Parylene Coating Process
Molecular Layer Deposition
Parylene is deposited through the process of Chemical Vapor Deposition (CVD) which gives it many unique benefits compared to dip and spray coatings. Parylene films are “grown” as vapor deposits molecule by molecule in a room temperature vacuum chamber. Thin film deposition occurs directly on parts, anywhere the vapor reaches. Because the vapor is able to get in all the nooks and crannies, parylene thin film coatings are truly conformal.
The end result is a pinhole free coating without any by-products. Parylene protects the most complex structures at a microscopic level. It can encapsulate complex shapes and evenly cover sharp edges. The thin film is highly uniform, ranging from hundreds of angstroms to a hundred microns.
Table of contents:
- Parylene Coating Process
- Parylene Removal
- Engineering Notes
Before we begin depositing parylene, we take a few steps to ensure success. When parts arrive, we perform an incoming inspection to make sure parts are in good condition and clean. After incoming inspection, we prepare the parts to be coated. Most parts go through a parylene adhesion promotion step. This process encourages the parylene layer to adhere when deposited on the part substrate.
Next, any areas that need to remain free of parylene are masked and sealed so that parylene can not deposit on the masked area. The masking step is a very important step for many applications. VSI Parylene has developed novel techniques that make us the industry leader for precise, small part applications.
After all the above steps are taken, parts are then carefully fixtured into the vacuum deposition chamber.
After the parts are prepared and placed into the chamber, a calculated amount of dimer is loaded into the vaporizer. Dimer, or paracyclophane, is the solid white granular powder that is the source material of parylene . Parylene dimer contributes to the process’s raw materials costs. The system is then sealed and put under vacuum. There is a start-up period we call “chilling and grilling”. The cold trap needs to “chill” to its cold setpoint. The furnace and gauge heaters need to “grill” to their hot operating temperature. The pressure in the system needs to reach the set base pressure. The time needed to reach the vacuum base pressures depends on the amount of outgassing from the chamber load and the size of the chamber.
Once all of the startup conditions are met, the three-stage parylene vapor deposition process begins…
Parylene is applied to parts using the three-stage vacuum deposition process shown above.
Stage 1 – Sublimation: The vaporizer heats to a set point of around 150°C. With the right temperature and pressure conditions in the vaporizer, the dimer will start to turn from solid to a gas. The system is designed so the gas will flow directly into the next stage, the pyrolosis furnace. It is interesting to note that as the dimer is vaporized and turned into gas, it causes the pressure in the system to rise. The change in pressure is used to control the parylene vacuum deposition rate. Once the pressure reaches the high set-point, the vaporizer heater will throttle off and the vaporization will slow. If the deposition rate is too fast, it can result in a lower quality film. VSI uses settings that are focused on quality.
Stage 2 – Pyrolysis: The dimer gas enters the pyrolosis furnace which is set around 690°C. As the dimer gas flows through the furnace, the high temperature causes the gas to transform. The double-molecule structure in the dimer gas splits into single monomer vapor. The monomer vapor leaves the pyrolosis furnace and enters the deposition chamber in an excited state.
Stage 3 – Polymerization: The magic happens when the monomer vapor enters the room-temperature deposition chamber. The monomers are looking to bond to other monomers. When the monomers bond, they form long molecular chains to grow the polymer layer directly on ambient temperature surfaces. Parylene deposits molecule-by-molecule onto everything in the chamber. There is no liquid phase and no byproducts that result from the polymerization. The resulting film is clear, thin and truly conformal.
The vacuum pressure in the deposition chamber is maintained in the range of 30-70 Millitorr depending on the application. With the chamber under vacuum, the gas molecules enter the chamber and bounce around until they lose enough energy to deposit. This means that the coating is not line of sight like many metallization CVD processes. The coating is genuinely conformal and will deposit uniformly on all sides of a part and also enter any holes or crevices.
An additional, critical element, is a cold trap trap between the deposition chamber and the vacuum pump. In order to keep the system under constant vacuum, there is an outlet from the deposition chamber to the vacuum pump. The cold trap is kept below -85°C and the extreme cold forces complete deposition to ensure the vacuum pump remains clean and free of parylene. Understanding parylene’s properties can help you understand the coating process.
