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PARYLENE CONFORMAL COATINGS USED BY VITEK CORPORATION
   FOR ELECTRONIC AND ELECTRICAL APPLICATIONS

THE DEPOSITION PROCESS OF PARYLENE

PROPERTIES OF PARYLENE COMPARED WITH CONVENTIONAL COATINGS

CHEMICAL AND SOLVENT RESISTANCE

CONTAMINATING IMPURITIES AND PARTICLES IMMOBILIZATION

A RATING COMPARISON OF THE CONFORMAL COATINGS

PARYLENE CONFORMAL COATINGS USED BY VITEK CORPORATION FOR ELECTRONIC AND ELECTRICAL APPLICATIONS

Parylene is a commercial name for a unique polymeric family: poly-para-xylylene. It is considered by many as the ultimate conformal coatings for protection of sensitive devices and components from adverse environments such as: moisture, salt spray, dust, fungus, corrosive vapors and liquids.

Parylene has been successfully used as conformal coatings in many electronic and electrical applications:

• Printed circuit boards - Hybrid circuits - Microelectronics -Ferrite cores and bobbins
• Miniature servo motors -Capacitors -Small rotors and stators -Pressure transducers
• Thermistors -Relays -Memory heads -Automotive sensors
• Memory cores -Potentiometers -Disk drives -Implanted bioelectrodes
• Thermocouples -Switches -Semiconductors -Computer keypads

Other successful applications include such diverse end-uses as Optical devices, Radiography equipment, Digital displays, Nuclear energy equipment, Biomedical devices and instruments and Space equipment.

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THE DEPOSITION PROCESS OF PARYLENE

The basic member of the poly-p-xylylene series is Parylene N - a completely linear, highly crystalline material. The other commercially available members are Parylene C and Parylene D which originate from the same original monomer, modified only by the substitution of one or two aromatic hydrogens with chlorine atoms. The deposition process is not line-of-site and since all sides of the substrate are uniformly impinged by the gaseous monomer, the final polymeric layer deposits as a true conformal coating.

The deposition process require a special machine and consists of three steps:

1. Sublimation under vacuum at approximately 120 deg.C of the stable crystalline dimer di-p-xylylene to produce vapors of this material
2. Pyrolysis of the vapors at approximately 650 deg.C to form gaseous reactive monomer para-xylylene.
3. Deposition and simultaneous polymerization to form poly-p-xylylene or Parylene N. The monomer enters the deposition chamber where it is adsorbed and polymerizes under vacuum at room temperature on the surface of the component intended to be coated.

A diagram of the parylene process

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PROPERTIES OF PARYLENE COMPARED WITH CONVENTIONAL COATINGS

CONFORMALITY AND THICKNESS UNIFORMITY
In contrast with liquid coatings applied by spraying, dipping or brushing, Parylene does not pull away from edges or exhibit meniscus forces. Compared to the liquid coatings that when flow between and beneath components tend to glue them to one another, Parylene does not bridge even in apertures as narrow as 7 microns (0.25 mils) and has the ability to penetrate beneath and around closely spaced components. And because the buildup is consistent and even, physical and electrical protection can be achieved with a substantially thinner layer than conventional coatings, which often require heavier applications to overcome coating imperfections.

Another important advantage of Parylene is the fact that it can be deposited in very thin layers: 0.1 to 50 microns (0.004 -2 mils), while the typical thickness of the conventional coatings is within 25 to 250 microns (1-10 mils).

STRESSES
Unlike conventional coatings, especially acrylic and silicone based, which can introduce stresses to the electrical components during the curing or drying cycles, Parylene can be stretched up to 200% before breaking and retains the ability to remain flexible over a wide temperature range. For instance, in the coating of ferrites, the elasticity of Parylene avoids any changes in the magnetostrictive properties of the ferrite material.

ELECTRICAL PROPERTIES
Overall, Parylene's electrical properties are superior to the electricals of conventional coatings: it has high dielectric strength and volume resistivity and low dissipation factor and dielectric constant.

An important characteristic is the fact that its dielectric constant and dissipation factor are practically insensitive to increased frequency.

RESISTANCE to HUMIDITY
Parylene's insulating resistance exhibits little deterioration after several humidity and temperature cycles of a circuit-board screening test per MIL-STD-202,method 302.

