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Parylene Conformal Coatings Specifications and Properties I. Introduction Parylene is the generic name for members of a unique polymer series. The basic member of the series, called Parylene N, is poly-para-xylylene, a completely linear, highly crystalline material.
Parylene D, the third member of the series, is produced from the same monomer modified by the substitution of the chlorine atom for two of the aromatic hydrogens. Parylene D is similar in properties to Parylene C with the added ability to withstand higher use temperatures. Parylene N is a primary dielectric, exhibiting a very low dissipation factor, high dielectric strength, and a dielectric constant invariant with frequency. This form has the highest penetrating power of all the Parylenes. Parylene C has a useful combination of electrical and physical properties plus a very low permeability to moisture and other corrosive gases. Along with its ability to provide a true pinhole free conformal insulation, Parylene C is the material of choice for coating critical electronic assemblies. Due to the uniqueness of the vapor phase deposition, the Parylene polymers can be formed as structurally continuous films from as thin as a fraction of a micrometer to as thick as several mils. II. The Deposition Process The Parylene polymers are deposited from the vapor phase by a process which in some respects resembles vacuum metallizing. Unlike vacuum metallization, however, which is conducted at pressures of 10-5 torr or below, the Parylenes are formed at around 0.1 torr. Under these conditions the mean free path of the gas molecules in the deposition chamber is in the order of 0.1 cm. Therefore, unlike vacuum metallizing, the deposition is not line of sight, and all sides of an object to be encapsulated are uniformly impinged by the gaseous monomer. This is responsible for the truly conformal nature of coating.
The process consists of three distinct steps as outlined in Figure 2 for Parylene N. The first step is vaporization of the solid dimer at approximately 150° C. The second step is the quantitative cleavage (pyrolysis) of the dimer at the two methylene-methylene bonds at about 680° C to yield the stable monomeric diradical, para-xylylene. Finally, the monomer enters the room temperature deposition chamber where it simultaneously adsorbs and polymerizes on the substrate. No liquid phase has ever been isolated and the substrate temperature never rises more than a few degrees above ambient. A necessary fourth component in this system is the mechanical vacuum pump and associated protective traps. The equipment can be modified to meet the requirements or configuration of the items to be coated. Present coating chambers vary in size from 23 cm in diameter and 31 cm long to a 85 cm high hemi-cylindrical unit. One particular configuration, 45 cm in diameter and 60 cm high, was designed specifically for circuit boards and can accommodate up to two hundred 12 cm wide by 16 cm long boards. The process is inherently simple and can be conducted with a minimum of supervision. Compared with vacuum metallizing, deposition rates are fast, especially for Parylene C, which is normally deposited at about 0.2 mils/hr. The deposition rate of Parylene N is somewhat slower. Kinetic studies have established that this rate is directly proportional to the square of the monomer concentration and inversely proportional to the absolute temperature. III. Properties
The electrical, mechanical, thermal, barrier, optical, and other properties of Parylene N, C and D are discussed below. These properties are compared to those reported for other conformal coating materials such as epoxides, silicones, and urethanes. A.
Electrical Properties 1. Thin
Film Dielectric Properties
One of the features of the Parylenes is that they can be formed in extremely thin layers. The breakdown dc voltages of both Parylene C and N films have been determined as a function of polymer thickness, a plot of which is shown in Figure 3. Parylene C is superior to N in this respect for films under 5 micrometers (0.0002 in). These data show that both Parylenes have excellent dielectric withstanding voltages even below the 1 micrometer thickness range, and the voltage breakdown per unit thickness generally increases with decreasing thickness. 2.
Circuit Board Insulation Resistance
In brief, this test consists of ten cycles (one cycle per day), with each cycle consisting of seven steps. The seven steps range from low temperature, low humidity (25° C, 50% RH) to more severe conditions (65° C, 90% RH). Resistance readings are taken initially and at the 65° C, 90% RH step for each cycle of the ten day test. Results are shown in Table 2 for Parylene C coating thicknesses from 0.002 in to 0.0001 in. It is interesting to note that even for the very thin coatings (0.0001 in), the insulation resistance values are about one order of magnitude above the prescribed specification. B. Physical
and Mechanical Properties
Impact resistance is high when the Parylene polymers are supported on test panels. Gardner falling ball impact tests on 0.001 in thick Parylene C coated steel "Q" panels are in the 250 in-lb range. Wear index values (measured on a Taber Abraser machine using CS-17 "Calibrase" wheel with 1000 gram weight) were 22.5 for Parylene C and 8.8 for Parylene N. By comparison, polytetrafluoroethylene (Teflon®) is 8.4, high impact polyvinylchloride is 24.4, epoxide is 41.9 and urethane is 59.5. C. Thermal, Cryogenic, and Vacuum Properties 1.
Thermal
Parylene may be annealed to increase cut-through resistance, increase hardness, and improve abrasion resistance. This is the result of a density and crystallinity increase. General thermal properties are summarized in Table 4. 2.
