Conformal Coating Testing - Investigation & Methodology

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:

• Good electrical properties
• Low moisture permeability
• Good physical characteristics
• Excellent adhesion to all board materials

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

Basic tests:

• Electrical performance and accelerated humidity testing.

Advanced testing:

• Severe conditions such as salt mist, temperature extremes or rapid environmental changes.

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.

Performance Criteria

Surface Insulation Resistance

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.

Flexibility

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.

• A strip of copper is coated and bent around either a 3mm mandrel to test.

The coating must not crack, delaminate from the substrate

Adhesion

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.

• A special tool is used to score lines in the coating in both directions, forming a

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

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:

• Humidity Exposure and Salt Mist Testing
• Thermal Cycling, Shock and Aging
• These parameters are either tested individually or combined depending on the requirements

Environmental Cycling

Based on UL746 test methods, the following environmental cycling profile can also be utilised:

• 24 hours immersed in water, followed by
• 24 hours at 105°C, followed by
• 96 hours at 90%RH, 35°C, followed by
• 8 hours at -70°C – end of cycle
• 3 cycles

Humidity Tests

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

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

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:

Electrical Testing

Evaluation of electrical properties is essential in all conformal coating applications. Some typical tests include:

• Dielectric Strength
• Dielectric Constant
• Dissipation Factor
• Surface Insulation Resistance
• Comparative Tracking Index (CTI)

Water Immersion

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.

• Coated boards were immersed for 7 days and the SIR results compared.
• For continuous or frequent immersion in water we advise Electrolube Encapsulation Resins

Solvent Resistance

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.

Thermal Cycling / Thermal Shock

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.

Thermal Cycle Test Example:

A thermal cycling profile was set up as per IEC 60068-2-14:

• -55°C to +125°C, 25 minutes at each temperature
• 12°C/min rate of temperature change
• 20 cycles

Coated tin, copper, aluminium and FR4 panels were subjected to the cycling and then tested for adhesion (BS EN ISO 2409) and flexibility (3mm mandrel – IPC-TM 650 2.4.5.1)

Thermal Shock Test Example:

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.

UV Resistance

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.