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 MIL-HDBK-419A
If a joint in a current path is not securely made or works loose through vibration, it can behave like a set of
intermittent contacts. Even if the current through the joint is at dc or at the ac power frequency, the sparking
which occurs may generate interference signals with frequency components up to several hundred megahertz.
Poor bonds in the presence of high level rf fields, such as those in the immediate vicinity of high powered
transmitters, can produce a particularly troublesome type of interference. Poorly bonded joints have been
shown to generate cross modulation and other mix products when irradiated by two or more high level signals
(7-1). Some metal oxides are semiconductors and behave as nonlinear devices to provide the mixing action
between the incident signals. Interference thus generated can couple into nearby susceptible equipments.
7.3 RESISTANCE CRITERIA.
A primary requirement for effective bonding is that a low resistance path be established between the two joined
objects. The resistance of this path must remain low with use and with time. The limiting value of resistance
at a particular junction is a function of the current (actual or anticipated) through the path. For example,
where the bond serves only to prevent static charge buildup, a very high resistance, i.e., 50 kilohms or higher, is
acceptable. Where lightning discharge or heavy fault currents are involved, the path resistance must be very
low to minimize heating effects.
Noise minimization requires that path resistances of less than 50 milliohms be achieved. However, noise
control rarely ever requires resistances as low as those necessary for fault and lightning currents. Bond
resistance based strictly on noise minimization requires information on what magnitude of voltage constitutes
an interference threat and the magnitude of the current through the junction. These two factors will be
different for every situation.
A bonding resistance of 1 milliohm is considered to indicate that a high quality junction has been achieved.
Experience shows that 1 milliohm can be reasonably achieved if surfaces are properly cleaned and adequate
pressure is maintained between the mating surfaces. A much lower resistance could provide greater protection
against very high currents but could be more difficult to achieve at many common types of bonds such as at
connector shells, between pipe sections, etc. However, there is little need to strive for a junction resistance
that is appreciably less than the intrinsic resistance of the conductors being joined.
Higher values of resistance tend to relax the bond preparation and assembly requirements. These requirements
should be adhered to in the interest of long term reliability. Thus, the imposition of an achievable, yet low,
value of 1 milliohm bond resistance ensures that impurities are removed and that sufficient surface contact
area is provided to minimize future degradation due to corrosion.
A similarly low value of resistance between widely separated points on a ground reference plane or network
ensures that all junctions are well made and that reasonably adequate quantities of conductors are provided
throughout the plane or network. In this way, resistive voltage drops are minimized which helps with noise
control. In addition, the low value of resistance tends to force the use of reasonably sized conductors which
helps minimize path inductance.
7-3
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