Conformal Coating Application Guide 

This guide will cover a description of parylene, the parylene deposition process, and detailed descriptions of the properties and characteristics of parylene. Learn more about our company, facility, and capabilities here. 

INTRODUCTION

This application guide is provided to assist in the selection and application of Parylene Conformal Coating for a wide array of uses.

The data provided herein is of a general nature in order to accommodate the wide application variations encountered with this type of product.

Specific application criteria may already be available for your specific intended purpose. Typically, performance characteristics and application design can be easily obtained for variant uses through physical inspection and testing procedures accomplished by Advanced Coating.

For Technical Questions and/or to arrange an applications performance analysis contact the following:

Advanced Coating
Attn: Engineering Dept.
10723 Edison Court
Rancho Cucamonga, CA 91730
909-481-1400/Fax 909-481-1427
E-Mail: engr@advancedcoating.com

DESCRIPTION OF PARYLENE

Parylene is a common generic name for a unique series of polymers based on paraxylene. The three most common Parylene types are Parylene N, Parylene C, and Parylene D.

In actual usage, Parylene C is the greatly predominant type of Parylene used for almost all types of applications and as such is usually the type of material associated as “Parylene.”

The basis for the Parylene family is the Poly -para-xylylene monomer which comprises Parylene N. Parylene C and D are created by the substitution of single chlorine molecule (C) or two [double] chlorine (D) (See Figure 1).

The Parylenes are formed by the pyrolisis of a di-para-xyxlene (dimer) in a vacuum environment which is then deposited on a cooler substrate (i.e., room temp.) under continuous vacuum.

Vapor phase deposition of the Parylene polymer allows it to be formed as a structurally continuous film that is truly conformal to the design and structure of the substrate for which it is being deposited on.

( See Figure 2 ).

Parylene can be effectively deposited with excellent accuracy in the thickness range of 0.1 Mils to over, 2 mils.

PARYLENE DEPOSITION PROCESS

The parylene polymers are deposited from the vapor phase by a process that in some respects resembles vacuum metalizing. 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 metalizing, 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 the coating. The process consists of three distinct steps as shown in Figure 3.

Since Parylene is non-liquid, it does not pool, bridge or exhibit meniscus properties during application. No catalysts or solvents are involved, and no foreign substances are introduced that could contaminate coated specimens.

In contrast to Parylene, the thickness of liquid coatings is related to viscosity, working temperature/humidity, and application process (spray or dip) and can only be controlled to a tolerance of approximately +/- 50% of final thickness.

Parylene thickness is a function of the amount of vaporized dimer and chamber dwell time and can be controlled accurately to within +/- 5% of targeted thickness for most typical applications.

Figure 1. Parylene Molecules

PARYLENE C

The most widely used dimmer, providing a useful combination of properties, plus a very low permeability to moisture, chemicals, and other corrosive gasses.

PARYLENE N

Provides high dielectric strength and a dielectric constant that does not vary with changes in frequency. Best selection where greater coating protection is required.

PARYLENE D

Maintains its physical and electrical properties at higher temperatures.

Figure 2. Parylene Conformal Characteristics Vs. Conventional Coatings

Figure 3. The Parylene Deposition Process

PROPERTIES AND CHARACTERISTICS OF PARYLENE

General Properties

General properties and characteristics of Parylene are listed in Table 1. 

Typical Specifications for Parylenes C, N & D are shown in Tables 2, 3 & 4 respectively.

TABLE 1 - PARYLENE GENERAL PROPERTIES/BENEFITS

• MIL-I-46058C, Type XY Approved
• FDA Approved - USP XXII, Class VI Bio-Compatibility Rating
• Completely Pin-Hole Free Barrier Coating
• Fully Conformal on Any Type of Surface Material or Design
• Inert Transparent Polymer
• Barrier To Oxygen, Moisture, Chemicals, Solvents and Carbon Dioxide
• Thermal Mechanically Stable Between -200^o C And 150^o C
• Extremely High Dielectric 5,000 Volts per Mil ( 0.001”) Minimum
• Excellent Adhesion Properties
• Low-Stress Coating That Does Not Form Sites Prone To Crack Initiation
• Low/Minimal Impact On Package Cooling
• Hydrophobic
• Barrier to Ionic and Moisture Species
• Chemical and Fungal Resistance
• Non-Contaminating Coating and Coating Process. No Solvents, Catalysts or Other By-Products Are Introduced During Coating
• Entire Process Is Accomplished At Room Temperature. Alleviating Temperature Stresses On Parts
• Particle Encapsulation/Immobilization
• No Outgassing (NASA Approved)
• Environmentally Safe
• Superior Abrasion Resistance

TABLE 2 - TYPICAL SPECIFICATIONS OF PARYLENE C

TABLE 3 - TYPICAL SPECIFICATIONS OF PARYLENE N

Table 4 - Typical specifications of Parylene D

Electrical

Thin Film Dielectric Properties
One of the features of parylene is that it can be formed in extremely thin layers. The DC breakdown voltage of parylene films has been determined as a function of polymer thickness, a plot of which is shown in Figure 4. Parylene C is superior to N in this respect for films under 5 microns (0.0002 inches). These data show that both parylenes have respectable dielectric withstanding voltages even below the 1-micron thickness range and the voltage breakdown per unit thickness generally increases with decreasing thickness.

