Materials to Insulate a PCB in a Metal Enclosure

Anytime you place a board into an enclosure, there will be exposed leads and solder points that create a risk of shorting. If the enclosure is totally made of plastic, and there are only plastic pieces of hardware around the PCB, then your board is already well protected. In many cases though, the enclosure is made of metal or it has metal pieces inside it. Exposed contacts and solder joints on a PCB need to be insulated from mounting hardware and from the enclosure itself.

There are several ways to provide this insulation, both as added materials or as mounting methods that provide adhesion as well as insulation. Some of these can be purchased off the shelf and do not require specialized materials, manufacturing, or assembly. Let's take a look at some materials that can be used to insulate a pcb.

What Do We Need to Insulate?

The main elements that may need to be insulated on a PCB are the following:

• Exposed solder joints near metal surfaces

• Long through-hole pins or header pins

• Gull wing leads on low-profile PCBAs

• Enclosures with hanging metal parts

In these instances, it is desirable to protect, at minimum, the exposed conductors on the PCB that could experience a short circuit. In some cases, the entire PCB might need to be protected from shorts, which would provide many other benefits.

There are some ways to implement this kind of insulation.

Kapton Insulators

Adhesive-coated polyimide strips, better known as kapton, can be used to apply targeted insulation. Kapton tape is one option for placing insulation directly on to exposed solder joints. Any wiring or fasteners that might be exposed can also be protected with a small amount of kapton tape.

Kapton comes in rolls of adhesive tape or as films that can be cut to shape.

Kapton tape is a bit of a poor man's method for applying insulation, but it works fine for prototypes. In production, it is also possible to manufacture die-cut kapton films. These films can be applied to the entire surface of a PCB or as a spacer on a metal enclosure. For example, if a PCB is mechanically fixed directly to a metal enclosure, the kapton spacer protects the entire bonded surface of the PCB. Die-cut kapton spacers are very inexpensive to produce at volume and they bring all the other benefits of polyimide films.

RTV Silicone

RTV silicone can be used as a spot adhesive, or as a conformal coating or encapsulant. These materials are available as insulating materials, and they are sometimes used in electrical systems as sealants or for waterproofing.

Heavy-duty RTV silicone sealants can be used as insulators.

RTV silicone is most often used as a sealant, but it can be used as an insulator and an adhesive. Silicone can bond to many surfaces or to scuffed surfaces, and insulating silicon can protect exposed conductors on a PCB.

RTV silicone materials can also act like a potting material by using it to encapsulate circuitry that would otherwise be exposed to the environment. One potential problem with RTV silicone is thermal degradation and outgassing. Environmental testing should be used to investigate these materials before use in certain application areas.

Double-Sided Adhesives

Another adhesive option that can be used in place of kapton or silicone is commercial double-sided tapes. These are commonly used in construction and can provide very strong adhesion once fully bonded and cured.

VHB double-sided tape

Just like RTV silicone, there may be outgassing when the enclosure is used in a low pressure/high temperature environment. Thermal stability also varies widely across different products, particularly because many of these products are not always designed for use in electronics. To determine if a particular material is acceptable, it should be environmentally tested first.

Encapsulation

The easiest way to protect a circuit board is to fully encapsulate it inside its enclosure with a potting material. Encapsulants can be mixed and poured into a sealed enclosure, and this will completely protect all surfaces of the PCB. It also helps protect against vibration, and the encapsulant adds thermal mass around the PCB. For more information on potting materials and encapsulation, read this article.

Conformal Coating

While not normally used as an insulator for an enclosure, conformal coating does provide protection from exposed conductors and components when applied to a PCB. A conformal coating material can be used as an insulating material that protects exposed conductors against dust or contact with other conductors. However, when the PCB is mounted on a metal surface, a conformal coating will normally be very thin and could rub off against the surface. Therefore, a conformal coating may need to be used with another method to fully protect a PCB.

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