During the PCB fabrication process, you’re likely to encounter a number of conformal coating problems.
That’s why with the help of this guide you will find the ultimate solution to all problems you have during the conformal coating process.
Let’s dive right in.
- What are the Qualities that Determine a Good Conformal Coating?
- What are Conformal Coating Quality Tests?
- Can the Method of Application Affect the Quality of a PCB Conformal Coating?
- What is Surface Insulation Resistance in Conformal Coating?
- What does an Environmental Cycling Test for Conformal Coatings Entail?
- Why does the Conformal Coating need to be Flexible?
- What are the Limitations of Water-based Conformal Coatings?
- How is Adhesion a Problem during Conformal Coating?
- What are the Drawbacks of Employing Solvent-based Conformal Coatings?
- What problem does humidity cause to a PCB without proper coating?
- What are the Major Causes of Concern for using Conformal Coating?
- Why is Salt Mist a Problem Source for Conformal Coating?
- Why would a Conformal Coating Fail?
- What Electrical Parameters are Covered in the Electrical Testing of the Conformal Coating?
- Do PCB Conformal Coatings Resist Water and Solvents?
- How can Thermal Cycles Present a Problem to Conformal Coating?
- What is Conformal Coating Flow?
- What Problems are Associated with the Application of Coatings?
- How are Problems Related to UV Exposure Tested?
- What are the Board Design Challenges Facing the Coating Process?
- Why are Thick Coatings Prone to Failure?
- What Problems are Associated with the Viscosity of Conformal Coatings?
What are the Qualities that Determine a Good Conformal Coating?
The use of a particular coating is dependent on the area of application.
However, there are general features a good conformal coating should possess.
Some of these features include:
- Impressive adherence to the board features
- High resistance to moisture penetration
- Remarkable electrical ability
- Reasonable physical quality
Applying conformal coating on PCB
What are Conformal Coating Quality Tests?
The testing of conformal coatings requires subjection to a variety of environments under particular conditions.
These conditions you find are representative of the environment of use of the conformal coatings.
Furthermore, they help to determine the response of the coating and subsequently its abilities and failings.
There are two types of tests carried out on conformal coatings:
- Basic tests are used to establish the coating’s electrical capabilities and performance under increased humidity levels.
- Advanced tests that seek to determine the coating resistance to extreme conditions including sudden changes, temperature extremities, and heavy mist.
Can the Method of Application Affect the Quality of a PCB Conformal Coating?
Yes, it can.
Several factors such as the environment of use and the area of application can affect the quality of a conformal coating.
Furthermore, the material employed will also determine the performance of a coating due to the qualities they possess.
However, you also find that the application methodology can influence the coating’s quality and performance.
You find some coatings adhere better to the PCB when certain application methods are used.
Moreover, since coating materials are formulated by different companies, they can be tailored for particular coating processes.
You can find similar materials from different companies that have differing conformity levels when used with a single application process.
What is Surface Insulation Resistance in Conformal Coating?
The surface insulation resistance of conformal coatings is the value of electrical resistance that is taken between any two points.
The surface insulation resistance of a conformal coating should have a pretest value of not less than a thousand ohms.
While it decreases posttest, it should not fall beyond a hundred ohms.
When the resistance values are below the stated values, it signifies a problem with the coatings resistance capability.
What does an Environmental Cycling Test for Conformal Coatings Entail?
An environmental cycling test is used to determine the overall capacity of a conformal coating to withstand general environmental conditions.
You use this test to identify a coating’s response to humidity, thermal changes, and salt mist for instance.
In this test, you find the coated board is subjected to three cycles involving various conditions.
For every cycle, four conditions are provided with each successively following the previous condition.
The first condition involves submerging the coated board in water for a day.
Thereafter, it is kept for another day under temperatures of a hundred and five degrees.
The third condition is a four-day exposure to a relative humidity of ninety percent at thirty-five degrees.
Finally, the board is stored for eight hours at negative seventy degrees Celsius.
The cycle is then repeated for a second and third time.
Why does the Conformal Coating need to be Flexible?
Achieving the right flexibility of a board’s coating is vital to prevent peeling and crevice formation during the curing process.
Flexible coating warrants impinged movement without causing damage to the coating.
Furthermore, having a flexible board provides room for the expansion and contraction of the boards as it responds to thermal changes.
What are the Limitations of Water-based Conformal Coatings?
Water-based coatings have been developed as a response to the environmental concerns raised by the use of solvent-based coatings.
You find while solvent-based coatings are organic, they are also deemed to be volatile compounds.
These solvents are said to contribute to the depletion of the ozone layer when they are evaporated.
Water-based coatings are an environmental-friendly alternative whose use is encouraged to reduce solvent-based coating use.
However, the use of these water-based solvents is faced with a few limitations as described.
Conformal coating on PCB
- As their name suggests, these coatings comprise water whose structure allows movements of ions that conduct electrical charge.
Consequently, you find coatings of this kind to be conductive an aspect that is undesirable especially for high-frequency applications.
In such instances, signal quality concerns such as crosstalk are experienced.
- Additionally, you find that for the polymer resins to be used with water, they have to be chemically congruent.