Electrical properties of Parylene C compared to typical values of epoxies, silicones, and urethanes are shown in Table 5.

Insulation Resistance
A critical test of the protection afforded by the parylene coating is to coat circuit board test patterns (as described in MIL-I-46058) and subject them to insulation resistance measurements during a temperature-humidity cycle (MIL-STD-202, methods 106 and 302). In brief, this test consists of ten cycles (one cycle per day), each cycle being in seven steps. The seven steps range from low temperature, low humidity (25 ^oC, 50% RH) to more severe conditions (65^o C, 90% RH). Resistance readings are taken initially and at periodic times at 65^o C, 90% RH during the ten-day cycle.

Results are shown in Table 6 for Parylene C coatings 0.0001 inches to 0.002 inch in thickness. Even for the very thin coating (.0001 inch), the insulation resistance values are about one order of magnitude above the prescribed specification.

Figure 4. Breakdown Voltage of Parylene C & N Vs. Thickness

Table 5 - EleCtrical properties of parylene c vs. other coating types

(1) Properties measured on parylene films, 0.001 inches thick.

Table 6 - Parylene C Insulation Resistance

[ (Ohms) MIL-STD 202 Method 302]

Mechanical

Physical and Mechanical Properties
Physical and mechanical properties of the parylenes are summarized in Tables 2 - 4.

Table 7 shows a comparison of physical and mechanical properties of Parylene versus typical values of Epoxies, Silicones, and Urethanes.

Because of their high molecular weight (~ 500,000) and because the melting temperatures and crystallinity are high, the parylenes cannot be formed by conventional methods such as extrusion or molding. Solubility in organic or other media, except at temperatures above 175^o C, is so low that they cannot be formed by casting.

Impact resistance is high when the parylene polymers are supported on test panels. Gardiner falling ball impact tests on 0.001 inches thick parylene C coated steel “Q” panels are in the 250 inch-pound range.

Wear index values (measured on a Tabor Abraser machine using CS-17 Calibraser wheels and 1000 gram weights) were 22.5 for parylene C and 8.8 for parylene N. By comparison, “Teflon” is 8.4, high impact PVC is 24.4, epoxy is 41.9 and urethane is 59.5.

Parylenes may be annealed to increase cut-through resistance, increase hardness and improve abrasion resistance. This is the result of a density and crystallinity increase.

TABLE 7 - PHYSICAL AND MECHANICAL PROPERTIES OF
PARYLENE C VS. OTHER COATINGS

(1) Properties measured on parylene films, 0.001-0.003 inch thick, except where specified.

(2) Properties dependent on deposition conditions. 

Abrasion Resistance

Coatings are used on circuit boards to provide protection from hostile environments, fungus, shorting and arcing. A stringent test of the ability of the coating to protect the board and its components is the military specification, MIL-I-46058, which qualifies a limited number of epoxy and urethane materials at specified thickness’. These thicknesses’, for sprayed and/or dipped coatings, are measured on flat surfaces and in reality, might have little bearing as to what the actual coating coverage is on the most critical areas of the board; edges, corners, depression walls, and points.

This section describes a technique and the results for measuring the amount of mechanical properties one can expect from various coatings and their methods of application when applied to a critical area of a circuit board, a sharp point. A comparison is made between the vapor deposited parylene C, a multiple dipped epoxy, and a sprayed urethane. 

By comparison of abrasion results acquired with these materials and by photomicrograph comparison of the coverages obtained, it is demonstrated that a 2.5 um coating of parylene C will afford the same protection as will an epoxy or urethane coating at more than ten times this thickness.

Abrasion-Test

Several standard methods for estimating the mechanical protective ability (handleability) of a conformal coating material exist, among them being Tabor Abrasion drop tests, cut-through tests, puncture tests, Sward hardness, etc. None of them, however, specifically relate the particular material property to its performance when applied to the circuit board. Therefore, a method was devised to measure the abrasion of the conformal coating at the most critical area of the circuit board, on a sharp point. For test standardization purposes, a metal phonograph needle was used to simulate a typical sharp point.

