If you're reading this article on ensuring good Parylene adhesion, chances are you already know a bit about Parylene and its properties, and now you're more curious about the process. As the Process Engineer Manager at HZO, I can't stress enough how beneficial streamlined, optimized production processes are when bringing our customer concepts to life.
Read more about Parylene masking
Our customers come from different industries but tend to want one thing in common—a durable, high-performance coating. Parylene needs good adhesion to do that.
You already understand the importance of a clean, well-prepared surface from daily life. When was the last time you waxed your car without washing it first? If you applied wax over a dirty, dusty, or greasy surface, the wax wouldn't bond adequately; there would be streaks, uneven coverage, and poor protection.
Similarly, if a PCBA isn't adequately cleaned before applying a Parylene coating, contaminants prevent it from adhering well, leading to weak coverage and potential failure. Surface preparation is understood and used across various industries—from chefs thoroughly cleaning a non-stick pan of debris before cooking to nail salons, filing and buffing nails, and swabbing them with acetone before applying paint.
Yet, it is a concept that I feel isn't discussed enough when we talk about coating reliability. I'm going to explain why surface prep and cleaning might be more important than you think and give you an overview of the process so you can walk away with some actionable tips to use on your production line. I will go into precise cleaning techniques, so if you are an assembler and your goal is to clean your PCBAs and components before sending them to be coated, you will also find value in this post.
But first, let's talk about why your assemblies and components need surface prep.
Careful! That Sample May Not Be As Clean As You Think
Surface preparation and board cleaning are hot topics in our industry, and an entire history of why this is a debate would be beyond the scope of this article. If you are looking for that type of deep dive, Aqueous has a great reference.
I've observed, anecdotally, in my career that prospective customers often choose to skip surface preparation during sampling, and this can correlate with poor coating adhesion. Despite explaining the importance of surface preparation for achieving strong adhesion, we've encountered situations where, when we first recommend cleaning and outline the risks of not doing so—such as encapsulating contaminants like dust or dirt—prospects often decline to proceed with cleaning in order to save on costs or processing time.
While there are cases where the outcome is acceptable, failures frequently occur due to contaminants trapped on the board. Given that not all potential sources of contamination on the manufacturing floor are immediately visible to the naked eye, our partners may be misled into skipping this crucial step. This can lead to issues down the line, as even invisible contaminants can compromise the quality of the coating adhesion and ultimately can lead to corrosion and other reliability issues.
This can frustrate our partners because of wasted time and resources, as we will have to start again, this time with surface preparation. However, it's somewhat understandable if you don't consider all of the possible sources of contamination on the manufacturing floor that may not even be visible to the human eye.
Contamination Sources
● Flux residues
● Material handling
● Upstream/ Downstream
● Touch up and Repair
● Fingerprints
● Dirt
● Lint
● Skin oils
● Hand Lotions
● Salts from worker perspiration
● Particulate matter in the air from cardboard
Think of these tiny contaminants as ticking time bombs when they come between the Parylene coating and your component. The coating may not lift right away - causing delamination - but only time will tell if you have a huge issue on your hands. If you're not a fan of risk, read about how to ensure good Parylene adhesion with surface prep and precise cleaning techniques.
Ensuring Adhesion with The Surface Prep Process
Again, proper preparation of the substrate is crucial for the coating to adhere correctly. The surface should be clean (as possible) and free of contaminants that could interfere with the bonding process. First, use appropriate cleaning agents for the substrate to remove grease, dirt, oil, and other contaminants.
The choice of cleaning agent depends on the type of contamination and the material of the substrate. Common cleaning agents include:
Solvent and Aqueous-Based Cleaners
● Isopropyl Alcohol (IPA): Effective for removing light oils, grease, and flux residues.
● Alkaline Detergents: Solutions containing surfactants, such as sodium hydroxide or potassium hydroxide, break down organic contaminants.
● Citrus-Based Cleaners: Environmentally friendly options that dissolve oils and grease.
● Deionized (DI) Water with Detergents: Used for water-soluble contaminants and light residues.
At HZO, we commonly recommend an ultrasonic wash and bake for precision removal and less abrasive cleaning. The ultrasonic wash uses high-frequency sound waves in a cleaning solution to remove contaminants such as dust, oils, and residues from the substrate at a microscopic level.
This method provides deep, uniform cleaning, especially for intricate or delicate components. Following this, the bake step involves heating the substrate to eliminate moisture and volatile residues that could interfere with the adhesion and uniformity of the parylene coating. Together, these steps enhance coating reliability, ensuring a clean and stable surface for deposition.
Specific pre-treatment surface modification steps may be required depending on the material, such as aluminum, steel, or plastic. You can use physical surface modification to give the Parylene physical roughness to latch onto. At HZO, we use plasma to roughen the surface and increase the surface area for adhesion. In this process, bombardment by active Argon atoms causes roughening at the molecular level.
You can also use chemical surface modification methods. These give coatings like Parylene a chemically attractive surface on which to bond. We use adhesion promotors such as silane to create a stronger bond.
