Disadvantages of Using Parylene on Electronics

While Parylene offers various advantages as a conformal coating, it comes with inherent drawbacks that warrant consideration prior to its application. The emergence of failure mechanisms associated with Parylene coatings has limited its wider scale application in comparison to liquid conformal films such as acrylic, epoxy, silicone and urethane. In numerous scenarios, wet coatings may deliver superior performance and cost-effectiveness, or even both, across a wide array of applications.

The Cost of the Parylene Process

Applying Parylene conformal coatings generally costs more than liquid coatings like acrylic, epoxy, silicone and urethane. The costliness of Parylene dimer, the raw material for Parylene films, is a significant factor, ranging from a few hundred dollars to several thousand dollars per pound. Each Parylene production run typically requires a pound of dimer, leading to high material expenses, especially for small coating projects. 

Parylene, applied via a chemical vapor deposition (CVD) process, coats all surfaces, including critical components like the inner diameter of a printed circuit board (PCB), which must remain film-free for proper functionality. To prevent unwanted coating, masking and other protective measures are necessary. These conditions render the Parylene process inefficient and wasteful, driving up production costs for customers. Additionally, substantial investments in new production equipment are often required.

Other Parylene Disadvantages

In addition to higher production cost, other disadvantages of Parylene conformal films include:

• Batch processing: CVD requires a batch production process. The production chamber’s confined physical space restricts the quantity of items that can be efficiently coated in a single machine-run. The main goal is to maximize the number of coated items in the chamber while upholding high-quality conformal film. Insufficient coated items can significantly raise the price-per-piece, just as overcrowding the chamber can compromise film quality. These issues elevate production costs and time.
• Chemical inertness: Parylene is often sought after as a conformal coating because it doesn’t react to many chemicals; in this respect, its inertness is highly-prized property. This can become problematic when coated PCBs and other assemblies need rework. Parylene, resistant to solvents and relatively heat-resistant, proves challenging to eliminate. The most consistently reliable method for removing Parylene remains the time-consuming micro-abrasion process.
• Delamination: Delamination occurs when a film finish is of poor quality, characterized by torn, unattached and non-conforming coating that separates from the substrate, thereby defeating the purpose of conformal coating. Before the coating process, it is crucial to complete all necessary preparations such as cleaning the substrate and masking. It is also essential to verify the materials’ compatibility with the substrate and ensure moisture impermeability where necessary. These aspects enhance adhesion by facilitating better interaction of surface energies between the Parylene and the substrate.
• Limited throughput: Operating CVD deposition chambers comes with a high cost. Due to their compact size, they are only suitable for small-batch production. The amount of product coated in a single session is constrained, making the process time-consuming, often taking between 8 to 24 hours to finish.
• Masking/other prep: During the process of chemical vapor deposition (CVD), Parylene vapor can seep into uncovered areas of an assembly, necessitating the meticulous masking of electrical components. The preparation of these exposed regions further hinders the production timeline. Moreover, ensuring surface cleanliness is crucial in the fundamental production stages. Contaminants can disrupt the favorable interaction between vapor-phase chemicals and hinder the formation of a stable solid film on the substrate surface.
• Physical resilience: Parylene is notably soft, exhibiting minimal durometer value. Due to its softness, this conformal film is susceptible to damage from regular handling.
• Poor adhesion to metals: Without proper adhesion techniques, Parylene adheres poorly to gold, silver, stainless steel and other metals, a problem since they are frequently used in PCBs to support conductivity. The introduction of adhesion promotion methods to enhance metal adhesion can be costly and labor-intensive.
• Solder joint defects: Improperly applied, Parylene can stimulate a 300% expansion of solder joint fatigue.
• Tin whiskers: Inadequate application of Parylene film can cause the growth of tin whiskers on coated assemblies.
• UV resistance: Affordable Parylene dimers (priced under $1,000 per pound) offer minimal UV resistance and yellow when exposed to outdoor conditions.

Summary

Parylene is widely recognized for its exceptional conformal coating capabilities, but it does not guarantee flawless performance. While its downsides can be addressed, it is crucial to acknowledge and mitigate them. Factors such as cost, the impact of CVD processing, delamination/adherence issues, coating durability, material suitability and tin whiskers must all be considered. This approach ensures that the numerous performance benefits of Parylene are fully leveraged and effectively implemented.