BARRIER PROPERTIES
The permeability of Parylene to moisture and most gases is extremely low and substantially lower than most conventional coatings, in some cases by many orders of magnitude.

PHYSICAL AND THERMAL PROPERTIES
The Parylene film has very high tensile strength but maintains its flexibility over a wide temperature range. Its surface is lubricious (it has a low coefficient of friction) and at the same time its abrasion resistance is one of the best of any organic coatings.

The Parylene's water absorption is good and it is lower than the urethanes' water absorption.

The Parylene polymers have some of the highest melting temperature of any plastics (in range of 300 deg.C), are good thermal insulators and at the same time their thermal coefficient of expansion are much lower than those of the conformal coatings based on urethane and silicone.

Typical Properties of Commercial Parylenes versus other coatings
Property	Parylene N	Parylene C	Epoxies	Silicones	Urethanes
ELECTRICAL PROPERTIES
Dielectric Strength, short time, volts/mil ( on .001" thick film)	7,000	5,600	-	-	-
Volume resistivity @ 23deg.C 50% RH, ohm-cm	1017	1016	1012-1017	1015	1015
Surface resistivity @ 23deg.C 50% RH, ohms	1013	1014 -	-	-	-
Dielectric Constant 60 Hz 1,000 Hz 1,0000,000 Hz	2.65 2.65 2.65	3.15 3.10 2.95	3.15-5.0 3.5-4.5 3.3-4.0	2.75-3.05 - 2.6-2.7	4.0-7.5 4.0-7.5 6.5-7.1
Dissipation Factor 60 Hz 1,000 Hz 1,0000,000 Hz	0.0002 0.0002 0.0006	0.020 0.019 0.013	0.002-0.01  0.002-0.02 0.003-0.05	0.007-0.001  - 0.001-0.002	0.015-0.017  0.05-0.06 -
PHYSICAL AND MECHANICAL PROPERTIES
Secant (Young) Modulus,psi	350,000	400,000	350,000	900	1,000-10,000
Tensile Strength,psi	6,000-11,000	10,000	10,800	800-1,000	175-10,000
Yield Strength,psi	6,100	8,000	-	-	-
Elongation to break,%	20-250	200	3-6	100	100-1,000
Density,g/cc	1.10-1.12	1.289	1.11-1.40	1.05-1.23	1.10-2.5
Index of refraction	1.661	1.639	1.55-1.61	1.43	1.50-1.6
Water absorption/24 hrs.,%	less than 0.10 %	less than 0.10 %	0.08-0.15	.12 (7 days)	0.02-1.5
Rockwell Hardness	R85	R80	M80-M110	40-45 Shore A	10-250 Sh
Coefficient of Friction Static Dynamic	.25 .25	.29 .29	- -	- -	- -
THERMAL PROPERTIES
Melting Temperature, deg.C	405	280	cured	cured	170 or cured
Heat distortion Temperature,degC	-	-	up to 220	up to 300	-
Linear Coeff. of expansion, 10-5 /degC	6.9	3.5	4.5-6.5	25-30	10-20
Thermal Conductivity, 10-4cal/sec/sqcm/cm	3	-	4-5	3.5-7.5	5
BARRIER PROPERTIES
Moisture transmission, g/mil/100in/24 hrs	1.6	0.21	1.79-2.38	4.4-7.9	2.4-8.7
Gas permeability, cc-mil/100sqin/24 hrs. to: Nitrogen Oxygen Carbon Dioxide Hydrogen	15 55 420 540	0.6 5 14 110	4 5-10 8 110	- 50,000 300,000 45,000	80 200 3,000 -

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CHEMICAL AND SOLVENT RESISTANCE

Due to its unique molecular structure, the Parylene polymers are practically insoluble in all known organic solvents up to 150 deg.C and very resistant to the most of the inorganic reagents including strong acids and alkali. They are also effective barriers to corrosive agents.

In contrast, some of the polymers used in conventional conformal coatings, especially the ones based on acrylics and silicones have relatively low to moderate chemical resistance and some, such as silicones are permeable to corrosive chemicals.

ADHESION
In principle, its special chemical composition causes the Parylene to more difficult to be bonded to various substrates versus the conventional coatings.

