Conformal Coating and Thermal Shock Resistance
When electronics have to withstand harsh environments and impact, protection is provided with conformal coatings or potting compounds. Potting is the process of filling a complete electronic assembly with a solid or gelatinous compound for protection but is heavy in comparison to conformal coating and is harder to inspect, test, and repair.
A conformal coating is a protective chemical coating or polymer film 25–75 micrometers thick (50 micrometers is typical) that conforms to the contours of a printed circuit board. It protects the board's component from moisture, dust, chemicals, and temperature extremes that, if uncoated (non-protected), could result in damage or malfunction.
By providing electrical insulation, it maintains long-term surface insulation resistance (SIR) levels and ensures the operational integrity of the assembly. It also provides a barrier to airborne contaminants from the operating environment, such as salt spray, preventing corrosion.
Traditional Conformal Coating Materials
Conformal coatings are used across a diverse selection of electronics, from everyday household appliances and automotive to spacecraft applications, so selecting the correct conformal coating for the job is vital. Traditional coating materials include acrylic resin, silicone resin, and urethane (or polyurethane) resin.
Thermal Shock Testing
With the drive for increased functionality for end devices, and increased life cycles being placed on all electronic assemblies, the need to have materials endure more stringent testing becomes increasingly vital. Thermal shock testing is widely used as the most stringent indicator for qualification purposes of today’s most reliable printed circuit boards.
Standard technology conformal coatings are not designed to meet those needs and new technologies. Next generation systems, such as those found in the new synthetic rubber and UV offerings, are needed to assure success. These relatively new product technologies provide resistance to long-term thermal shock induced defects through improvements in the thermally stable physical properties such as CTE, Tg, elongation, and modulus.
Thermal shock testing has traditionally been used to allow OEMs to simulate long-term life cycle performance. These tests are widely used in various industries where the normal operating temperature can fluctuate greatly, such as automotive circuit boards, outdoor lighting, and agricultural applications. Standard testing involves chamber temperature fluctuations between 40C to 85C, with temperature change gradient of 20C per min. There are some thermal shock tests with greater and lesser temperature changes, however, the -40C to 85C seems to be most widely used.
Long-term thermal shock is considered to be greater than 1,000 cycles, with some reaching up to 3,000 cycles. Most existing chemistries will not pass such stringent test requirements, so next generation conformal coating technologies are needed.
Commonly observed thermal shock defects include coating cracking, loss of adhesion, and blistering. These are caused by stresses that manifest as a result of the thermal excursions. Plastic deformation progression and adhesion loss are exaggerated with the acceleration of temperature induced stresses. For example, blistering may start at a small localized point as particulate or residue on board which creates a lower point of adhesion. This small point is then stressed during the thermal cycle and becomes a blister.
New technology coatings counteract the stresses created by thermal shock by remaining more flexible and less susceptible to thermal stress induced defects. They include synthetic rubber, UV-curable and VOC free coatings, epoxy resins, parylene coating, and fluorocarbon (or ‘nano’) coatings.
Epoxy resins are available as a single part or two-part compound. Single part compounds are cured thermally or by UV exposure. Two-part compounds begin to cure as soon as they’re mixed together. Both systems provide the same benefits. They have good moisture and dielectric resistance. They also have excellent temperature and chemical resistance. Epoxy also has good resistance to abrasion and is rigid. However, it is nearly impossible to rework. As with acrylic, polyurethane, and silicone, epoxy can be applied by brush, spray, or dipping.
Fluoropolymer coatings are composed of polymers that contain fluorine. Fluoropolymer coatings are costly compared to analogous hydrocarbon acrylates, epoxies, silicones, and other coating types. In addition, special equipment is required to manufacture most of these fluorinated compounds. Fluoropolymer surface modifier coatings are used in environments that require high reliability, high water oil, and silicone resistance, high resistance to microbiological attack, a very low dielectric constant, a very low ion migration rate, and weatherability.
Parylene is considered by many to be the ultimate conformal coating for protection of devices, components and surfaces in electronics, instrumentation, aerospace, as well as medical and engineering industries. Parylene is unique in being created directly on the surface at room temperature, with no liquid phase. It is chemically stable and makes an excellent barrier material, has excellent thermal endurance, as well as excellent mechanical properties and high tensile strength. However, the material is expensive, has poor bonding characteristics with certain metals, and the process requires special equipment to produce and can be very time-intensive.
Synthetic rubber copolymers contain specifically formulated blocked alkenes. These materials are extremely flexible and yield under various temperatures. Additionally, they offer superior moisture protection. This translates to greater reliability testing performance as the material flexes and recovers under various stresses during thermal shock and thermal excursion testing. Furthermore, susceptibility to dendritic growth is minimized with the use of synthetic rubbers due to superior Moisture Vapor Permeability (MVP) resistance performance. The combination of water vapor and ionic contamination leads to dendritic growth, and synthetic rubber conformal coatings provide the industry’s lowest moisture infiltration.
UV dual-cure elastomeric acrylate and acrylate polyurethane conformal coatings exhibit excellent flexibility, moisture resistance, and electrical insulation properties as well as good chemical resistance and give improved performance during thermal cycling tests.
Where most applications require a maximum constant operating temperature of 125°C, the solvent based acrylics and polyurethane technologies are well suited. The synthetic rubber constant operating temperature range varies greatly up to 150°C. These materials behave very well in a wide range of temperatures due to the unmatched thermo-mechanical properties.
Synthetic rubber materials are solvent based and they can be applied using the same process = as used with existing acrylic/polyurethane. Therefore, the upgrade to a higher performance material can be done without the need to change process equipment and, unlike UV curable conformal coatings, synthetic rubbers do not require capital investment in a UV oven, special handling requirements, or lighting.
With so many materials to choose from, it’s not easy to decide which route to take. It’s the interaction between multiple properties that create improvement in performance. The fact remains, though, that product selection is your first and most important step towards achieving maximum thermal shock resistance of your circuit board assembly.
For more information call Circuits Central at (888) 602-7264 or contact us here.