An abrasion testing apparatus was assembled which resembles a phonograph. The turntable portion maintains constant speed under a wide variety of load conditions and is fabricated from chromium plated brass. The arm is electrically insulated from the turntable and holds the needle being tested; it is counterbalanced by a weight on the opposite side of the fulcrum. A table above the needle is used to apply a known load to the 60um radius point.

A continuity meter is attached between the needle and turntable; this meter automatically turned off the apparatus when continuity between the two was established. Since the coatings tested are dielectrics, ohmic contact would occur only when the coating was worn away (thereby exposing the metal needle tip) by the rotating turntable. At this point, failure is indicated and the turntable revolutions and time to failure were recorded.

The needles were coated with either Parylene C, Epoxy (dip coat) or Urethane (sprayed).

The epoxy was applied to a set of needles attached to the periphery of a cork by dipping as a set into the liquid. The needles were removed to cure with the points facing up. Two sets of epoxy-coated needles were evaluated; one set using three such dip and cure steps and the second set using four. Sets evaluated with only one or two epoxy applications indicated no coating on the point (as evidenced by ohmic continuity between the needle and turntable), although a build-up of epoxy nearly 3mm in diameter had accumulated at the base of the needle.

Additional attempts to obtain an epoxy-coated point by other means were also made. The needles were (a) coated and cured points down, (b) coated and cured in a silicone mold, and (c) in a continuous dipping and curing cycle with the needle held at 45o to the horizontal (point down). The first two methods were completely unsuccessful (no coating at the point), the third did coat the point but required such a mass of epoxy as to make the method untenable.

The urethane was applied in several spray cycles. The needles, set in a rubber cork, were rotated during the spray operation. The procedure was to use two wet passes, a cure cycle, two additional wet passes, and a second cure cycle. The thickness of the urethane on a flat surface coated concurrently was 90-100 microns. (≈ 4 Mils) However, as indicated by continuity testing and micro-graphic cross-sectioning, the tip of the needle was bare. No further attempt to obtain a coating on the needle was made because of the extremely large amount of material that would have to have been deposited on the concurrently coated flat surface.

Coating Thickness and Coverage

The coating thickness was determined by potting and cross-sectioning the needles and then measuring the thickness on a calibrated micrograph. For the parylene-coated needles, the substrates were sectioned perpendicular to the long axis of the needle. Since the coating is uniform, the point of sectioning was relatively immaterial. Three points were randomly selected from each cross-section and three needles were used (nine points total ).

Obtaining the thickness of the epoxy on the needles was more difficult because of the extreme non-uniformity of the coating; i.e., the coating at the point was much thinner than the coating further away from the point. For example, it was found in one instance that for a needle that had an epoxy thickness of 8.6 um at the tip, the epoxy thickness just 1mm away from the tip was 108 um.

For this reason, the epoxy needles had to be sectioned parallel to the long axis of the needle, a very tedious and delicate task. The thickness of the epoxy was then measured as cited above for parylene.

This disparity of coverage uniformity for sprayed/dipped coatings is not peculiar to the tip of a needle. Circuit boards have several areas which will not receive an even coating of applied protective material; the corner or edge of an object, the space between two closely spaced components, the walls of a hole, etc. A good illustration is the edge of the Y shaped conductor pattern which is used as the test vehicle for MIL-I-46058. A cross-section of such a pattern shows where the epoxy material is shown to be non-uniform in coverage at the copper conductor edges and between the two conductors whereas the thickness of the parylene is the same on all surfaces. 

Abrasion Model and Results

The rate of abrasion of a coating will be a function of (a) the force applied to the coated object and (b) the surface area presented. Logically, the greater the force causing abrasion, the faster will be the rate of wear. Also, the smaller the area being abraded, the faster the rate of wear (constant load conditions). 

If “W” is the weight of coating removed in distance “X” by the application of a load, “F”, then

where “k” is a constant. The weight removed can be related to the volume removed by the material density (p) and in turn, the volume is relatable to the area presented (A) and the thickness removed (y).

Substituting for the “W” and differentiating leads to

where “k” is being defined as the “wear index”.

In the case of a needle abraded in the vertical position, it can easily be shown that the area presented at any time is

where “R” is the radius of curvature of the needle tip and “t” is the total thickness of the conformal coating.

Equation (3) can be substituted into Equation (2), which upon integration yields:

From Equation (4 ) it can be seen that when the coating thickness is insignificant compared to the radius of curvature, the time to abrade varies with the square of the coating thickness. When the coating thickness is large in comparison to the radius of curvature, the abrasion time increases with the cube of the coating thickness. In any instance, for a constant coating thickness, the abrasion time is inversely proportional to the force applied to the point.

