Parylene for Defense Applications
Posted by Sean Horn
Wednesday, February 4, 2015 9:54
@ 9:54 AM
What is Parylene?
Parylene is a conformal protective polymer used for coating, first postulated by Michael Szwarc in 1947. His early experiments involved the decomposition of the solvent p-xylene. His worked proved that when the vapors of the p-xylene reacted with iodine, para-xylyene di-iodide was the only resulting product. The reaction produced only a low yield and the process was later made more efficient by William F. Gorham.
Today, Parylene coating is used to uniformly protect any component configuration. Its unique features allow it to be used effectively on a diverse range of materials, including metal, ceramic, paper, plastic, ferrite, resin and silicon. It can be made to conform to any shape, including sharp edges, crevices, points, flat and exposed internal surfaces. Parylene works under pressures, stresses and in environments that would be detrimental to other types of coatings.
What are main uses of Parylene within a defense context?
The main purpose of a conformal coating is to encapsulate critical components and protect them from external hazards such as harsh weather conditions, chemicals and physical damage. The versatile nature of Parylene means that it has a wide range of uses within the defense industry. It can be used on components that are put under constant extremes and are contact with changing contaminants. It also has dielectric properties that allow it to protect against signal interference. Its ability to do this whilst being both light and thin means that it does not add to the overall size and weight of the circuit board. Parylene’s main uses within the defense industry include:
Circuit boards and Hybrid Circuits
Parylene coatings are thinner than other forms of protective coating. They are also stress free and highly resistant to attack from fungi and solvents. The excellent adhesion of Parylene to ceramic substrates aids in minimizing the ionic conduction at the coating-substrate interface. Parylene coating also strengthens solder joints, which in turn strengthens the circuit links and wire bonds, increasing the lifespan of the circuit and its components. The Parylene’s low dielectric constant also minimizes loading in high frequency applications.
Parylene coating can be attached in exceptionally small spaces and has been used within spaces as narrow as 10µ. Due to its nature, parylene can also be used to coat interior elements through a hole of just 1mm.
Using parylene coating causes the termination of movement in particles such as ball of solder, left over from the creation of the circuit board.
The reduction of friction is a key concern in manufacturing equipment and technologies for defense. Minimal friction is essential to ensuring smooth operations and minimizing overheating and fire risks.
Elements such as ferrite cores and bobbins require coating to reduce friction. Parylene is highly effective at reducing friction and at minimizing abrasion during coil forming. It has the additional benefit of preventing dust forming on components and protecting them from chipping. Parylene coating also provides electrical insulation and protection against corrosion to cast metal magnets. It is an ideal coating system for components such accelerometers, strain gauges and pressure sensors that require environmental protection.
Rubber and plastic
Defense and military equipment needs to work effectively in any situation and under a variety of extreme conditions. It needs to be able to come into contact with chemicals and solvents without the equipment being compromised. Parylene coating protects even the smallest elements.
The parylene coating process allows it to penetrate the surface of rubber and plastic components. Adhesion is exceptional and gives a surface covering that is free of air bubbles, which minimizes opportunities for the external environment to compromise the component. A 2µ coating creates wear resistance and applies dry lubricant feel and appearance to the surface. Chemical and solvent resistance can be increased by using a thicker coating.
Optics and instruments
Moisture can cause havoc with sensitive elements. A Parylene coating creates a barrier that is moisture resistant as well as hard wearing.
The specific properties of Parylene, including its ability to work under extreme pressures and stresses makes it perfect for coating elements in optics and instruments that are in contact with the external atmosphere or which need to work in extreme temperatures and potential condensation. Parylene can be used to coat solid rocket fuels and explosives. The coating not only makes the material less sensitive to moisture, but also allows other characteristics to be modified.
How is Parylene coating engineered and tested
Engineering Parylene coating is an intricate multistage process. Many aspects of the process will be specific to the coated material. These aspects will include the type of coating, thickness of the coating and the time taken to apply the coating effectively. The process will begin before the materials have even been received:
Stage one: Information collection
Before the coating can be applied, as much information as possible is gathered about the item that is to be coated. This information will come in the form of written descriptions and drawing and will include, although is not necessarily limited to, the following:
- Dimensions of item
- Composition of the item
- The stresses / environment it is likely to be subjected to
- Coating free areas
- Any specific customer requirements
Stage two: Inspection
The item to be coated is carefully inspected for any damage that may have occurred during transit. The inspection team will also ensure that it meets the specifications that have been previously discussed. Cleaning will also be undertaken at this time if it is required or has been requested.
Stage three: Masking / Prep
Once inspection, and if necessary cleaning, has been completed the item will be masked. The item is masked in accordance with the customers’ specifications, to ensure that the coating is only applied in the requested areas. The item will be inspected again once masking is completed, to ensure that it meets the customers’ requests. Any surface preparation is also completed during this stage.
Stage four: Coating
The item is now ready to be coated. The solid state raw dimer is placed in a loading boat, which is then placed into a vaporizer. The variant of raw dimer will be dependent on the item being coated (the main variants are Parylene C, Parylene D, Parylene N and Parylene AF-4). The dimer is heated to a minimum of 100°C (212°F) and the vapor pulled through vacuum into a furnace. Here it is heated to a higher temperature, allowing the molecules to be split and the monomer to be created.
The monomer, still in gas form, continues to be pulled through the vacuum in single molecules onto the item. This is done in the coating chamber at ambient temperature. The process is completed in the cold trap, with the temperature between -90°C (-130°F) and -120°C (-184°F). Any residual coating material is removed at this stage.
Stage five: Post coating
Once the coating is completed the masking is removed. This needs to be done with extreme care so that the newly applied, very thin coating is not damaged. This is followed by a final inspection of the completed item, during which the item will be compared to the original customer request and specifications. Once the item has passed the inspection, it is sent to the shipping department and returned to the customer. Close contact is kept with customers to ensure that the item or materials have met their needs and are working as expected.
Why Parylene coating from Diamond-MT
Parylene is an expensive and labor intensive form of coating. However, done effectively, the coating saves medium and long term costs as the coated items perform exceptionally well and have an increased life span. Diamond-MT’s manufacturing processes make the coating process even more cost effective:
- Care is taken to ensure the quality of the product from the moment it arrives
- Cleaning processes add to the quality of the coating and improve its effectiveness
- Masking, and careful inspection of the masking, minimizes mistakes
- The size of the coating chamber allows for multiple items to be coated simultaneously
Combined with Diamond-MT’s unparalleled customer service and aftercare, customers are assured that their orders are treated with the upmost care and respect.