Perhaps the most reliable of the conformal coatings, parylene (para-xylylene di-iodide) is also one of the more expensive coating options. Production costs typically encompass three primary expense categories -- raw materials, labor, and lot volume. Of the three, labor expenses are generally the most costly, but raw materials can add significantly to production overhead; materials’ costs can be largely attributed to the raw parylene dimer required to make conformal coatings.
Parylene CVD and Dimer Requirements
The parylene chemical vapor deposition (CVD) process is unique and generates the most reliable of conformal coatings for an abundance of products and purposes. CVD causes a gaseous parylene to seep deep within the substrate surface, producing a micro-thin protective film of exceptional durability. But parylene does not start as a vapor; its begins as a solid state raw dimer (di-p-xylene) that is transformed into a gas during CVD.
A chemical compound in a powder form, raw dimer is the essential ingredient of parylene conformal coatings. During CVD, the dimer undergoes pyrolysis in a vacuum environment, maintained throughout the process. It is then transformed from a solid state into a monomeric vapor. Cooled to room temperature, gaseous parylene dimer polymerizes at a very low pressure, and becomes a structurally continuous film that entirely conforms to the physical configurations of the substrate surface. Polymerization occurs at near-room temperature.
In addition to completely uniform, pinhole-free coverage, parylenes typically provide exceptional resistance to solvents and moisture, as well as incredible dielectric strength. Not all parylenes are equal; while all share nonpareil barrier properties, each type has a distinct set of coating properties compared to other parylenes. Major dimer types include Parylene C, Parylene N, Parylene D, and Parylene AF-4, among the most frequently used parylene compounds. Other variants are also possible; their selection and use depends upon the necessities of the specific coating assignment.
For instance, required coating thickness factors into the kind of parylene dimer used and its quantity. Regarding cost, the amount of raw dimer needed to produce the required film thickness across the assembly needs to be carefully calculated for optimal results. This issue emerges because parylene can be effectively deposited in thicknesses ranging from angstroms to well over 2 millimeters; determining precise project thickness helps ensure application of the correct dimer type and its expense to the client/manufacturer.
In addition, each of the different types of raw dimer has its own associated manufacturing processes and costs. Thus, different projects and their diverse assembly/component functions are best served by different grades of parylene coating. Other necessary project properties, such as
- the level of dielectric strength,
- the degree of barrier protection, and
- overall coating flexibility,
further add to the need to appropriately select the correct dimer type used to initiate CVD and the coating process. These conditions combine with the cost of the CVD itself to sum total expenses. Depending on the coating type and required thickness, typical parylene deposition rates are about .2/mils per hour, so machine runs can vary from as little as 1 hour for small production batches to over 24 hours for larger-scale projects.
The majority of parylene product runs -- wherein the dimer is polymerized and applied as conformal coating to the selected substrate -- require about a pound of raw parylene dimer. Depending upon the project and the type of dimer necessary for effective completion, dimer costs between $ 200.00 to $ 10,000,00 per pound, a sizable expenditure range. Selecting the appropriate dimer type is paramount to successful coating. Saving money on dimer cost – opting for a less expensive dimer variant – is never recommended. The resultant conformal film will inevitably malfunction, performing well below standard and leading to assembly breakdown.
The extra cost for parylene coatings is well worth it, considering their exceptional operational versatility. Dimer costs are high, but their appropriate selection leads to longer-lasting, better coating quality and more reliable performance, the most recognized and respected characteristic of parylene coatings.
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