The validity of Equation (4) was tested by plotting the distance to failure against (1) the thickness function and (2) the reciprocal force. Figure 5 shows the former at a constant load of 5.0 grams for parylene thickness’ of 3.3 um to 38.3 um and for epoxy thickness’ of 8.6 um and 22.4 um. These thickness’ were those measured at the needle tip as previously described. The two lines are parallel but have a slope slightly higher than unity because the needles were abraded somewhat off the vertical position.

As previously stated, at constant thickness, the distance to failure should vary inversely with the force applied (F). The data is plotted in Figure 6 showing this to be the case. Both constant thickness lines extrapolate to the origin indicating to time to failure would be instantaneous at an infinite loading (which is not quite true as there is a finite load which would puncture the film).

Where T = π + ² (Rt 2/3 t) and is defined as the “thickness function”.

Figure 5. Parylene C Wear Index Vs. Epoxy

Figure 6. Comparison of Parylene Wear Index at Two Different Thickness

From Figure 5, the wear index, k, can be calculated as follows:

These values are valid when the force applied is expressed in grams, the coating thickness is in microns and the distance to failure is in centimeters. Therefore, in the thickness level range tested, at the equal thickness and equal force, the parylene C is about seven times more wear resistant than is the epoxy.

Abrasion Test Applied Results

It would be useful to translate these results into a meaningful comparison. For example, how far can one abrade a circuit board along a hard surface before the coating will wear through and expose the metal tip of a clinched wire.

Let us assume that a typical circuit board, with all components, weighs 60 grams. Each feedthrough connecting the wire on the board measures 0.5mm diameter, thus the minimum radius of curvature of a clinched point would be R = 0.25mm. The minimum contact of the board, when set onto a flat surface, would be three points, therefore, each point would support 1/3 of the weight or 20 grams. Figure 7 relates the distance this board may be moved with various parylene and epoxy thickness’ using the above data and the values of “k”, the wear indices, found in this study.

From Figure 7, one can see that parylene coated boards can be moved much farther than the epoxy at equivalent thickness’. For example, at 25 microns, the parylene C coated board may be abraded through a distance of almost 9 meters before the clinched metal wire is exposed; the epoxy at 25 microns would be able to travel only 1.2 meters before failure. 

MIL-I-46058 qualifies epoxy systems from a minimum of 25microns to a maximum of 75 microns ( 0.002 +/- 0.001 inch ).

This, of course, assumes that when the circuit board is coated with epoxy, the points will be coated to the same thickness’ as the flat land areas. As evidenced by photomicrographs and by the experience with the urethane, this is hardly the case. In fact, from the micrograph, it would be safe to assume that there is a minimum of a 10:1 disparity between the point thickness and the thickness at a very small distance from the point (<lmm). If these criteria were used, then the maximum allowable 75-micron epoxy coating would produce no more than a 7.5.micron coating on the points in question. From Figure 7, the abrasion resistance of 2.5 microns of parylene C would be equivalent to 7.5 microns of epoxy on the point or 75 microns of epoxy on a flat surface.

Abrasion Test Summary

The ability of a conformal coating to protect a circuit board from hostile chemical, biological and mechanically damaging environments will depend upon the coverage which can be obtained with the coating as well as its inherent protective properties. It has been demonstrated in this article that the spraying or dipping of epoxy and urethane systems leave the most critical areas of the circuit board with the least amount of protection. The vapor deposited parylene C, however, is shown to afford an equivalent amount of mechanical protection at less than one-tenth the thickness of a dip-coated epoxy because of its superior coverage and thin film abrasion.

Mechanical Characteristics of Parylene versus Other Coating Types

Figure 8 shows a comparison of Parylene C, Epoxy and Polyurethane hardness as measured using the “ Sward “ scale.

Table 8 lists the standard “ Taber “ wear index for both parylene C and N in relation to

Urethane and Epoxy. Table 9 Lists “ Pencil Hardness “ for Parylene C, Urethane and Epoxy and Table 10 compares impact resistance for these same materials.

By analysis of this data, it is evident that Parylene C provides superior mechanical protection at thickness’ utilized for conformal coating applications.

Figure 8. Parylene C Hardness Vs. Polyurethane and Epoxy (Sward Number)

TABLE 8 - TABER “WEAR INDEX” FOR CONFORMAL COATING MATERIALS

TABLE 9 - PENCIL HARDNESS FOR CONFORMAL COATING MATERIALS

TABLE 10 - IMPACT RESISTANCE OF PARYLENE C VS. URETHANE & EPOXY

Thermal

The coefficients of thermal expansion of the parylenes are similar to epoxies: approximately 35 ppm/degrees C vs 27 to 30 ppm/degrees C for most epoxy molding compounds.