Ruggedization and Conformal Coating
Posted by Sean Horn
Friday, January 5, 2018 7:35
@ 7:35 AM
Conformal coatings are non-conductive dielectric film-coverings applied over printed circuit boards (PCBs) to protect them from damage caused by chemical incursion, corrosion, current-leakage, dirt/dust, extreme temperatures, fungus, moisture, rain, salt-spray, wind and persistent, intensive vibrations both within and external to the device. These failure mechanisms can soon lead to PCB malfunction and eventual breakdown. Rugged coatings’ exceptional performance durability and versatility protect delicate, finely-tuned components.
The Role of COTS Electronics
Because of the operating conditions they must endure, defense and aeronautic PCBs benefit from ruggedization. However, defense electronics are frequently costly; reliable substitutes are sought by the budget-conscious military establishment. Commercial off-the-shelf (COTS) assemblies are increasingly used for military purposes.
COTS electronics are generally NOT designed for military use. The custom design that typified most military devices in the past has been modified for COTS-adaptation for many non-critical products:
- Budgetary ceilings within the Department of Defense have limited available funds for purchasing specialized systems.
- Stringent Restriction of Hazardous Substances’ (RoHS) directives encourage introduction of less dangerous products.
Conformal coatings improve the performance of COTS assemblies ruggedized for defense and similar specialized uses. Their protective properties upgrade conventional COTS’ consumer applications for reliable operation in far more punishing working environments.
Ruggedized systems are designed to safeguard PCBs’ internal components, maintaining expected performance levels in punishing operational ecosystems. Military embedded systems requiring protracted in-field and on-site functioning are a major beneficiary of product ruggedization. Other devices profiting from rugged systems include:
- aerospace computing,
- emergency healthcare/public safety situations, and
- specialized scientific research or undersea/arctic exploration/surveying.
All encompass performance environments typified by radical changes in temperature, and variable atmospheric conditions demanding specialized, in-depth protection for electronic systems. Conformal coatings provide resistance to harsh working environments supporting functionality where unprotected devices would fail. To this extent, conformal protection is a major element of most dependable ruggedization projects.
The most significant professional standard for ruggedization is MIL-STD-810F, which provides guidelines for device-testing to assure it functions:
- under low pressure/high altitude situations,
- through temperature extremes,
- in rain/humidity,
- through shock, gunfire vibration or vehicle acceleration,
- in the presence of salt fog or fungus, floods or desert conditions.
Meeting MIL-STD-810F specifications is the basic requirement for reliable product ruggedization.
Conformal Coatings for Electronics’ Ruggedization
To varying degrees, conformal coatings can protectively insulate and ruggedize PCBs, supporting systems’ operation, without fail during extreme duress. Present applications undergo ongoing review to develop enhanced performance parameters.
Applied by brush, dip (immersion), and spray methods, liquid acrylic coatings meet approval standards for:
- UL 746C, regulating use/ performance criteria/material property considerations of polymers in electrical equipment, and
- MIL-I-46058C, regulating application of insulated coatings for PCBs.
Humidity resistant, acrylic films offer good moisture protection and dielectric properties, with low glass-transition temperatures. Acrylics work best as secondary protection material for ruggedized uses, minimizing component condensation during operation, while offering relative ease for repair/rework.
Another liquid coating, epoxy is known for its strength and durability. It can meet MIL-I-46058 and IPC-C-830 performance standards. Able to withstand prolonged salt air exposure and high-level vibration, epoxy is also:
- resistant to scratches, tears, and corrosives,
- chemical/solvent resistant,
- watertight, and
- temperature independent between −20°C/54°C.
It can serve as a primary protection material for ruggedization, but can become brittle over time, lessening anti-vibration capacities.
Tolerant of high temperatures and hydro-/oleophobic, silicone is inert biologically and chemically. While these properties make it useful for coating assignments untenable for other liquid coatings, they also interfere with silicone’s ability to bond to other materials; ruggedized applications are limited by a tendency toward delamination. Chemically-resistant silicone generates thick, rubbery films, requiring mechanical removed for rework.
Very hard and resistant to chemical solvents and mechanical wear, liquid urethane coatings offer good humidity protection and dependable dielectric properties, withstanding prolonged exposure to harsh chemical solvents. Urethane excels is tin whisker mitigation, lessening the impact of the electrically conductive, crystalline structures within a component. Applied at 2 mm. thickness, urethane provides a dependable tin whisker resolution strategy, sufficiently strong to inhibit coating-penetration, a prime military fail-mechanism.
Unfortunately, urethane’s high solvent resistance makes it difficult to remove/rework. Also, products with outgassing oil-modified or alkyd chemistries disrupt coatings’ long-term performance. Prone to cracking during prolonged thermal exposure, urethane often fails in high-vibration/high-heat environments, limiting ruggedized coating use.
In contrast to liquid coatings, parylene’s unique chemical vapor deposition (CVD) application method deposits gaseous parylene deep within substrates on a molecule-by-molecule basis, generating a superior degree of dielectric, non-conductive, insulating performance. For ruggedized purposes, parylene is
- IPC-CC-830 compatible, and
- appropriately itemized on the Defense Supply Center Qualified Parts List (QPL) for MIL-I-46058.
More important, parylene meets all essential MIL-STD-810F specifications for COTS assemblies and product ruggedization, with flexible, ultra-thin, uniform and pinhole free coating. Entirely conformal, it doesn’t decompose at upper-range temperatures, or become brittle like liquid coatings under severe temperatures. Parylene coating remains adherent and intact, preserving the dielectric and insulation properties essential to military component performance.
While useful for property-specific applications, wet coatings do not offer parylene’s versatility for ruggedization. Mil-spec approved parylene enhances the integrity of COTS’ devices for ruggedized performance without adding high cost. They represent the optimal, primary-service conformal choice for ruggedized electronics.
To learn more about how conformal coating can protect COTS electronics, download our whitepaper now: