Acrylic and urethane conformal coating are widely used conformal coatings. While they share the same methods of applications and ultimate end item uses, there are some differences that should be addressed prior to deciding what conformal coating is the best for your project.
Acrylic conformal coatings are a liquid conformal coating and can be applied four ways: spray, dip, brush, or robotics. Which method selected will vary by a couple of factors including: Quantity of products, complexity of masking requirements, and skill level of operators. They are usually applied between .002” and .005” in accordance with IPC-610.
Urethane conformal coatings are applied in the same manner as acrylics. A difference between the two coatings in this stage is the cure. While acrylics typically cure in as little as 30 minutes, most urethanes need a longer cure time. Some urethanes even require a thirty day air-cure! While most are not as extreme as that, it is a good idea to take into account the cure times if you have a high volume application and tight schedules are a must.
EXAMPLES OF PRODUCTS
Some examples of acrylic conformal coatings are:
- MG Chemicals 419C
- HumiSeal 1B31
- HumiSeal 1B73
- MG Chemicals 4223
- HumiSeal 1A33
- Humiseal 1A20
A common use of acrylics is on printed circuit boards for moisture protection.
While urethanes are also widely used for printed circuit boards, they are mainly used for protection against solvent exposure. Urethanes can also be used for moisture protection and instances where a more durable, rigid coating is needed.
In applications that have an exposure to solvents, acrylic conformal coating is not the best choice. Acrylic conformal coating can be removed with a weaker solvent such as isopropyl alcohol or xylene. Whenever it faces even stronger solvents, it will not offer the protection that is needed, especially if your product is a mission critical device.
For products that require a high temperature application, acrylic coatings will fall short of expectations. For HumiSeal 1B31, arguably the most popular acrylic coating, the max continuous operating temperature is 125ºC. Compare this to silicone conformal coating, whose operating temperature can exceed 200ºC.
One disadvantage of urethanes is the difficulty of rework. Because urethanes have such a high tolerance against solvents, removal of the coating to rework components or the coating itself can be quite a lengthy process involving harsh strippers. Often times, during the removal process, the PCB itself or the components soldered to it can be damaged.