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 MIL-HDBK-419A
It should be recognized that a low dc bond resistance is not a reliable indicator of the performance of the bond
at higher frequencies. Inherent conductor inductance and stray capacitance, along with the associated standing
wave effects and path resonances, will determine the impedance of the bond. Thus, in rf bonds these factors
must be considered along with the dc resistance.
7.4 DIRECT BONDS.
Direct bonding is the establishment of the desired electrical path between the interconnected members without
the use of an auxiliary conductor. Specific portions of the surface areas of the members are placed in direct
contact. Electrical continuity is obtained by establishing a fused metal bridge across the junction by welding,
brazing, or soldering or by maintaining a high pressure contact between the mating surfaces with bolts, rivets,
or clamps. Examples of direct bonds are the splices between bus bar sections, the connections between
lightning down conductors and the earth electrode subsystem, the mating of equipment front panels to
equipment racks, and the mounting of connector shells to equipment panels.
Properly constructed direct bonds exhibit a low dc resistance and provide an rf impedance as low as the
configuration of the bond members will permit. Direct bonding is always preferred; however, it can be used
only when the two members can be connected together and can remain so without relative movement. The
establishment of electrical continuity across joints, seams, hinges, or fixed objects that must be spatially
separated requires indirect bonding with straps, jumpers, or other auxiliary conductors.
Current flow through two configurations of a direct bond is illustrated in Figure 7-2. The resistance,
of the
path through the conductors on either side of the bond is given by
(7-1)
where
is the resistivity of the conductor materials,
is the total path length of the current through
conductors, and A is the cross-sectional area of the conductors (assumed equal). Any bond resistance at the
junction will increase the total path resistance. Therefore, the objective in bonding is to reduce the bond
resistance to a value negligible in comparison to the conductor resistance so that the total path resistance is
primarily determined by the resistance of the conductors.
Metal flow processes such as welding, brazing, and silver soldering provide the lowest values of bond resistance.
With such processes, the resistance of the joint is determined by the resistivity of the weld or filler metal which
can approach that of the metals being joined. The bond members are raised to temperatures sufficient to form
a continuous metal bridge across the junction.
For reasons of economy, future accessibility, or functional requirements, metal flow processes are not always
the most appropriate bonding techniques. It may then be more appropriate to bring the mating surfaces
together under high pressure. Auxiliary fasteners such as bolts, screws, rivets or clamps are employed to apply
and maintain the pressure on the surfaces. The resistance of these bonds is determined by the kinds of metals
involved, the surface conditions within the bond area, the contact pressure at the surfaces, and the cross-
sectional area of the mating surfaces.
7-4
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