Enhanced protection for enhanced PCB performance
Conformal coatings are designed to protect printed circuit boards and related equipment from their environment. Typically applied at 25-75μm, these coatings ‘conform’ to the contours of the board allowing for excellent protection and coverage, ultimately extending the working life of the PCB.More info > Download product selector chart > Download product brochure >
The use of conformal coatings is particularly important in automotive, military, aerospace, marine, lighting, industrial and green energy applications. Due to the rapid expansion of the electronics industry, conformal coatings are also finding their way into the domestic and mobile electronics industries, providing the necessary combination of high performance and reliability within a vast array of electronic devices.
Conformal coatings can be used in a wide range of environments to protect printed circuit boards from moisture, salt spray, chemicals and temperature extremes in order to prevent corrosion, mould growth and electrical failures, for example. The protection provided by conformal coatings allows for higher power and closer track spacing, in turn enabling designers to meet the demands of miniaturisation and reliability.
Electrolube is among the world’s foremost experts in the formulation and application of conformal coatings designed to meet international approvals (including European and American military specifications). The range of products currently available comprises acrylics, silicones, polyurethanes, hybrid chemistries and environmentally friendly options.
Electrolube can offer both transparent and pigmented coatings to improve or camouflage the appearance of printed circuit boards. The range also includes a number of ancillary products to complement the use of our conformal coatings, including thinners and removers, peelable coating masks and thixotropic materials for dam and fill applications.
Electrolubes Conformal Coatings Technical Director Phil Kinner recently wrote a micro e-book on conformal coatings for harsh environments. It’s a great little insight into the selection, implementation and testing of protective coating process.
The requirements of conformal coatings can differ from application to application. The most important properties that any conformal coating must have should be discussed first.
The ideal coating should offer a combination of:
To ensure the coating meets the desired characteristics, the coating needs to be exposed to a range of environments via appropriate test conditions to establish its performance range and limitations
Electrolube put all of their conformal coatings through the test conditions outlined in these specifications and therefore, many other coatings from the range also meet the requirements of these standards. For full info on Product test results, please view the product TDS’ available from downloads.
The electrical resistance of an insulation material measured between two contacts.
When a board is coated it must have an electrical resistivity of 1010 ohms before environmental testing starts. After the tests, this value should not decrease below 108 ohms.
A coating must have a certain level of flexibility to ensure that all areas of the board will be coated sufficiently without any cracking or chipping upon cure. This will also ensure that the board can be moved and handled without damaging the coating and that an adequate level of flexibility is achieved in order to allow the PCB to expand and contract.
The coating must not crack, delaminate from the substrate
Without adequate adhesion to the board the coating will not provide full protection. The cross-hatch test is a simple but effective way of measuring the adhesion of a conformal coating to standard circuit board materials.
Cross-hatch pattern. An adhesive tape is then applied to the cross-cut area and carefully peeled away then the coating is inspected to see if any of the squares produced have been removed from the substrate.
Environmental testing is essential to ensure the required level of protection is achieved. End-use conditions should be replicated or accelerated, however care must be taken to ensure accelerated tests are suitable for comparison with end-use conditions:
Based on UL746 test methods, the following environmental cycling profile can also be utilised:
Humidity is probably one of the most obvious causes of corrosion for PCBs as moisture in the atmosphere can react with metal components and connections on the board. Humidity testing is carried out on a conformal coating to make sure that the resistance to this humidity is maintained.
Salt mist tests are important for any board which will be operating in an environment where there will be salt present. An obvious example of this is a marine environment such as electronics on shipping. Salt and water combined is one of the most highly corrosive combinations so it is imperative that adequate protection from this is provided.
Corrosive gas testing involves exposing PCBs to a mixed gas, environment combing hydrogen sulphide and sulphur dioxide – BS EN 60068-2-60, method 1.
Surface insulation resistance (SIR) was used to determine the performance of each coating in this environment:
Evaluation of electrical properties is essential in all conformal coating applications. Some typical tests include:
Immersion in water is an extremely harsh test for a conformal coating to pass. Most coatings will resist immersion for short periods of time however prolonged exposure can highlight issues.
During its lifetime a PCB may come into contact with any number of solvents, whether in the atmosphere of its working environment, or in direct contact.
Solvent resistance tests can be carried out in accordance with IEC 61086-2. The performance of the coating will largely depend on the solvents used during the test and the chemical makeup of the coating.
Changes in temperature happen to a varying degree across most electronic applications due to fluctuations in the operating temperature or the surroundings.
As an example, if you think of a car starting up in winter, the standing vehicle in Europe can be as cold as -40°C, but once the engine is running the temperature of the engine quite rapidly rises to somewhere around 100°C (depending on the vehicle). The car could go through this thousands of times within its lifetime so it is important that products go through a thermal cycle test, or the more extreme thermal shock test (changing the temperature rapidly) to ensure the coating maintains the level of protection offered.
A thermal cycling profile was set up as per IEC 60068-2-14:
Thermal shock is similar to thermal cycling, but over a much faster time frame – Electrolube have recently bought a new machine to test for thermal shock, initial testing has taken place cycling a change of 190 °C in less than a minute.
Electrolube have carried out weathering resistance tests on a number of available conformal coatings. Tests were in accordance with ISO 4892, Part 3, Cycle 1: ‘Plastics Methods of Exposure to Laboratory Light Sources’ and carried out in a QUV SE Accelerated Weathering Tester. After 1000 hours exposure, the results indicated that Electrolube acrylic coatings, APL, HPA and TFA, have superior resistance to UV light, maintaining their clarity throughout the exposure testing.
Exposure intensities will vary depending on geographical locations and therefore it is important to establish the correct accelerated exposure time for your region. As an example, this test is roughly equivalent to 4 years weathering resistance in a typical Northern European climate.