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 MIL-HDBK-419A
7.4.2.3 Soft Solder.
Soft soldering is an attractive metal flow bonding process because of the ease with which it can be applied.
Relatively low temperatures are involved and it can be readily employed with several of the high conductivity
metals such as copper, tin and cadmium. With appropriate fluxes, aluminum and other metals can be soldered.
Properly applied to compatible materials, the bond provided by solder is nearly as low in resistance as one
formed by welding or brazing. Because of its low melting point, however, soft solder should not be used as the
primary bonding material where high currents may be present. For this reason, soldered connections are not
permitted by MIL-STD-188-124A or the National Electrical Code in grounding circuits for fault protection.
Similarly, soft solder is not permitted for interconnections between elements of lightning protection networks
by either the Military Standard, the National Fire Protection Association's Lightning Protection Code or the
Underwriter's Master Labeled System. In addition to its temperature limitation, soft solder exhibits low
mechanical strength and tends to crystallize if the bond members move while the solder is cooling. Therefore,
soft solder should not be used if the joint must withstand mechanical loading. The tendency toward
crystallization must also be recognized and proper precautions observed when applying soft solder.
Soft solder can be used effectively in a number of ways, however. For example, it can be used to tin surfaces
prior to assembly to assist in corrosion control. Soft solder can be used effectively for the bonding of seams in
shields and for the joining of circuit components together and to the signal reference subsystem associated with
the circuit. Soft solder is often combined with mechanical fasteners in sweated joints. By heating the joint hot
enough to melt the solder, a low resistance filler metal is provided which augments the path established by the
other fasteners; in addition, the solder provides a barrier to keep moisture and contaminants from reaching the
mating surfaces.
7.4.2.4 Bolts.
In many applications, permanent bonds are not desired. For example, equipments must be removed from
enclosures or moved to other locations which require that ground leads and other connections must be broken.
Often, equipment covers must be removable to facilitate adjustments and repairs. Under such circumstances, a
permanently joined connection could be highly inconvenient to break and would limit the operational flexibility
of the system. Besides offering greater flexibility, less permanent bonds may be easier to implement, require
less operator training, and require less specialized tools.
The most common semipermanent bond is the bolted connection (or one held in place with machine screws, lag
bolts, or other threaded fasteners) because this type bond provides the flexibility and accessibility that is
frequently required. The bolt (or screw) should serve only as a fastener to provide the necessary force to
maintain the 1200-1500 psi pressure required between the contact surfaces for satisfactory bonding. Except for
the fact that metals are generally necessary to provide tensile strength, the fastener does not have to be
conductive. Although the bolt or screw threads may provide an auxiliary current path through the bond, the
primary current path should be established across the metallic interface. Because of the poor reliability of
screw thread bonds, self-tapping screws are never to be used for bonding purposes. Likewise, Tinnernman nuts,
because of their tendency to vibrate loose, should not be used for securing screws or bolts intended to perform a
bonding function.
7-14
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