The way parylene is deposited give 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. A parylene film grows directly on parts, anywhere the vapor reaches. Because the vapor is able to get in all the nooks and crannies, parylene films 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 very uniform, ranging from hundreds of angstroms to a hundred microns.
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 to 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.
Parylene Coating Process
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 . 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 coating process begins…
Parylene is applied to parts using the three-stage vapor deposition process shown above.
Stage 1: 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 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: 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: 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.
The 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, they are made of a known substrate and we can save them 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 customers requirements. Quality documents are provided with each shipment and documents can be customized according to customers request.
The parts are returned to the customer’s facility ready for use!
Ask an Expert about your own application and let us impress you! 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 choice than VSI Parylene.