showing thickness of 300nm (top) and 600nm.

It is possible to image the coatings deposited in the plasma using high resolution optical microscopy and by scanning electron microscopy (SEM). Electronic assemblies have been coated using this plasma polymerization process and then potted and microsectioned to inspect the conformal nature of the coating. Figure 5 shows the presence of a thin conformal coating across the surface of a soldered pad on a PCB assembly. The thin coating can be seen to follow the contours of the metal along the edge of the pad, and then continues along the surface of the PCB and over the soldermask. The lighting and contrast in the image have been modified and enhanced to highlight the presence of the thin film. The coating thickness in this case was approximately 1.5 micrometers.

Figure 5: Optical micrograph of a cross-section of a soldered pad on a PCB which has been coated with 1,500nm conformal coating using plasma polymerization.

Figure 6: SEM micrograph of a cross-section of a soldered pad on a PCB which has been coated with 1,500nm conformal coating using plasma polymerization.

Figure 7: SEM micrograph of a section of a 1,500nm conformal coating using plasma polymerization.

A higher-resolution scanning electron micrograph (Figure 6) again shows the uniformity of the coating coverage and the conformal nature of the coating as it covers the metal pad and the adjacent soldermask. It is important to note how the coating easily covered the complex contours of the interface between the solder pad and the soldermask. The coating replicated the bumps in the surface of both materials and even filled in a small divot in the solder. The final SEM micrograph (Figure 7) details a section of the surface of a sample where the coating has been removed. This image shows a coating that is relatively uniform in thickness and density and has grown uniformly from the surface. The coating was seen to be continuous and defect free for all samples that were inspected.

Corrosion Resistance of Plasma-Deposited Conformal Coatings  Previous work has shown how plasma-deposited fluoropolymers can be used as board-level protective coatings for PCBs [4]. These coatings have been shown to be highly effective at preventing oxidation and corrosion on PCBs exposed to harsh environments. The plasma-deposited fluoropolymer coatings have been shown to be particularly effective at preventing corrosion driven by high-sulphur environments. The fluoropolymers have even been shown to prevent creep corrosion when applied over an immersion silver surface finish which is known to be extremely susceptible to creep corrosion [5]. Figure 8 shows micrographs of immersion silver finished electronic circuits which are uncoated and coated using plasma polymerization after exposure to a high-sulphur, high-humidity environment for seven days. The uncoated sample shows severe creep corrosion while the plasma-coated sample looks pristine.

Figure 8: Optical micrographs of immersion silver-finished electronic circuits which are uncoated (top) and coated using plasma polymerization. The uncoated sample shows severe creep corrosion while the plasma-coated sample looks pristine.

An important characteristic of a many conformal coatings is the ability to protect the circuitry in an electronic assembly from exposure to the combination of moisture and corrosive elements when the circuit is in operation. These conditions can arise when the product is placed in a high-humidity environment, if the sample is exposed to a condensing environment, or if any water accidentally reaches the surface of the circuit. An unprotected electronic assembly can often suffer from catastrophic failure in these conditions. One failure mechanism is the growth of dendrites across the surface of the electronic assembly. These dendrites are conductive metal salts formed by the combination of water and corrosive elements on the surface, and are driven to grow by applied voltages across the circuit.

It is possible to show the impact of this dendritic growth when a circuit is exposed to tap water and a voltage is applied. There are enough salts in the tap water to set up an electrochemical cell which results in the growth of dendrites between the two electrodes of the circuit. In this experiment one set of samples was coated with a 1 micron thick conformal coating using plasma polymerization, while a second set of samples was left uncoated. Bare copper and immersion silver-finished circuits were used in these sample sets. The samples were then connected to a voltage source and immersed in tap water under a microscope so that they could be observed. The first set of images in Figure 9 shows low and high magnification images of an unprotected copper circuit and a copper circuit which has been coated with a plasma deposited conformal coating. The uncoated sample clearly shows the growth of dendrites between the electrodes, while the plasma-coated sample shows no damage to the circuit.

Figure 9: Low-magnification and high-magnification images of bare copper (left) and plasma polymer coated copper after applying voltage while immersed in tap water. The uncoated samples show the presence of dendrites between the electrodes.

Figure 10: Low-magnification and high-magnification images of bare immersion silver (left) and plasma polymer coated immersion silver after applying voltage while immersed in tap water. The uncoated samples show the presence of dendrites between the electrodes.

Conclusion 

Plasma deposition has been shown to be a very attractive coating method for the protection of electronic products. The deposition process itself is very simple and can be accomplished with minimal handling or sample preparation, in some cases even eliminating the requirement for masking contacts and connectors prior to coating. Plasma polymerization offers access to an incredible range of coating materials, as virtually any material which can exist in the gas phase can be polymerized in the plasma. We expect that use of plasma polymerization for the conformal coating of electronics will enable a new generation of improved reliability electronics as the ease of use and low cost allow more and more products to be conformally coated.

Acknowledgements

We would like to acknowledge the contributions of Jim Scott for microscopy and SEM work, Randy Scheuller of DfR for creep corrosion testing, and James Getty and Bob Condrashoff of Nordson MARCH.

References:

1. Yasuda, H. “Plasma Polymerization,” Academic Press Inc.: Orlando, Florida, 1985.2. N. Morosoff, “An Introduction to Plasma Polymerization,” Plasma Deposition, Treatment, and Etching of Polymers.3. H. Biederman, “Plasma Polymer Films,” Imperial College Press, London, 2004.4. von Werne, T. Ph.D., Brooks, A., and Woollard, S., “Latest Developments in Surface Finishing of PCBs Using Plasma Deposition,” SMTA International Proceedings, 2010.5. von Werne, T. Ph.D., Brooks, A., and Woollard, S., “Inhibition of Creep Corrosion using Plasma Deposited Fluoropolymer Coating,” SMTA International Proceedings, 2011.

Andy Brooks has been senior process engineer at Semblant for three years. During this time he has refined the plasma coating process for use in the electronics industry. He graduated from University of Essex with an honors degree in electronic engineering in 1989 and is a registered Chartered Engineer with the Institution of Engineering Technology.

Siobhan Woollard has worked for Semblant for three years as process engineer using her experience in CVD technology to help develop new coatings for the electronics industry. After graduating in chemistry from the University of Exeter, she developed her skills with Element Six Ltd., the world’s leading manufacturer of synthetic diamond, as part of their CVD division.  

Gareth Hennighan, laboratory manager, is responsible for running the UK R&D laboratory and transferring that knowledge to customer installations. Prior to Semblant, he worked for 21 years at Plasmon Datasystems, where he was senior mastering engineer for a department responsible for developing optical media masters for internal use and for external customers. Hennighan holds a HnC in physical chemistry from Hatfield Polytechnic. Tim von Werne, Semblant’s CTO, is responsible for the company’s technical roadmap, research and development strategy, and new product and process development. Prior Semblant, he spent seven years at Plastic Logic in Cambridge, where he was director of research, responsible for development of an innovative application of polymer thin film technology. Von Werne holds a Ph.D. in organic chemistry from the University of California, Davis, a BS in chemistry from Florida International University, and continues his studies in technology and innovation management at the Judge Business School at Cambridge University.