Ideally, this is usually not the case requiring an alteration of the resin polymer structure or the use of an emulsifier.
This typically makes water-based coatings marginally costlier than their solvent-based counterparts.
- Furthermore, even when the resins have been modified and complemented to allow their use with water, the bond weakens over time.
Ultimately, you find their protective capacity is limited where the environment is highly humid.
How is Adhesion a Problem during Conformal Coating?
Adhesion refers to the adherence or conformity of the coating to the board features on the application.
Achieving good adhesion is paramount to prevent bumps and gaps under the coating.
To prevent such occurrences the board surface has to be cleaned before the coating process.
Cleaning eliminates contaminants such as dust particles and moisture.
Adhesion of the conformal coating to the board can be measured by a cross-hatch test.
In this test, a line pattern is made on the coat by employing a special marker.
This pattern is then taped and then removed.
The pattern should not be visible on the substrate surface.
What are the Drawbacks of Employing Solvent-based Conformal Coatings?
Solvent-based coatings are popular for their qualities such as good penetration, flexibility, and high resistance to several environmental elements.
However, there are a few drawbacks associated with the use of these types of coatings.
- Solvent-based coatings have a strong smell that can cause health concerns to people when inhaled.
As such, you find the use of protective gear such as face masks is required when handling these products.
- You find the solvents used in these coatings can catch fire and require proper storage to avoid the risk of major fires.
However, when used on PCBs, curing alleviates the danger of flammability.
- The solvent-based coatings are considered to be volatile organic compounds. Subsequently, you find these substances can cause emissions that are harmful to our environment.
- When using solvent-based coatings in two-part applications, using thicker coatings can handicap other qualities of the coating’s performance.
For instance, a thicker coating will hamper the coating’s flexibility resulting in cracking on the thermal strain.
What problem does humidity cause to a PCB without proper coating?
Humidity is the water content present in the atmosphere.
Humidity is a major reason for corrosion on the circuit board.
The water molecules interact with the metalized parts of the board causing oxide formation.
To avoid such scenarios, the coating’s resistance to humidity has to be ascertained through a humidity test.
What are the Major Causes of Concern for using Conformal Coating?
Conformal coatings are provided mainly to serve a protective purpose.
Protection is usually from environmental elements present in the areas of application to ensure the PCB’s durability.
Some of these elements that are a cause of concern on a board’s performance are:
PCB conformal coating
- Temperature extremities that can be attributed to the seasonal changes.
For instance, winter is associated with very low temperatures while summer experiences temperature spikes.
- Temperature fluctuations that result in the consistent subjection of the board to contrasting temperature values. You find this present in machinery where temperature rises during operation and drop when shut down.
- Salt mist and fog which are prevalent in humid conditions of coastal areas. The presence of salt accelerates the corrosion process for metallic parts.
- Conditions with saturated and high humidity levels such as equatorial and mountainous regions. In this case, the moisture presence is elevated causing a possible board efficiency concern.
- Conditions adequate for the growth of fungus which can occur on an unprotected board resulting in performance issues.
- Areas with elevated gas levels that have a corrosive effect such as industrial zones on interaction with water. This may happen when it rains.
- Excessive exposure to solar radiation in areas with low cloud cover such as deserts, arid areas, and space.
- Extreme atmospheric pressures that can affect the board connections. You can find equipment subject to such conditions in astrophysics and space-related developments. Underwater applications are also subject to this concern.
Why is Salt Mist a Problem Source for Conformal Coating?
Without a salt mist test, the protective performance of a conformal coating is put in doubt.
You find this true, especially in coastal regions.
Salt mists contain water molecules and salt combinations that exhibit high corrosion levels.
Consequently, when allowed to penetrate a coating, they can cause massive damage to the PCB’s metallic parts.
Why would a Conformal Coating Fail?
An applied conformal coating would fail to meet its protective function based on several factors other than the application method.
You find the following aspects will contribute to the failure of a coating:
- Sometimes you find the formula for the resin coating is inadequate to withstand saturated environments resulting in condensation.
Consequently, extra care needs to be taken when choosing a particular material.
- A coating can also fail because of inadequate spread and thickness.
You find this common on complex board designs with pronounced boundaries and differently sized populates.
In this case, proper selection of the material and method of application will be vital.
- When the curing process is not comprehensively done, you find the coating will fail to fully achieve its capabilities.
As a result, its protective capacity will be greatly diminished.
- Coating failure can occur when the coating material gets in contact with other circuit board related material such as solder flux.
You find this waters down the quality of the coat at the point of interaction reducing its protective capacity.
You, therefore, need to ensure the board is cleaned before application of the coating.
What Electrical Parameters are Covered in the Electrical Testing of the Conformal Coating?
Testing for the electrical capabilities of conformal coating seeks to establish its performance across various aspects, these include;
- The value of dielectric constant
- The dissipation factor of the coating
- The coating’s comparative tracking index
- The coating’s dielectric strength
- The resistance value of the coating’s surface insulation
Do PCB Conformal Coatings Resist Water and Solvents?
The ability of coatings to resist water molecules will depend on the nature of interaction and duration.