The thin film deposition process ends when all of the loaded dimer has been vaporized. The coating process is complete but there are a few important steps remaining.
No cure time means that parts can be removed from the chamber immediately. First the coating thickness is measured. The parylene thickness layer is measured on sample coupons placed throughout the deposition chamber. The sample coupons provide a very repeatable and easy way to measure the thickness. The coupons are small squares made of a known substrate that we can save for our records. It is possible to measure the coating thickness directly on parts in some applications.
We use spectral reflectance to measure parylene thickness. Spectral reflectance is a non-contact, optical measurement method that is ideal for measuring thin films. Spectral reflectance measures the amount of light reflected from a thin film over a range of wavelengths. When the material under the coating is known, the thickness can be obtained by measuring the reflected light.
After the thickness has been measured, the parts are carefully removed from the coating fixture. If the application requires that some areas do not have coating, the sealed masking will be removed. For some applications parylene can be ablated or removed with other methods.
Next, parts are inspected to ensure they meet customer specifications. The inspection requirements vary depending on customer’s workmanship standards. It is common to inspect the quality of parts under a microscope.
Parts are then packaged and labeled for shipping. Packaging systems are important to protect parts and allow for efficient packing and unpacking. Custom labeling can be provided depending on individual customer’s requirements. Quality documents are provided with each shipment and documents can be customized according to customer requests.
The parts are then returned to the customer’s facility ready for use! For an overview of the advantages parylene coating provides, head to our parylene benefits page.
Occasionally we are asked “is deposition a reversible process”? For some conformal coatings, removal is a straightforward process. However, the removal of parylene is difficult. Parylene removal solvents do exist but are challenging to implement in most applications. Because of the difficulty in removing parylene, it’s best to ensure there are no missteps during the coating process so that removal is not needed. If you have parts that need parylene removed, we recommend you speak directly to one of our engineers.
Achieving adhesion between the parylene coating layer and the component substrate is critical to the effectiveness of the coating. VSI Parylene offers a variety of solutions including parylene primers that allow us to achieve the results you are looking for.
A note on cleanliness…
In order to achieve repeatable and reliable adhesion, we work with customers to ensure parts come to us free of any greases, oils, fluxes or other contaminants. The cleaner the surface we start with, the better adhesion we can achieve.
A-174- Wet Solution
The most common adhesion promotion used is a liquid A-174 silane and isopropanol mixture. During this process parts are submerged in a series of liquid baths that rinse and modify the surface energy of the substrate. A-174 works well from a wide variety of surfaces, but it does have some limitations. Some products can’t be submerged in liquid and certain substrates require more suitable chemistries.
A plasma treatment can also be used to activate and prepare a surface before coating. Plasma preparation doesn’t expose the parts to liquid and we can customize recipes for specific components and assemblies. Note that we can also apply plasma after parts have been coated to promote adhesion to the parylene layer, this is useful when the next operation requires over molding with silicone or gluing with epoxy.
Kaphesion for Kapton® and Polyimide
Problems with parylene adhesion to Kapton® and Polyimide? VSI Parylene has developed a proprietary adhesion prep process we call KAPhesion™ specifically for Kapton® and Polyimide. This allows parylene to provide long-term barrier protection on flex circuits. KAPhesionTM promotes superior adhesion of parylene onto polyimide and Kapton® substrates. VSI’s exclusive adhesion prep process will advance your product reliability for flex circuits, medical devices (for biocompatibility), IoT and wearable technology.
For wet adhesion promotion, A-174 silane is widely employed due to the ability of the alkoxysilane functional group to crosslink with hydroxyl groups, which commonly occur and are easily generated on many substrates. However, polyimide substrates lack available hydroxyl groups and cannot cross-link with alkoxysilane functional groups. As a result, with the use of A-174 adhesion promotion solution, poor adhesion of parylene onto polyimide may lead to delamination, as can be shown in Figure 2b.
The R&D engineers at VSI Parylene conducted a series of experiments to create the optimal adhesion promotion solution for parylene onto polyimide substrates. A variation of chemistries, solvents, concentration, time and temperatures were variables in the experimental design. The ASTM D3359-09ε2 adhesion measurement by tape test method was used to quantify parylene adhesion onto polyimide substrates after adhesion promotion and parylene coating. A formula with an average of 90% adhesion (3B classification) was achieved, with samples of up to 100% adhesion (5B) demonstrated, see Figure 1. While A-174 samples result in complete delamination (0% adhesion, 0B), KAPhesionTM provides a verified solution to the polyimide adhesion promotion problem.
