Popular Shielding Types
In situations with long signal runs or complex circuits, shielding becomes a crucial part of reducing interference and maintaining signal integrity. Metallic braids and foil shields are options for wire shielding that most people are likely familiar with. But there are additional options available that may fit your application better. We’ll explore the most common shielding types and their pros and cons.
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Metallic Braid Shields
Metallic Braid Shields are usually formed from bare, tinned or silver plated copper strands. Steel and other metals can be used for physical protection. They are woven over a conductor or conductors in the same manner as a textile braid.
Braid shields effectiveness is proportional to the amount of braid coverage over the wire. 75-85% coverage is standard, 100% coverage is technically unobtainable. The greater the braid coverage, the slower it is to produce, the more it costs and the less it will flex. Braids provide mechanical strength as well as flexibility and flex life and are most effective at low frequencies.
Often a braid shield is supplemented by a foil shield to increase shield coverage and still provide the benefits of a braid shield and maximum shield coverage. Braid shields are specified on all mil spec applications.
To terminate a braid shield it is necessary to unbraid the braiding material and hand terminate to ground. An uninsulated ground wire can be included, in contact with the braid for easy termination. The normal drain wire in electronic applications is tinned copper to prevent the drain from corrosion. The braid can be "pig tailed" for termination.
Key factors to consider when using metallic braid shields include:
- Greater flex life and flexibility
- Increased mechanical strength
- Offers physical protection
- Effective at all frequencies
- Braids can be combined with foil for maximum performance
- 40% to 96% coverage
- Difficult to terminate
- Slow and costly to produce
- More costly than foil
Foil Shields
Foil Shields are usually constructed of an aluminum and mylar composite tape. The appearance is much like the foil in a gum wrapper. Foil shields are light weight, inexpensive and easy to apply. Foil can face in or out. The drain wire must be in contact with the metallic part of the shield.
Foil shields will provide 100% shield coverage and are normally terminated (grounded) with a drain wire. The drain wire provides easy termination. Foil shields, especially in multi pair constructions can be color coded. Foil provides good flexibility but limited flex life.
One side of a foil shield is metallic and the opposite side is non-conductive. The drain wire must be in contact with the aluminum (metallic) side of the shield to provide for proper grounding of the shield. The drain wire is usually one size smaller than the conductors it is terminating the shield for. The shield can face in towards the conductors or face outward toward the jacket.
Key factors to consider when using foil shields include:
- Thin and lightweight
- Good flexibility, mediocre flex life
- Can reach 100% coverage
- Easy to produce
- Less costly than braids
- Easy to terminate
- Braids can be combined with foil for maximum performance
- No mechanical strength
- Little physical protection
- Requires a drain wire
Spiral or Serve Shields
Spiral or Serve Shields are usually formed from bare, tinned or silver plated copper strands. Steel and other metals can be used for physicalprotection. The main advantage to a spiral or serve shield is that they are more flexible than braid shields and provide for easy termination. These shields are constructed by wrapping the shield material in one direction around the conductors. A reverse spiral shield is formed by wrapping shielding material in both directions on separate layers with no interweaving of wires.
Although spiral or serve shields can obtain shield coverage of 95% and greater they are recommended only for audio applications due to the frequencies they shield against.
- Inexpensive to produce
- Good flexibility
- Easy to terminate
- Cannot reach full coverage (up to 95%)
- Can be delicate
- LImited frequency coverage - audio use only
Tape Shields
Tape Shields are formed from a variety of materials, most common are copper, aluminum and bronze. Tape shields are applied at cabling and are wrapped around the conductors that are being shielded. A drain wire is required for easy termination. Copper and aluminum are usually combined with a backing such as mylar or polyester, much like foil shields. Tape shields can reach 100% coverage. Properties are similar to foil shields.
- Thin and lightweight
- Good flexibility
- Can reach 100% coverage
- Easy to produce
- Less costly than braids
- Easy to terminate
- Braids can be combined with foil for maximum performance
- Mediocre flex life
- No mechanical strength
- Little physical protection
- Requires a drain wire
Combination Shields
Combination Shields are formed of two or more shields combined in the same cable. The most common combinations are a braid shield over a foil shield or a braid shield over a braid shield.
Key factors to consider when using combination shields include:
- Maximum shield effectiveness
- Good physical protection
- Easy to terminate
- 100% foil coverage
- Good flexibility
- Low DC resistance
- Good flex life
- Good mechanical strength
- Costly to produce