When a PCB with a coating is immersed in water, it is bound to succumb within a short amount of time.
You find most PCB coatings are not waterproof.
Concerning solvents, the resistance achieved depends on the chemical composition of the solvent.
Solvents with strong molar concentrations are much more damaging on prolonged exposure.
PCB protected with conformal coating
How can Thermal Cycles Present a Problem to Conformal Coating?
Thermal cycles encompass the temperature fluctuations experienced in environments that the PCBs are operated in.
PCBs can be used in environments where temperature values can fluctuate several times over in its life.
Subsequently, the coatings to these PCBs have to be subjected to temperature cycling tests to ensure their reliability in such situations.
Without proper capability, the coating can succumb to the temperature cycles that can cause cracks and peel off.
Consequently, contaminants such as moisture and solvents can find their way through to the board surface where they’ll initiate corrosion.
A common test is to subject the coating to temperatures between -55 oC and +125oC for a sustained period.
What is Conformal Coating Flow?
Coating flow occurs during the coating application process when the coating solvent flows from the areas of deposition to undesignated areas.
Coating flow is a problem especially for boards with connecting vias as the coating can find its way into these vias.
To mitigate this issue, it is advisable to seal these vias by filling them or covering them with a solder mask.
It is also important to provide separation of those components to be coated and those not to.
What Problems are Associated with the Application of Coatings?
The application of coatings is an important process that can be done in different ways.
Some common methodologies are by hand using a brush or a spraying device or automatically.
The choice of application depends on factors such as the cost needs and the coating material.
When carrying out the application of a coating, you can face several difficulties.
Achieving a complete coverage of the board area to be coated is usually desired in theory.
However, managing to apply a coating all over is hindered by the board design and complexity.
It is especially difficult when many board areas are not to be coated and positioned all over.
Furthermore, the problem is exacerbated when you are manually applying the coating via the use of a brush.
You find this requires the painstaking task of masking all the areas not to be coated.
This is before carefully stroking unmasked areas to prevent smudging and splashing.
Additionally, in applying coatings, another difficulty is in determining the appropriate thickness and evenness.
While you can achieve surface evenness and uniform thickness with the automated coating processes, it is much difficult by hand.
Consequently, the resulting coat will be irregular.
How are Problems Related to UV Exposure Tested?
Ultraviolet exposure can hamper board functionality, especially when used for high-frequency applications.
To avert problems related to such exposure, coatings are subjected to weathering tests to establish their resistance level.
A typical test will involve the coated boards subjected to UV light sources for over a thousand hours.
The amount of time signifies the intensity of the actual environment the board will be used in.
A board with remarkable protective qualities will retain its clarity under such conditions.
What are the Board Design Challenges Facing the Coating Process?
Conformal coating on PCB
The coating is done to offer PCB protection from elements such as dust, moisture, and radiation.
Most coatings are solvent-based requiring a curing process to form the protective layer which is usually non-conductive.
However, some board areas are not coated as a matter of form or function.
You find that coating PCBs with many areas that are not to be coated is challenging.
You find this especially true for large volume productions.
Application techniques such as selective coating remedy the process eliminating the need for masking all these areas.
However, the time involved in completing a single coat is still a lot.
Consequently, you find the need to locate the features not to be coated along one board side to allow faster processing.
Furthermore, having less of these features onboard helps shorten the coating cycle time.
Why are Thick Coatings Prone to Failure?
While thicker coatings are efficient in some applications such as two-part coatings, many times overly thick coatings are predisposed to fail.
You find a common failure mode of thick coatings is cracking.
The result is the formation of gaps through which environmental elements such as moisture can seep onto the board surface.
You observe the failure of thick coatings many times as a result of thermal-induced strains.
Such strains can be initiated by extreme temperature environments for extended periods and continuous temperature fluctuations.
The level of damage caused by the thermal strain will also depend on the resin material used for the coating.
Furthermore, when you use thick coatings, you find that during that curing process, some solvents will be trapped in the layer.
The amount of solvent trapped will differ from the material used.
The solvent present in the coating layer will decrease the resistance of the coating to elements such as temperature and humidity.
What Problems are Associated with the Viscosity of Conformal Coatings?
Viscosity is the resistance to the flow of a fluid.
Coatings are provided as solvents whose flowability is important in determining the coverage and thickness of the coating.
Furthermore, coatings with very low viscosity are prone to coating flow.
Coating flow can hinder the performance of surface mounted components such as ball grids.
You find a coating with a low viscosity can trickle underneath these components interfering with their electrical connection to the board.
Additionally, a coating with low viscosity will take considerably longer to cure.
Alternately, coatings with a high viscosity will be much difficult to apply especially when using a brush.
Moreover, the thickness achieved with such a coating material will be highly irregular.
To achieve evenness, it is important to determine an optimum material viscosity.
You can improve a conformal coating material’s viscosity by altering its manufacturing process to suit your flow needs.
Another approach is to employ the use of a primer on the coating.
Equally important in establishing the right viscosity is maintaining optimal temperature values when developing the material’s viscosity.
We hope this guide solved all your conformal coating problems.
However, in case you have any question, Venture Electronics is here to help – contact us now.