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In the previous parts of this article, we read about high-speed PCB design, multi-layer PCB design and grounding for electromagnetic compatibility. Let us now look at electrical bonding.
Electrical bonding is a process by which parts in an equipment assembly or sub-systems within a system are connected electrically by their joints or by any low-resistance bonding media. The main purpose of a bond is to make a structure homogenous with respect to the flow of radio frequency (RF) current, so that it would experience minimum barrier as it crosses one surface to the other without developing electrical potential at the crossover point.
To ensure electromagnetic compatibility (EMC) in an equipment or a system, the various components, modules, cable shields, connectors, etc that make up the system need to be connected to a chassis or reference ground via a low-impedance bond that should provide near-zero impedance at all frequencies, or at least over those frequencies for which EMC standards require measurements to be carried out.
Good bonding is required, for example, for mounting line-filter modules on the chassis that serves as a drain for EMI currents, or for connector shells to equipment enclosures to ensure shielding integrity of cable shields that are terminated on these connectors, or for ensuring shielding integrity over seams and joints to avoid leakage of RF energy.
Behaviour of a bond at RFs
At direct current (DC) or power frequencies, bond resistance should be less than one milli-ohm. Even if this is ensured, behavior of the bond at RF can be very different, since parasitic effects set in and the bond is no longer purely resistive. Fig. 21 represents the bond at RF where you can see that, in addition to resistance, there is now self-inductance in series and residual capacitance in parallel.
This now becomes a parallel resonant circuit and at its resonant frequency (fr) it exhibits a high impedance. In addition, at higher frequencies, due to skin effect, current tends to flow via the outer periphery of the conductor, and the bond offers a high resistance.
Types of bonding
The intention of bonding is to prevent a voltage difference between the members being joined. To perform effectively at DC or low frequencies, including 50Hz or 60Hz power frequencies, a simple (but durable and permanent) low-resistance joint is adequate.
To accomplish a low DC resistance bond between two adjoining cabinets, bolt holes can be drilled in the adjoining parallel cabinet walls and the two cabinets can be bolted together with star washers at the points of contact between the cabinet walls.
But such a bond is deficient at high frequencies because of higher resistance and inductive reactance. This is more marked where the bond involves dissimilar metals where non-linear junctions can develop at the bonds, leading to the generation of spurious and harmonics, especially under the presence of strong RF fields, causing interference.
To reduce these problems, a radical approach is required if the bond is to work at high frequencies. The most effective method is to split the bonding process into two—mechanical and electrical. Mechanical part of the bond provides only the physical strength to make it robust mechanically, while electrical part of the bond provides low-impedance connectivity to make the bond electrically robust.
Bonds can be broadly classified as direct bonds and indirect bonds. Direct bonding is where specific portions of the surface areas of the members are placed in direct contact by permanent or semi-permanent bonds. Permanent bonds can be achieved by welding, brasing or soldering. Semi-permanent bonds are bolted connections that provide flexibility and accessibility. For satisfactory bonding, the bolt or screw should serve as a fastner, and should maintain a pressure of 85kg/cm2 to 110kg/cm2.
An indirect bond is used in situations where the metal-to-metal contact is not electrically reliable, like for parts that are removed frequently, parts that have relative motion like hinges, parts that use dissimilar metals and parts that are exposed to corrosion. Here, in addition to the primary bond (which provides for the mechanical joint), an indirect bond in the form of straps or jumpers (thick wires) is incorporated (Fig. 22).
Jumpers are employed in low-frequency scenarios and can be short or stranded conductors, generally round in cross-section. These jumpers, unfortunately, exhibit self-inductance and residual capacitance at high frequencies due to the skin effect. In such cases, special flat braids or sheet-metal straps called bond-straps are used. For bond-straps to work effectively, width-to-thickness ratio should be ten or more, while length-to-width ratio should not exceed five.
Bonding of protective earth wire. In a typical electronics cabinet, a protective earth wire (green or green-yellow wire) is used for safety reasons to connect the cabinet to external earth.
As shown in Fig. 23, this is usually done by providing a bonding terminal—essentially a bolt screwed to position and held in place by a captive nut or a stud welded to the enclosure cabinet, the latter being a better option. While fixing the earth wire, any insulating surface finish (paint or powder coating) is removed from the contact area.
The contact area should be slightly larger than the surface area of the lug. A spiky or shake-proof washer is first put into place to ensure a good lifetime bond. Over this is placed the lug. Another shake-proof washer is then placed over the lug after which the nut is tightened. If the bond is likely to be exposed to moisture or corrosive environment, a coating of paint or grease is applied over the bond.