Precise Cleaning Techniques
Cleaning is not just about removing visible dirt; it also involves eliminating microscopic contaminants that could prevent the coating from bonding or make customer parts appear aesthetically unappealing. We tailor the cleaning method to the contamination and the substrate. Sensitive substrates cannot handle all cleaning methods.
For example, isopropyl alcohol is sometimes used to clean surfaces. We avoid solvent cleaning unless we know it's the proper solvent for the material to prevent damage or ensure no residues that could affect adhesion. After using solvents, we recommend using dry, filtered air to blow off any remaining residues or moisture.
The following chart describes types of substrate materials and their reactions to cleaning methods.
| Substrate Materials | Reactions to Cleaning Methods |
| Electronic Components | Generally tolerate all cleaning methods, but items such as displays, MEMS, and optical components can be sensitive. |
| Solder Mask | Very robust but can be discolored by plasma. A known point of delamination and contamination in surface energy. |
| Other Conformal Coatings | Some conformal coatings have very low surface energy, and some can inhibit cure of other coatings (acrylics and silicones, for instance). |
| Solder Joints | Combined with the standoff gap of components and chips, these are areas most likely to hold contaminants both before cleaning (flux) and after (the solvent or detergent). |
| Other Metallic Surfaces | Copper and silver can be discolored by aqueous/acidic/basic solutions in cleaning, as well as possible oxidation or tarnishing. |
Compressed Air and Ionizers
Compressed air and air ionizers play a crucial role in the final stages of surface preparation for Parylene coating by ensuring no residual contaminants remain on the substrate. Dry, filtered compressed air, used in short bursts to avoid dislodging components, can remove any remaining dust, lint, or microscopic debris that may have settled after previous cleaning steps. This step is crucial for intricate or delicate components where trapped particles could compromise coating uniformity.
Alongside compressed air, air ionizers are often used to help neutralize static charges that attract dust and contaminants to the surface, especially in environments with sensitive electronics or plastic components. By eliminating electrostatic forces, air ionizers prevent airborne particles from re-adhering, ensuring a clean, contamination-free surface for optimal Parylene adhesion and performance.
Environmental Considerations and Safe Handling Procedures
Environmental conditions, such as room cleanliness, can affect the quality of a coating application. Specks of dirt, lint, or other particulates in the air could easily land on the components or assemblies. Keep items such as cardboard boxes off the floor and the production line orderly.
The process is always essential. To avoid contamination before coating, ensure that the coated parts are not exposed to contaminants like dust, as contaminants on the surface may interfere with adhesion.
Additionally, industry standards, such as IPC-A-610, provide recommended practices for safely handling electronic assemblies to minimize the amount of contamination. Recommendations include:
- Keep workstations clean and neat. Do not eat, drink, or use tobacco products in the work area.
- Minimize the handling of electronic assemblies and components to prevent damage.
- When gloves are used, change as frequently as necessary to prevent contamination from dirty gloves.
- Do not handle solderable surfaces with bare hands or fingers. Body oils and salts reduce solderability, promote corrosion and dendritic growth. They can also cause poor adhesion of subsequent coatings or encapsulates.
- Do not use hand creams or lotions containing silicone since they can cause solderability and conformal coating adhesion problems.
- Never stack electronic assemblies, or physical damage may occur. Special racks may be provided in assembly areas for temporary storage.
- Always assume the items are ESDS (Electrostatic Discharge Sensitive) even if they are not marked.
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Personnel must be trained and follow appropriate ESD practices and procedures.
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Never transport ESDS devices unless proper packaging is applied.
Parylene Adhesion Tests
Regarding customer projects, we never make any assumptions, no matter how optimized or streamlined the process is. Test everything. We use cross-hatch testing internally to ensure our coating bonded well.
Cross-hatch testing is a widely used method for evaluating the adhesion strength of coatings to a substrate. This test involves making precise cuts in the coating, typically in a grid or cross-hatch pattern, using a sharp blade or specialized cutter. After scoring the surface, adhesive tape is applied and quickly removed to assess how much the coating detaches. The adhesion is then rated based on the percentage of coating removed, with classifications ranging from excellent adhesion (no coating loss) to poor adhesion (significant peeling or flaking). Cross-hatch testing provides a quick and reliable way to assess coating durability and performance, ensuring that coatings meet quality and industry standards before being subjected to real-world conditions.
A Good Cleaning Process Brings Parylene to Life
When people contact us about Parylene coating, it's typically about a property they’ll need to meet a spec or solve a problem. For example, they need superior corrosion resistance. They'll also need good adhesion, which means they'll need good surface prep.
My team and I work daily to bring customer visions to life through streamlined, dependable processes. Our problem-solving skills save our customers money and time and enable and enhance our amazing technology at HZO.
In the coming months, I'll share more about how you can use the process to do the same for your company. Of course, I can't share the custom solutions we've developed for our partners, but I'm happy to provide value where I can.
Do you have questions about process engineering in Parylene or surface preparation? Use the contact form to send me a message, and I'll get back to you. Otherwise, learn more about the Parylene coating process in its entirety.