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CONFORMAL COATINGS USED BY VITEK FOR ELECTRONIC
AND ELECTRICAL APPLICATIONS (Condensed version)
TABLE OF CONTENTS: Click title to view topic
APPLICATIONS AND FUNCTIONS OF CONFORMAL COATINGS
APPLICATIONS AND FUNCTIONS OF CONFORMAL COATINGS
The organic coatings provide an extremely important and economical means of protecting electronic and electrical devices and components by encapsulating and acting as a barrier against adverse environments.
One of the most important application is to protect PRINTED CIRCUIT BOARDS and MODULES from airborne contaminants, moisture, vapors, fungus, salt spray and corrosive chemicals. Printed circuit boards are usually coated as the last step, after the components have been assembled and joined. Application of a conformal coating greatly improves the reliability of the entire assembly. This is particularly important for printed circuit boards used in certain industrial, commercial, military and space applications, where a long operational life is expected and the expense of replacing with a completely new assembled board may be very high.
Besides printed circuit boards, the conformal coatings are been used to protect many other electronic and electrical devices and components:
- INTEGRATED CIRCUITS
- HYBRID CIRCUITS
- SEMICONDUCTORS
- DISK DRIVES
- TRANSDUCERS and SENSORS
- MINIATURE INDUCTORS and MOTORS
- FERRITE CORES of PULSE TRANSFORMERS
- MAGNETIC RECORDING HEADS
- ELECTRETS
- THERMOCOUPLES, CAPACITORS, RELAYS, SWITCHES, etc.
It has often been demonstrated that unprotected electronic assemblies exposed to severe environments fail because of moisture penetration, degradation of insulation properties, electrical shorting or corrosion.
PARAMETERS AFFECTING ELECTRONIC AND ELECTRICAL DEVICES
Stresses and Stress Relief
Coatings can be used to minimize or eliminate the stresses imposed by manufacturing processes, such as molding and casting or during handling, installation and service and to protect electronic components from vibration and shock. The stress-relief coatings must, however, be flexible in addition of being intrinsically stress-free in order to avoid cracking.
Purity The degree of purity is especially critical for materials that are to be applied directly to active microelectronic devices. The most critical impurities are ions, both cations (positively charged) and anions (negatively charged ions) which can severely impair the insulating properties and drastically change the electrical characteristics of a device, and under certain conditions cause corrosion of its metal portions.
Other impurities consist of unreacted organic chemicals, outgassing products and additives.
Moisture Resistance and Transmission
Moisture can enter a plastic encapsulated device by: absorption and transmission through the coating polymeric layer, by transmission through coating's pinholes, voids or microcracks or by penetration along the device leads. The amount of moisture that will diffuse depends on several factors including: the coating thickness, the type of polymer used in the coating and its permeability, the undesired existence of pinholes and voids, the exposed surface area and the duration of exposure.
The effect of the presence of moisture cab be observed by an initial decrease of the insulating characteristics and ultimately an electrical failure and even the corrosion of metal portions of the device.
For instance literature sources have shown that while the uncoated circuit boards have a relatively poor resistance to humidity (as measured by insulation resistance in ohms after several cycles in a humidity chamber), the insulation resistance of the same boards coated with conformal coatings is substantially higher (by as many as 4 to 5 orders of magnitude).
Adhesion
Strong adhesion of plastic conformal coatings to substrates is essential both to create a satisfactory seal against moisture and contaminants and to allow the encapsulating plastic to dissipate mechanical stresses.
Particle Immobilization
It is desired that any particles that remain on the assemblies after cleaning process or that sloughs from the devices after sealing do not migrate and cause electrical shorts. The function of the coating is to freeze in place all loose particles and to protect the circuit from any particles that may subsequently be generated by vibration, testing or actual use.
