Custom Search
|
|
|
||
 MIL-HDBK-419A
The bonding path established by a rivet is illustrated in Figure 7-8. The current path through a rivet is
theorized to be through the interface between the bond members and the rivet body. This theory is justified by
experience which shows that the fit between the rivet and the bond members is more important than the state
of the mating surfaces between the bond members. Therefore, the hole for the rivet must be a size that
provides a close fit to the rivet after installation. The sides of the hole through the bond members must be free
of paint, corrosion products, or other non-conducting material.
For riveted joints in shields, the maximum spacing between rivets is recommended to be approximately 2 cm
(3/4 inch) or less (7-7). In relatively thin sheet metal, rivets can cause bowing of the stock between the rivets
as shown by Figure 7-9. In the bowed or warped regions, metal-to-metal contact may be slight or nonexistent.
These open regions allow rf energy to leak through and can be a major cause of poor rf shield performance. By
spacing the rivets close together, warping and bowing are minimized. For maximum rf shielding, the seam
should be gasketed with some form of wire mesh or conductive epoxy to supplement the bond path of the rivets.
7.4.2.6 Conductive Adhesive. Conductive adhesive is a silver-filled, two-component, thermosetting epoxy
resin which when cured produces an electrically conductive material. It can be used between mating surfaces
to provide low resistance bonds. It offers the advantage of providing a direct bond without the application of
heat as is required by metal flow processes. In many locations, the heat necessary for metal flow bonding may
pose a fire or explosion threat. When used in conjunction with bolts, conductive adhesive provides an effective
metal-like bridge with high corrosion resistance along with high mechanical strength. In its cured state, the
resistance of the adhesive may increase through time. It also tends to adhere tightly to the mating surfaces and
thus an epoxy-bolt bond is less convenient to disassemble than a simple bolted bond. In some applications, the
advantages of conductive adhesive may outweigh this inconvenience.
7.4.2.7 Comparison of Techniques. Table 7-2 shows comparative ratings of the most commonly used bonding
methods. In this table a rating from zero to 10 is assigned to each method for each performance parameter. A
rating of 10 means that the method is suitable from the standpoint of the specific parameter listed in the
extreme left hand column of the table. Lower ratings mean that the method is less suitable. A zero rating
implies the method is a poor choice, while the dash means it does not apply. One-hundred percent consistency
in ratings is impossible because any given method may vary widely in workmanship. A low-rated method
expertly performed, will work better than a high-rated poorly performed method. When using the table assume
that all methods are equally well implemented.
7.5 INDIRECT BONDS. The preferred method of bonding is to connect the objects together with no
intervening conductor. Unfortunately, operational requirements or equipment locations often preclude direct
bonding. When physical separation is necessary between the elements of an equipment complex or between the
complex and its reference plane, auxiliary conductors must be incorporated as bonding straps or jumpers. Such
straps are commonly used for the bonding of shock mounted equipment to the structural ground reference.
They are also used for by-passing structural elements, such as the hinges on distribution box covers or on
equipment covers, to eliminate the wideband noise generated by these elements when illuminated by intense
radiated fields or when carrying high level currents. Bond straps or cables are also used to prevent static
charge buildup and to connect metal objects to lightning down conductors to prevent flashover.
7-16
|
 
|
|
|
||
