Silicone vs. Urethane Conformal Coating

Silicone and urethane stand out as prevalent conformal coating types due to their protective attributes. The superiority between the two coatings hinges on your specific application. Below is an exploration of the advantages and disadvantages of each type.

Silicone Conformal Coating

Benefits

Most common silicone coatings have continuous operating temperature ratings of 200ºC. This is far higher than most urethanes (125ºC). Some silicone coatings are rated as high as 600ºC for ultra-high temperature applications. There is a great deal of use for silicones in the automotive industry, where temperatures can get upwards of 175ºC in the engine compartment.

An additional benefit of silicone conformal coating is its excellent moisture protection. Silicones have been used in cases where there were extreme temperature differences, resulting in excessive moisture. Other conformal coatings failed within hours or days, while silicone, especially when applied thickly, has been proven to succeed. One example of an application that endured temperature extremes is electronically controlled heaters developed for artic temperatures. The heaters warm to almost 150°F, then cool to ambient temperature, reaching as low as -40°F. This wide temperature swing happens very quickly, resulting in a high moisture environment. Other coatings, such as urethanes, resulted in board failures, but silicones provided the needed protection in the tough environment.

Silicone conformal coatings are also among the easiest to apply and re-work. Silicone coatings are typically low on solvents, ensuring a smooth coating that cures very quickly (about one hour at room temperature). Silicones are quite flexible and soft, having a relatively weak resistance to solvents. For assemblies that will require work after coating, this is critical because it will keep labor time down without compromising the integrity of the coating.

When Not to Use

Silicone conformal coatings are required to be applied thicker than other conformal coatings. The target thickness is between 0.00197” to 0.00827”. If your application has tight clearance tolerances or if the solder joints cannot support the stresses placed upon it by the thicker layer of coating, it would be best to consider other coating options

Types of Silicone Conformal Coating

• HumiSeal^® 1C49
• HumiSeal 1C49LV
• HumiSeal 1C51
• HumiSeal 1C55
• DOWSIL™ 1-2577
• DOWSIL 3-1753
• DOWSIL 3-1765
• DOWSIL 3-1744
• DOWSIL 3-1953
• DOWSIL 3-1965
• DOWSIL 3-1944
• MG Chemicals 422B
• ELPEGUARD^® DSL 1705 FLZ
• ELPEGUARD DSL 1706 FLZ
• Electrolube SCC3
• Electrolube SCC4

Urethane Conformal Coating

Benefits

Urethane conformal coatings are very resistant to chemical solvents and are second only to Parylene conformal coating in their resistance. Applications that face prolonged exposure to harsh chemical solvents should consider urethane resins.

Long-term NASA studies have shown that urethane conformal coatings are one of the few ways to successfully mitigate tin whisker growth. Since there is no known way to completely eliminate tin whisker growth, a tin whisker mitigation strategy must be determined. Urethane conformal coatings are a great place to start.

Applications that may experience direct mechanical wear against the coating should also consider urethane conformal coatings. Urethane resins are very hard and resistant to mechanical wear. Their hardness is second only to epoxy conformal coating, but urethanes are much easier to be reworked than an epoxy.

When Not to Use

For products going into a high vibration environment, urethane conformal coatings are not ideal. Because of urethane’s mechanical strength and resistance to abrasion, a high vibration environment could ultimately end up weakening the integrity of this rigid coating. Alternatively, Parylene conformal coating’s completely conformal nature and flexibility makes it better suited for this type of environment.

If an application is going into a high-heat environment, urethanes will not provide the protection that is required. Leading urethanes, such as HumiSeal 1A33, offer protection to 125ºC. Silicone conformal coatings, on the other hand, offer protection of 200ºC or higher, making it better suited for applications exposed to high heat.

 Types of Urethane Conformal Coating

• HumiSeal 1A33
• HumiSeal 1A20
• HumiSeal 1A27
• HumiSeal 2A64
• HumiSeal 1A34
• Hysol^®  PC18M
• CONATHANE^® CE-1155-35
• CONAP^® CE-1170
• CONATHANE CE-1164
• Techspray Fine-L-Kote
• MG Chemicals 4223
• Electrolube PUC

Conclusion

There are many benefits to using either a silicone or urethane conformal coating. To select which coating is best for your application, you must understand your product’s protection needs and identify which coating best meets those needs.

SCS offers a comprehensive portfolio of protective coating solutions, including silicone and urethane conformal coatings. To discuss what coating is best for your application, call +1-317-244-1200 or Contact SCS online.