Acrylic films are usually the easiest to apply and work with. Acrylic removal involves a simple process. These coatings are robust, cheap, and transparent. They protect against moisture, fungi, acids, bases, and electricity.
Notable Acrylic Coating Properties
- Clear protective coating
- Relatively short drying times
- Good electrical and physical properties
- Low moisture absorption
Acrylics are one of the simplest conformal coating types to apply, and the films dry quickly. Application methods include brush, spray, and dipping. Acrylics are a good choice for low-cost, quick applications. Their coating process is simple and allows for cheaper application methods.
Silicone films protect components. They can absorb shock and vibration, relieving parts from physical stress. Silicones excel in extreme temperatures. They are stable at -65°C to 200°C, resisting ultraviolet and ozone degradation.
Notable Silicone Coating Properties
- Good humidity and moisture resistance
- Low toxicity
- Easy to apply
- Easy to repair
Silicone can be applied with dipping and spraying methods or simple flow coating. It is easy to repair, and its excellent heat cycling makes it suitable for automotive, aerospace, and industrial uses. However, moisture may permeate through the coatings, and they can exhibit poor mechanical strength.
Epoxy coatings protect against abrasion, corrosion, harsh chemicals, oil, and thermal shock. They also serve various other purposes. For example, thermally conductive epoxy protects circuit boards with heat-generating components. Meanwhile, flame-retardant epoxy is helpful for fire hazard protection.
Notable Epoxy Coating Properties
- Very robust, difficult to remove
- Excellent chemical and abrasion resistance
- Very rigid conformal coating
- High dielectric strength
Epoxies are easily applied and may be a two-part compound or a single compound to be cured with UV or heat exposure. Dip-coating, brushing, or spray methods are suitable for epoxy application. Film shrinkage may occur during polymerization. This can cause issues during coating. The toughness of epoxy may damage products if rework is needed.
Urethane, or polyurethane, is a conformal coating that makes tough, hard coatings. They resist solvents and abrasion. These sturdy films block airborne contaminants. They may help reduce tin whisker growth.
Notable Polyurethane Coating Properties
- Moisture and oil-resistant
- Fungicidal
- Good flexibility
- Class F temperature rating (160°C/320°F)
Urethane films can be applied by dipping, spraying, or brushing. Curing is required to complete the coating after application. Curing may take an hour to several days. This adds another step to the polyurethane coating process.
Parylene coatings are transparent, biocompatible barriers. They resist corrosion and are thinner than other coatings. Parylene's properties make it ideal for mission-critical applications.
Notable Parylene Coating Properties
- Biocompatible, biostable, “green polymer”
- Insoluble to common chemicals and conditions
- High dielectric strength
- Dry film lubricity
Brush coating is the easiest and cheapest application method and requires minimal maintenance. Equipment is inexpensive. There may be less need for masking due to the controlled nature of this technique. However, the quality and results of the brush conformal coating depend on the operator and tools. So, they may vary in quality and conformability. Contamination issues may also be a concern.
The spray coating technique can be manual or automated and is faster than brush coating. Spraying can give consistent results if it combines the right settings including pattern, solvent dilution, and nozzle pressure. Yet, PCB spray methods can entail more masking time than brushing methods. Spraying is cheaper than other methods, but needs humidity and temperature control, which can raise startup costs.
Dipping, like spraying, may be automated or manual. It is an effective way to coat components that are not too irregular or bulky in shape. The dip coating method fully encapsulates electronics. It is an efficient, low-cost process for high-volume applications. With proper implementation, dipping produces repeatable film thickness and uniform coverage. But, to avoid contamination, the material in the coating dipping machine must be purged and replaced periodically, increasing cost and complexity.
Of the listed coatings, CVD is used only with Parylene. The CVD process is done under vacuum. It uses specialized equipment, including a coating chamber. All surfaces can be coated evenly, no matter the chamber position. The coating deposits the same relative thickness on the objects. This vapor phase coating process also leads to pinhole-free coatings that are absent of defects. Masking and demasking are typically required, but no curing is necessary.
Coating material impacts thickness due to different material properties. For example, Parylene can pass IPC CC-830C at 50% of other coatings' thickness. Other coatings must be thicker to meet the standards. Accurately meeting the requisite thickness is crucial. If the coating is too thin, components remain vulnerable to the environment. But, if it is too thick, the insulating properties of the coating may render specific parts non-functional.
How to Measure Conformal Coating Thickness:
Several methods measure conformal coating thickness, including:
After the coating, inspect the components. Check for customer and coating standards. The conformal coating type may affect the inspection process. Different materials may be used to meet various standards and thicknesses. They may also need to meet different reliability levels.
Read the IPC A 610 standards for conformal coating inspection here.
Masking prevents coatings on components that must remain uncoated to function. For example, connectors. Successful masking requires: selecting the right materials, tailoring the masking process to the coating application, and using the best techniques. After the coating is done, the masking is removed. This is the demasking process. Masking and demasking can be more complex and critical with certain coating materials. Additionally, some application methods require more masking and demasking time than others. Thus, the best masking processes are tailored to the material.
Sometimes, the coating must be reworked due to poor adhesion, bad materials, repairs, or other reasons. Removal can add cost and complexity to the coating process. The type of coating you choose will affect the best removal methods.
Techniques include:
Conformal coating curing cross-links the film to give it peak properties. Curing is different from conformal coating drying. The latter means the coated component can be handled. Coating types require different drying and curing methods, with one exception. Due to its CVD coating process, Parylene does not require drying or curing.
Methods include:
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