However, after extensive R&D efforts the Vitek Research Corporation has been able to develop special techniques that insure a high degree of adhesion between the Parylene conformal coating and the surfaces of the electronic and electrical devices.

REPAIRABILITY
Since it is so different in chemical composition, coating technique, applied thickness and surface characteristics from the conventional resins type, Parylene presents unique set of circuit repair issues as well as offers certain benefits.

For example, the acrylic based coatings can be easily removed from circuit boards with proper solvents, using selective spot or overall dip exposure. However, this ease of repair is offset by the vulnerability of the acrylic material as well as of the material used to fabricate the circuit boards to potential solvent contaminants. Further, coating removal yields a contaminated solvent residue that is flammable and requires proper disposal. Also, the recoating of acrylic coated boards must be done in a special booth that is equipped to handle organic volatiles.

The urethane coatings are not vulnerable to solvents and their removal is generally accomplished by thermal softening followed by physical removal using special tools or by microblasting. However caution must be exercised in the case of heating or burning is used to remove them, because toxic fumes may be generated.

The epoxy coatings present the same removal issues as the urethane coatings, and are even more difficult to remove because they are more compact and have even higher hardness.

Silicone coatings are relatively easier to remove because are softer and are relatively vulnerable to some organic solvents. However, much attention has to be given to the silicone's known tendency to migrate and contaminate the adjacent manufacturing areas and negatively affecting the bonding processes.

In contrast, Parylene resists to all known organic solvents and its melting or burning temperatures generally exceeds that of plastics which form the circuits structure.

Typically Parylene can be removed by the following techniques (in some cases helped by heat softening): mechanical abrasion or air microblasting with a very focused strong stream of particles, incision and suction, plasma etching, or by excimer laser emission.

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CONTAMINATING IMPURITIES AND PARTICLES IMMOBILIZATION

One of the advantages of using Parylene as a coating in electronic circuit boards is its capability to immobilize loose particles, such as small fragments of conductive materials which can cause electrical shorts if they bridge wire bonds or conductor lines. These very fine particles are difficult to remove by conventional cleaning because of their strong electrostatic attraction to the circuit.

Insulation materials containing ionic impurities in contact with electrical devices can adversely affect their electrical parameters. These effects and the decrease in electrical insulating properties of coatings are augmented by an increase in mobility of the charged carriers, brought about by moisture, electrical stress or elevated temperature.

Most commercially available coatings contain ionic impurities as such or generate them in the presence of moisture or an electrolyte. The ions may be produced from the reactants used in the synthesis of the resin, catalysts, hardeners, from additives such as fillers, thixotropic agents or flame retardants used in the formulation or from reactive outgassing products that may evolve from the conventional coatings during the curing process. Also some coatings such as the silicone type tend to migrate and contaminate.

In contrast, Parylene polymerizes under vacuum from a gaseous monomer without the use of any solvents, catalysts or additives and therefore it produces a truly pure coating.

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A RATING COMPARISON OF THE CONFORMAL COATINGS

The following table compares the main properties of the conventional coatings: Acrylic, Urethane, Epoxy, Silicone and Parylene coatings in relation to the parameters that affect the most the proper function of the electronic and electrical devices.

PROPERTY	ACRYLIC	URETHANE	EPOXY	SILICONE	PARYLENE
Uniform, very thin, conformal layer	G	G	G	G	E
Low stress, pin-hole free layer	M	M	M	M	E
Dielectric properties	G	M	M	VG	E
Physical strength	G	VG	VG	M	v
Flexibility	M	VG	L	VG	VG
Wear and abrasion resistance	M	VG	VG	L	E
Thermal coefficient of expansion	G	M	VG	L	E
Water absorption	G	G	VG	M	E
Chemical, solvents, fungus resistance	L	VG	VG	M	v
Barrier to moisture, gases, liquids	VG	G	VG	M	E
Adhesion to substrates	VG	G	VG	M	G*
Repairability	VG	G	L	M	G*
No contaminating ingredients	G	G	G	L	E
Particles immobilization	L	L	L	L	E

The rating used are: E= excellent, VG= very good, G= good, M= moderate, L= low.

* Vitek Research Corporation has developed techniques which insure high degree of adhesion between Parylene coating and the surfaces of biomedical devices.

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