Cryogenic Unsupported 0.002 in films of Parylene C can be flexed 180° six times at -165° C before failure occurs. Comparable films of polyethylene, polyethylene terephthalate, and polytetrafluoroethylene fail at three, two, and one flexes, respectively. Neither electrical nor physical properties are affected by cycling from 2° K (-271° C) to room temperature. 3. Vacuum
Stability D. Barrier Properties and Chemical Resistance
1.
Barrier Moisture vapor permeability values have been measured at thicknesses below 0.1 micrometer. Normalized to equivalent thickness, the values are the same for all thicknesses. 2.
Chemical Resistance
E. Optical Properties, Radiation, and Weathering Resistance 1.
Optical Properties Parylene exhibits very little absorption in the visible region and is, therefore, transparent and colorless. Below about 280 nm both Parylene N and C absorb strongly, as shown in Figure 4. The infrared spectra for 1 mil films are shown in Figure 5, Figure 6, and Figure 7.
2.
Radiation and Weathering Resistance Although stable indoors, the Parylenes are not recommended for long term use outdoors when exposed to direct sunlight. IV. Adhesion Good to excellent adhesion of Parylene to a wide variety of substrates can be achieved by the simple treatment with a dilute solution of an organic silane prior to Parylene coating. Two silanes, vinyl trichlorosilane in either xylene, isopropyl alcohol, or Freon® and gammamethacryloxypropyltrimethoxysilane in a methanol-water solvent have been successfully used. The techniques were developed primarily for electronic components and assemblies and do not affect any of the substrate or coating properties. Silanes can also be vapor phase applied if non-liquid application is preferable. Generalized procedures for using these silanes are available upon request. V. Product Safety Specialty Coating Systems has compiled the information contained herein from what it believes are authoritative sources and believes that it is accurate and factual as of the date printed. It is offered solely as a convenience to its customers and is intended only as a guide concerning the products mentioned, Since the user's product formulation, specific use application, and conditions of use are beyond Specialty Coating Systems control, Specialty Coating Systems makes no warranty or representation regarding the results that may be obtained by the user. It shall be the responsibility of the user to determine the suitability of any products mentioned for the user's specific application. We urge you to review, prior to use, the Material Safety Data Sheets for any Specialty Coating Systems products mentioned herein. These documents are available from your Specialty Coating Systems sales representative. This information is not to be taken as a warranty or representation for which Specialty Coating Systems assumes legal responsibility nor as permission to practice any patented invention without a license. VI. Applications A. Conformal Coating of Printed Circuit Boards for Protection Against Moisture and Corrosive Environments
A number of printed circuit assemblies and conductor patterns have been cross-sectioned and polished after coating with Parylene. Figure 8. Scale drawing of 0.025 in wire coated with 0.001 in of Parylene - 50X (above-right) is a representation of a photomicrograph taken of a 0.025 in wire on a phenolic laminate, drawn to scale at 50X magnification.
Figure 9. Cross section of Conductor Pattern on Glass-Epoxy Laminate - 50X (left) shows a photomicrograph of a conductor pattern on an epoxy glass laminate coated with 0.002 in. of Parylene C. Replication of the step-edge is particularly impressive. B. Surface Passivation and Environmental Protection of Semiconductor Devices Parylene at thicknesses from 0.I micrometer to 0.002 in, has been found useful in this application. Again, the truly conformal nature of the coating allows bonded wires and evaporated conductor patterns to be coated at the same thickness as the silicon or other semiconductor surfaces. Parylene at thicknesses down to at least 0.4 micrometer is pinhole free as shown by HF and chlorine etch techniques. Thorough evaluation proves that Parylene does not alter the electronic parameters of devices. Furthermore, when Parylene coated devices, integrated and hybrid circuits, are encapsulated by transfer molded epoxide compounds their performance is equivalent to hermetically sealed units in accelerated aging tests (pressure cooker). C. Conformal Coating of Biomedical Devices for Environmental Protection and Electrical Insulation Requirements Thin Parylene films can be deposited on virtually any biomedical substrate. Parylene coated devices are stable, exhibit little or no change in response characteristics, and are electrically and chemically isolated from the body. D. Parylene has found acceptance in a myriad of industrial, aerospace, Chemical, automotive, consumer, pharmaceutical, and defense applications
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Parylene
C, the second commercially available member of the series, is produced
from the same monomer modified only by the substitution of a chlorine
atom for one of the aromatic hydrogens. The structures are shown in Figure
1. Parylene N,C & D Chemical Structures (right).
The
ability to deposit Parylene as a truly conformal, thin, continuous,
uniform adherent coating permits its application as a protective PCB
coating.
The
Parylene C coating was 0.001 in thick. The Parylene polymer penetrated
and coated uniformly even in the 0.002 in gap between the wire and
board.|
Specialty
Coating Systems, 7645 Woodland Drive, Indianapolis, IN 46278 |