Radiocommunications Agency  EMC Awareness

Bonding

What this technique is used for

Bonding is the practice of electrically connecting two or more conductors so as to equalise their electrical potentials.

Sometime the phrase ‘RF bonding’ is used to make the point that the technique is required to equalise voltages that are fluctuating at radio frequencies, which is much more difficult than equalising low frequencies such as 50Hz.

When two conductors are at different voltages an electric field is created between them. If the voltage difference fluctuates then there is also a current associated with each of the conductors which creates a magnetic field. The fluctuating electric and magnetic fields create an electromagnetic (EM) wave, possibly causing emissions problems.

Such a structure is an ‘unintentional antenna’. In the presence of an EM field it will carry an induced voltage and current, possibly causing immunity problems for the related circuit. Bonding is a means of reducing the efficiency of such ‘unintentional antennas’.

Bonding is used when shielding areas or volumes to allow internal and external surface currents to flow unimpeded between its various metal parts, so that the shield acts as a good shield and not as an assembly of unintentional antennas.

How this technique is used

Conductors are either brought into physical contact so that they make a reliable metal-to-metal contact, or they are connected together by an intermediate ‘bonding conductor’ which could be a piece of wire, a braid strap, a metal plate, or a conductive gasket.

Multiple bonds, or continuous seam-bonding, are needed for RF bonding.

Key issues in employing this technique

Don’t confuse it with ‘earthing’ or safety bonding

When used in an EMC context (as here), ‘bonding’ can be used on conductors at any voltage with respect to ‘earth’, ‘ground’ or 0V. It should not be confused with ‘earthing’ which is a safety engineering term. RF energy, and thus EMC, does not care about ‘earthing’ to the mass of the earth or the protective conductor in a mains cable – all such connections are simply conductors like any other conductors as far as RF is concerned.

Bonding is also a term used in electrical safety engineering, so care is needed to prevent misunderstandings. Bonding techniques used for safety purposes (e.g. a length of green or green/yellow insulated wire) usually have too much inductance to be effective above 1MHz in any case and should not be confused with bonding for EMC, or RF bonding.

Unfortunately, the terms ‘earth’ or ‘ground’ (and earthing and grounding) are widely misused by designers, leading to great deal of confusion and project delay. It is best to restrict the use of these two terms to electrical safety engineering and never use them in the context of an electronic unit – except for referring specifically to its protective earth/ground conductor or a protective bonding circuit.

Multiple bonds or seam bonds required for RF bonding

A single bond between two conductors forces the ‘potential equalising current’ to flow via a specific route, through a small contact area, and at RF this can have too much inductance. So multiple bonds are required for RF bonding, spread along the entire length of the joint between the two conductors.

To have any significant effect, the spacing of the bonds should be less than one-tenth of the wavelength at the highest frequency of concern. At UHF frequencies, or when very high performance bonding is required, continuous seam bonding is usually required.

Direct metal-to-metal bonding is the best technique, and seam welding is the best fixing method.

Conductive gaskets, sometimes called EMC gaskets, are mostly used where a metal joint may need to be opened from time to time, for example at an access panel or door, to avoid the time-consuming task of unbolting a number of wires or metal plates.

A short length of wire (whatever the colour of its insulation) will usually have too much inductance to be an effective bond above 1MHz. A short wide braid strap will usually be an ineffective bond above about 10MHz. However, placing multiple short wires or straps in parallel along the entire length of a joint helps to increase their bonding effectiveness at higher frequencies.

360° bonding

This is a term used to imply bonding all around the perimeter of a cable shield, around the periphery of an aperture in a shielded area or volume, or around the outside surface of a filter – even where the cable, aperture or filter does not have a circular cross section. Such bonds typically use conductive gaskets of some sort to provide a continuous seam bond.

Corrosion

When dissimilar metals are bonded, or when the bonding material is not the same as the metal being bonded, issues of galvanic corrosion arise – especially when liquids (such as condensation) might be present at the junction of the dissimilar metals. Corrosion will reduce the effectiveness of a bond, and eventually destroy it totally.

Corrosion at joints can also be a cause of intermodulation. Poor electrical joint quality is an issue for safety bonding too, and information on preventing corrosion may be found in textbooks and articles on electrical safety as well as EMC.