Using Parylene to coat Printed Circuit Boards (PCB)
PCB units are quite common in the industry and have been incorporated into many applications. In most cases, PCB units must be sealed in accordance with the complexity of the application they are implemented into; with qualities like:
- High dielectric strength
- Resistance to liquids and gases.
- No stress introduced by coating.
- Intrinsically safe (explosion proof).
Excellent mechanical characteristics such as:
- Low plasticity
- Stretching tolerance
- Fracture toughness.
All of these place Parylene at the forefront of circuit board sealing technology; moreover, Parylene increases the mechanical strength and endurance of the surrounding flexible connectors and wiring.
Due to its ability to shrink and expand after polymerization process, Parylene coatings forgo the need to be cured and decrease the potential damage to the circuit board and all of its components.
One of the undesirable characteristics of printed circuits is their tendency for moisture absorption, due to the hygroscopic nature of epoxy (the basic material for building 4FR circuit boards).
Water absorption is a combination of:
- Size and geometrical shape of the bundle
- The ratio between the amount of epoxy and metal components
- Type of epoxy
- Atmospheric conditions at time of operation
In cases where precaution and prevention measures are not exercised, absorbed moisture diffusion will occur and penetrate the interface between the Epoxy layer and the electrical processor. Surface cracks and delamination along connectors are the likely candidates for penetrating water and moisture. Post soldering exposure to heat is the likely cause weakening the components. More serious failure may occur if contaminated ions such as chlorine (Cl) or sodium (Na) were absorbed in addition to water, from sources such as flex rinse fluids.
Comparison tests performed for the U.S. Air Force in a combined effort with the Hughes Aircraft Company have shown that Parylene coating reduces electrical circuit failure. In fact, time to first failure point is improved by a factor of 2 using Parylene coating.
Other conventional popular coatings i.e. acrylic, silicone, epoxy and urethane applied in liquid state (submersion, brushing or spraying) tend to change their volume during vulcanization or polymeric processing. The crevices between the PCB components may accumulate some of the material thus causing components to lift, separate and possibly even break from the PCB unit. On the other hand, material that tends to shrink during the polymerization process will attract neighboring components towards each other, causing shortening or disconnection. In contrast to these conventional coatings, Parylene does not impose any stress load on the electronic parts.
Parylene’s unique application method using Chemical Vapor Deposition (CVD) differentiates it from other coating methods by its continuity, conformity and totality (pin-hole free). Parylene can coat sharp edges and corners and has proven to be able to penetrate areas with minor coating accessibility; making the coated device moisture and corrosion resistant and preventing unnecessary mechanical damage to the board.
Moisture protection and adhesion
Another important quality of Parylene is its ability to adhere to epoxy material. Parylene’s thermal expansion coefficient is roughly 35 ppm vs. other epoxy substances with typical thermal expansion coefficient of 27-30 ppm.
Protecting the conducting wiring in electrical boards from being exposed to moisture damage is critical; therefore, good adhesion is necessary. Proper adhesion will guarantee proper insulation between the wires and will help prevent moisture related shortcuts.
Parylene coating carries an additional unique and meaningful advantage: as a result of the fact that Parylene’s monomer is dissolved when used with epoxy material, a layer of Parylene polymer is generated underneath the epoxy surface area, creating a combined epoxy “net like” structure, which is an effective adhesion mechanism.
It is worth noting that in an experiment conducted by several companies, at Simtal, using a circuit board embedded with Parylene type C and placed in a sodium-mist compartment for a period of 144 hrs. Following standard ASTM B117 – 03, no apparent corrosion or sodium infiltrating the board was detected. Additionally, experimenting with various chemicals in temperatures up to 150° Centigrade, Parylene held its own and did not dissolve.
Using Parylene for wire bonding
The process of using Parylene and wire bonding has recently been gaining speed. It has become indispensable in applications that call for minimization/miniaturization of parts on one hand, and reducing over-all weight as a result of the tiny component sizes that make up the infrastructure, on the other.
The process of wire bonding incorporates the use of gold, silver or copper wires in widths ranging in 8-18µ. These fine wire widths impose a challenge on their ability to withstand the impact of missile or rocket acceleration and military aircraft deceleration upon landing (i.e. aircraft carriers) making them extremely vulnerable. The situation described calls for the need to strengthen the wires and in particular their soldering points (bonding – “mushroom and stitch”) on either side of the pad. Similar conventional coating products failed to meet the stringent specifications imposed by frequent modifications in volume/length that take place during the processes of polymerization and drying. In contrast with those conventional coating products, Parylene displayed qualities that greatly fortify the ability of the wire to resist breakage, along with fundamentally reinforcing the connection points with the underlying pad; all that without over-stressing the system, as the Parylene polymerization process does not affect changes in volume!