MAIN FUNCTIONS OF CONFORMAL COATINGS
To hold delicate components and fine interconnection wires together, allowing them to be handled, dropped or vibrated without electrical or mechanical damage. This function is called "ruggedization". To impart stress relief. Stresses arising from the molding operation, resin shrinkage, differences in coefficients of expansion between the components and substrates, thermal cycling and thermal shock may be equalized and dissipated by using barrier coatings. To provide some degree of moisture, gas, dust and chemicals protection, thus preventing corrosion and electrical malfunctioning. To provide an insulating barrier, thus preventing electrical shorting.
CONVENTIONAL CONFORMAL COATINGS
When applied properly, conformal coatings can also enhance the printed circuit reliability by eliminating detrimental conditions such as leakage from high impedance circuits, and allowing closer circuit traces required with high component density. The selection of the correct coating for a particular application can be a critical factor in the overall performance of the circuit or assembly.
The Military Specification MIL-I-46058, originally promulgated in 1966, established performance criteria for protective circuit boards conformal coatings used for military equipment. This specification has come to serve as the unofficial standard for circuit components used in physically challenging commercial and industrial applications as well.
The Qualified Products List (QPL) for MIL-I-46058C includes two categories of conformal coatings
Conventional Type: Acrylic, Epoxy, Polyurethane, Silicone and Paraxylylene (Parylene) and identified them as:
- Type AR Acrylic
- Type ER Epoxy
- Type UR Polyurethane
- Type SR Silicone
- Type XY Polyxylylene
MAIN CHARACTERISTICS OF CONVENTIONAL COATINGSS
PARYLENE BASED CONFORMAL COATINGS
Parylene is a commercial name for polymers which belong to a unique chemical family: poly-p-xylylene.
In contrast with conventional coatings, Parylene is using gaseous monomers which are polymerized when are applied on various substrates under vacuum at room temperature and provide the ultimate conformal coating with outstanding combination of physical, thermal, barrier and electrical properties.
MOST IMPORTANT FEATURES OF PARYLENE
Uniform thickness and true conformality - the process guarantees precise control of coating thickness and uniformity, especially critical in micro-electronics applications; no bridging, thin-outs, puddlings, run-offs which are common problem with conventional coatings. The polymerization from the gas is not line-of-sight and therefore can penetrate complicated substrate designs, that typical conformal coating will bridge-over.
Chemical and fungal resistance - Parylene resists attack from exposure from most solvents, acids and bases and inhibits the growth of fungus. Also its absorption and resistance to water is very good.
Impressive mechanical strength - Even if it is quite flexible, Parylene is used for encapsulating microcircuits because it has very high tensile strength and it is capable to increase the pull strength of wire and lead bonds, face bond chips and conductor bridges and therefore contributes significantly to device integrity.
Thermal stability - Parylene coatings have good mechanical properties from -200 to 275 deg.C and remain stable at continuous temperatures as high as 130 C in air or 220 C in the absence of oxygen.
Noncontaminating components - The polymerization process of Parylene occurs under vacuum using only a gaseous monomer, therefore it contains no potentially migratory ingredients, additives or by-products.
Pinhole free - tough coatings as thin as 0.1 microns can be achieved without any voids or pinholes.
Superior barrier properties - Parylene provides exceptional corrosion protection from moisture, salt spray, corrosive vapors and other hostile environments. Its water vapor transmission rate has been found to be significantly lower than of conventional coatings. Parylene also acts as a barrier to migrating ionic species.
High dielectric characteristics - Its extremely high dielectric strength combined with an electrical stability over a wide range of temperatures provide unique insulating properties. The resistivity is high and the dielectric constant and dielectric losses are low and unaffected by absorption of water vapors.
Stress-free - The coating process on the substrate takes place at room temperature, therefore there are no thermal or mechanical stresses introduced during application.
Particle immobilization - Since the polymerization of Parylene occurs by "growing" layer by layer, the coating film encapsulates all loose solders, wire particles and other mobile debris left from manufacture. Pressed powder parts, ferrites, ceramics, corrosive metals, glass and epoxy particulates can be positively stabilized.
* Vitek Research Corporation has developed techniques which insure high degree of adhesion between Parylene coating and the surfaces of biomedical devices.
