Although poly (p-xylene) has many advantages as a conformal coating, it has some disadvantages that should be recognized before use. Compared with liquid conformal films such as acrylic acid, epoxy resin, silicon and polyurethane, the failure mechanism that may occur in poly (p-xylene) coatings limits their wider application. In many cases, wet coatings can provide better performance and lower cost (or both) for many applications.
Cost of Parylene Process
The cost of poly (p-xylene) conformal coatings is usually higher than that of liquid coatings such as acrylic acid, epoxy resin, siloxane and polyurethane. On the one hand, the price of poly (p-xylene) dimers (the basic material for poly (p-xylene) conformal films) is quite expensive, ranging from $200 to $10,000 per pound. This factor greatly increases the cost of production before the start of the production process: the average production of poly (p-xylene) - running requires a pound of dimers, which from the beginning generates high material costs, especially if only one pound of dimers is used. Coating a limited number of items. Because poly (p-xylene) is applied by chemical vapor deposition (CVD), everything is coated. This includes the inner diameter of product components such as printed circuit boards (PCBs), which require no film to work properly. Masking and other corrective/protective procedures are required to ensure that unnecessary coating does not occur. The existence of these conditions makes poly (p-xylene) itself an inefficient process and wastes production materials, which increases the final cost of customers. High capital costs for new production equipment are also common.
Other Parylene shortcomings
In addition to higher production costs, other disadvantages of poly (p-xylene) conformal films include:
Batch processing: CVD requires batch production process. There is only limited physical space in the production room, which limits the total number of items that can be effectively coated during the operation of any machine. The main purpose is to maximize the number of items coated in the chamber without sacrificing the quality of the conformal film. Although the number of sub-optimal coatings can significantly increase the price of each item, the same is true of the film quality degradation caused by overcrowding in the room. These two problems will increase production costs and processing time.
Chemical inertia: Polyparaxylene is often sought after as a conformal coating because it does not react to many chemicals; in this regard, its inertia is a highly valuable asset. However, if the PCB needs to be reprocessed, this may be the problem of coating PCB and other components. Solvent-resistant and relatively heat-resistant poly (p-xylene) is difficult to remove. Time-consuming micro-wear is the only reliable way to remove poly(p-xylene)
Delamination: Delamination is the result of tearing apart from the base material, poor surface of the film represented by non-adherent and non-conformal coatings, which makes the purpose of conformal coatings ineffective. All appropriate preparations - clean substrates, masking, etc. - need to be completed before the coating process. The compatibility of the material with the base material and the applicable water impermeability must be verified. These factors support adhesion by improving the interaction of surface energy between poly (p-xylene) and matrix.
Limited throughput: CVD chambers are expensive to operate; although small in size, they are limited to small batch production. The total amount of products coated during any single coating period is also limited and time-consuming, requiring 8-24 hours to complete.
Masking/other preparation: During CVD, polyparaxylene vapor will penetrate into any uncovered area of the assembly, requiring labor-intensive masking of functional electronic components; preparation of free areas further slows down the production process. In addition, surface cleanliness is an important part of the basic production process, because the presence of any pollutant will interfere with the positive interaction between the gas-phase chemical reactants and the non-volatile solid membrane formulations on the substrate surface.
Physical elasticity: About as elastic as human muscles, poly (p-xylene) is very soft and has very low hardness. This soft shape-preserving film is often damaged during conventional treatment.
Suspicious Adhesion to Metals: Without proper bonding technology, Parylene has poor adhesion to gold, silver, stainless steel and other metals, which is a problem because they are often used in PCBs to support conductivity. Introducing adhesion promotion methods to enhance metal bonding may be expensive and labor intensive.
Solder joint defects: improper coating, poly (p-xylene) can stimulate 300% fatigue expansion of solder joints.
Tin whisker: The inadequate application of poly (p-xylene) film will lead to the growth of tin whisker on coated components.
Ultraviolet Resistance: Cheaper poly (p-xylene dimers) (less than $1,000 per pound) offer little resistance to ultraviolet light and yellow if outdoors.
Parylene's reputation as a high-quality conformal coating is well-deserved, but it is by no means representative of performance reliability. Parylene's shortcomings can be overcome, but need to be recognized, so they can be offset by paying due attention to cost, CVD processing effect, layering/adhesion, coating elasticity, material applicability and tin whisker, to ensure that many of the performance advantages of the coating can be achieved and implemented to the best extent.