Despite parylene’s numerous benefits as a conformal coating, it has several disadvantages that should be recognized before it is used. Failure mechanisms that can emerge from parylene coatings have limited its wider scale application in comparison to liquid conformal films such as acrylic, epoxy, silicon, and urethane. In many situations, wet coatings can provide better performance and lower cost (or both) for many applications.
The Cost of the Parylene Process
The cost to apply parylene conformal coatings generally exceeds that of such liquid coatings as acrylic, epoxy, silicone and urethane. For one thing, the price of parylene dimer, the essential raw material of parylene conformal film, is rather expensive, ranging from $200-$10,000+ per pound, a factor that adds significantly to production expense before the process begins: The average parylene production-run requires a pound of dimer, generating high material costs from the outset, particularly if only a limited number of items are being coated. Because parylene is applied through a chemical vapor deposition (CVD) process, everything is coated. This includes product components such as the inner diameter of a printed circuit board (PCB), which needs to be film-free to work properly. Masking and other corrective/protective procedures need to be implemented to assure unnecessary coating does not occur. The existence of these conditions make parylene an inherently inefficient process and wasteful with production materials, which escalates the end cost to the customer. High capital costs for new production equipment are also common.
Other Parylene Disadvantages
In addition to higher production cost, other disadvantages of parylene conformal films include:
- Batch processing: CVD requires a batch production process. Only a finite expanse of physical space is available in the production chamber, limiting the total number of items that can be effectively coated during any single machine-run. The primary objective is maximizing the number of items coated in the chamber, without sacrificing conformal film quality. While a suboptimal quantity of coated items can dramatically increase the price-per-piece, diminished film quality caused by chamber overcrowding will do the same. Both concerns escalate production costs and process 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. Yet, this can be a problem for coating PCBs and other assemblies, if a PCB needs to be reworked. Solvent-resistant and relatively heat-resistant, parylene is difficult to remove. Time-consuming micro-abrasion is the only consistently reliable method of removing parylene
- Delamination: Delamination is the result of a poor film finish typified by torn, unattached, and non-conformal coating, separated from the substrate, nullifying the objective of conformal coating. All appropriate preparation –cleaning the substrate, masking, etc. – needs to be completed prior to the coating process. Materials’ compatibility with the substrate must be verified, as should applicable moisture impermeability. These factors support adhesion by improving the interaction of surface energies between the parylene and the substrate.
- Limited throughput: CVD deposition chambers are costly to run; physically small, they are limited to small-batch production. Total quantity of product coated during any single coating session is similarly limited and is time-consuming, requiring between 8 – 24 hours to complete.
- Masking/other prep: Parylene vapor will penetrate any uncovered regions of an assembly during CVD, necessitating labor-intensive masking of functional electrical components; preparing free areas further slows the production process. In addition, surface cleanliness is an essential part of the basic production process, since the presence of any contaminants interferes with positive interaction between vapor-phase chemical reactants and formulation of a non-volatile solid film on the substrate surface.
- Physical resilience: Approximately as physically resilient as human flesh, parylene is very soft, with little durometer value. A conformal film this soft is often subject to damage during routine handling.
- Questionable 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: Less expensive parylene dimers (lower than $1,000/pound) provide little resistance to ultraviolet light, and yellow if situated outdoors.
Parylene’s reputation as a superior conformal coating is well-deserved, but in no way represents performance infallibility. Parylene’s disadvantages can be overcome but need to be recognized, so they can be offset by appropriate attention to such issues as cost, the effects of CVD processing, delamination/adherence, coating resilience, materials’ applicability, and tin whiskers, to ensure the coating’s many performance advantages are accessible and implemented to the optimal degree.