The phrase “in-line parylene processing" is deceptive because it does not accurately describe the method in which parylene (XY) is applied as a conformal coating. It is true that some aspects of the traditional production line are relevant, but primarily in a fractional way. without the traditional station-to-station regimentation of standard in-line manufacturing processes.
A conventional factory production line incorporates a structure wherein the object being manufactured – automobile, bookshelf, dishwasher, textiles, etc. -- is passed through a set linear sequence of mechanical or manual operations, from start-to-finish, until the product is complete. In-line manufacture began in the 18th century and, as production technology developed, spurred the Industrial Revolution and the growth of Western capitalism. Its impact remains strong, influencing how goods are made and sold in our consumer society.
Despite remarkable advances in production technology, the in-line structure continues to prevail today for many industries. Many of the computer-related items parylene is called on to coat are themselves manufactured through in-line processing methods. For instance, printed circuit boards (PCBs), a prime recipient of XY protection, are typically manufactured, stamped and soldered in a sequential manner, from process-to-process, in an ongoing procession of successive activities, without pause or intervention until the board is complete.
And, if the PCB manufacturer has its own conformal coating mechanism, applying the film can become part of the in-line process. However, this applies more for liquid materials, whose dip or spray coating methodologies are more readily incorporated into a sequential series of work processes, with perhaps a spray and curing function at the end of the line.
Yet, conformal coating processes require in-line methods of their own, apart from product design, construction and manufacture. For wet coatings like acrylic, epoxy, silicone and urethane, typical line processing includes:
- Cleaning, to remove contaminants from the PCB.
- Applying coating by dipping or spraying the liquid film material onto the substrate.
- Drying the wet component allows it to be handled for further use.
- Curing permits attainment of the product’s optimal electrical/mechanical performance properties and conformal protection.
- Inspection ensures the coating is conformally appropriate to the assembly’s function.
There is a sequential order to these production events that can be achieved through in-line processing; however, all occur after the PCB or other item-to-be-coated has been produced, and thus may be separate from the original in-line production sequence.
Parylene conformal coating requires specialized processes for application, which makes it even less adaptable to in-line production. XY's unique vapor-phase polymerization differs considerably from the application processes of other coating materials, eliminating the intermediate liquid deposition procedure necessary to wet coatings.
Called chemical vapor deposition (CVD), it is more complex than liquid methods, which apply pre-synthesized coating onto substrates. CVD synthesizes the conformal film in-process, depositing XY directly onto the substrate, while penetrating into its surface. Opportunities to enact in-line production methods are severely limited because, to be successful, CVD needs to be implemented in a specialized vacuum chamber. In addition to providing superior conformal film adaptable to a multiplicity of purposes, CVD offers these process advantages:
- elimination of specialized surface treatment prior to film deposition, causing
- a chemically stronger consistency than conventionally assembled monolayers.
The relative complexity of XY processing makes it more difficult to modify for in-line manufacture; once implemented, the process is self-governing, from beginning-to-end, more suitable to batch processing.
Parylene does require some extra steps to be used that are not for liquid conformal application. Cleaning substrates prior to application is recommended, and masking/de-masking almost always is required. However, XY only needs priming in the case of noble metal substrates, when a pre-application of silane can improve adhesion. CVD, a longer process more suitable to batch production, replaces liquid application methods, with generally better results. Parylene requires no drying/curing.
Thus, three of the in-line steps necessary for wet coating are eliminated, and the basic process – XY application – is another method entirely; CVD’s coating product offers greater performance reliability, but is also more costly and time-consuming. There seems to be no way to incorporate in-line methods to parylene coating processes; the requirements of successful CVD simply contradict the sequential, piecemeal approach of the production line. Once CVD is initiated, it proceeds autonomously; the series of processes for completion take place within the deposition chamber, without participation from human or automated workers. Perhaps future, unforeseen technical innovations will alter these circumstances. Until they do, CVD transformation to in-line processes will remain infeasible. The need for batch-processing negates reliance on conventional production line/in-line manufacture for parylene conformal film application.
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