EMC Bonding Basics | Bonding Types | RF Wireless World
This page covers EMC Bonding basics and EMC Bonding types.Behaviour of bond at RF as well as types of bonding are described.
Definition: The electrical bonding is a process in which parts of a circuit or sub modules of a system are connected together electrically or by low resistance media.
Function: The main function of bond is to create homogeneous structure which will allow smooth flow of RF current. The bond will be such that it will incur minimum resistance to the flow of current. It does not create any potential at the joint or crossover point.
In order to comply with EMC (Electromagnetic Compatibility) specifications all the components of a module or all the modules of a system are connected to a common chassis or common reference ground via low impedance bond. This kind of bond provided to meet EMC specifications is known as EMC bond and the process carried out is known as EMC bonding.
The main purpose of bonding is to prevent voltage difference between two parts being joinded.
Bond behaviour at RF or Microwave frequency
The figure-1 depicts behaviour of bond at Radio Frequencies (RF). Due to parasitic effects, bond is no longer purely resistive at RF. It exhibits series inductance and parallel capacitance also in addition to resistance.
The circuit acts as parallel resonance circuit. The circuit provides high impedance at higher frequencies. Moreover at high frequencies skin effect exists which results into flow of current via outer peripheral of the conductor. Due to this bond offers very high resistance.
To perform bonding at DC or low frequencies (i.e. 50 Hz or 60 Hz), low resistance joint can be adequate. This kind of bond is created using bolts and star washers at the point of contacts between the two metal walls. Such a bond is not suitable at high RF/Microwave frequencies due to high resistance and high inductive reactance. This generates strong RF fields due to harmonics and spurious.
To reduce above problems, bonding process is divided into mechanical and electrical bonding. Mechanical bonding ensures appropriate robust strength is being applied to the parts being joined. Electrical bonding ensures low impedance connectivity between parts to make them robust electrically.
Bonds are classified into two types viz. direct bond and indirect bond.
Direct Bond: In this bonding type, specific areas of members to be joined are placed in direct contact by permanent or semi-permanent bonds. Permanent bonds are carried out using welding, soldering or brasing. Semi-permanent bonds are carried out using bolted connections. For satisfatory direct bond, bolt/screw will serve as fastner and pressure of about 90 to 100 Kg/Cm2 need to be maintained.
Indirect Bond: This type of bonding is used in the applications where metal to metal contacts are not reliable to be used such as where parts are frequently removed, parts which use dis-similar metal types, parts which are exposed to corossion or parts which will have relative motion such as hinges. In this type of bond, other than primary bond with the help of mechanical joint, indirect bond with the help of jumper wires is being employed.
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