KAPhesion™ is available exclusively from VSI Parylene.
The components and assemblies we frequently work with are complex and built with a combination of materials which can introduce adhesion challenges. VSI Parylene’s engineers have successfully solved complex adhesion challenges with custom designed experiments and research into surface interactions.
The coating thickness is controlled by the amount of dimer that is loaded into the system relative to the load surface area. The coating process will run until all of the dimer is vaporized. The amount of dimer needed is carefully controlled and calculated based on the surface area of the load in the deposition chamber. Because of the relationship between dimer amount and surface area, the most consistent coating thicknesses are archived with similar lot quantities. During the process development, we work with customers to define the lot size and coating tolerance requirements.
The required thickness depends on the application and purpose of the coating. Parylene can be accurately deposited from 0.5 microns to well over 50 microns (2 mils) in a single coating run. For example, the IPC CC 830 and MIL-I-46058C standards specify a coating thickness around 15 microns. Typical thickness for a barrier layer ranges from 5 to 20 microns while a dry lubricity layer on silicone would require less than one micron of parylene.
How long does the parylene coating process take?
The deposition rate depends on the parylene type and the control system settings. Parylene C deposits much faster than Parylene N and other parylene variants. During the deposition process, the control system balances the deposition chamber pressure and vaporizer temperature with a feedback loop. Allowing higher chamber pressures will result in more aggressive vaporization and faster deposition. The problem with depositing at higher chamber pressures is the quality of the film can be degraded. VSI Parylene is very careful to always lean on the side of quality when determining the process settings because we understand that quality is what is most important to our customers.
The total coating time is a function of the deposition rate and the coating thickness required. Each coating cycle also requires time at the beginning for the vacuum system to pump down. The pump-down time can vary depending on the outgassing of the chamber load. The entire coating process can last anywhere from 2 hours to 40 hours.
Parylene Coating Equipment – Vacuum Chamber Size
The chamber size is determined based on the size of the part and the volume demand. A full chamber will provide a more efficient coating and we also take into account the workflow before and after coating. From a lean manufacturing standpoint, it helps to keep parts flowing and sometimes it makes sense to perform more frequent runs with a smaller chamber. Using a large chamber that is mostly empty can add cost to a project while sometimes a large chamber is required to fit a larger part.
VSI has a variety of chamber sizes that help us optimize lead time and production capacity. Small chambers can be used for rapid turnaround on engineering runs. Larger chambers up to 24” inches in diameter are used for larger parts and high volume applications.
History of Molecular Deposition – The Gorham process
Parylene was first formed in 1947 by a British chemist named Dr. Michael Szwarc. Dr. Szwarc discovered that it was possible to form a polymer film using para-xylene, a common solvent. Para-xylene was heated under vacuum and the vapors were mixed with iodine vapor. This did create a polymer film (para-xylylene di-iodide) but it was a very low yield process and it required very high pyrolysis temperatures from 700°C to 900°C. This was an important discovery but there was no commercial application for the inefficient and difficult process.
Twenty years later Dr. William Gorham, a scientist with Union Carbide, developed a method of depositing parylene films that is the foundation for modern day parylene deposition. The Gorham process was patented in 1967. Gorham started with di-para-xylene aka paracyclophane aka dimer. He discovered that he was able to vaporize and split the dimer into two monomers at lower temperatures around 550°C in pressures less than 1 Torr. When the monomer remained under vacuum and was exposed to room temperature surfaces, it polymerized very efficiently without any byproducts. Union Carbide continued to commercialize the process in the 1960’s to plant the seed for modern parylene technology. Parylene is becoming more relevant as electronics and medical devices are getting smaller and more complex with increased demands for durability and ruggedness. Learn more by reading the Gorham process patent.
Ask an Expert about your own application and let us help define your service needs! Because each customer’s part is unique, our engineers bring their extensive experience to ensure that every part is coated to your unique specifications. For products requiring innovative solutions, there is no better company than VSI Parylene.