Passive components are those that need
no power supply for their operation and whose action will dissipate
power, though in some cases the amount of dissipation is negligible.
No purely passive component can have an output that supplies more
power than is available at the input.
Active components, by contrast, make
use of a power supply, usually DC, so that the signal power output of
an active component can be higher than the signal power at the input.
Typical passive components are resistors, capacitors and inductors.
Familiar active components are
transistors and ICs. All components, active or passive, require to be
connected to a circuit, and the two main forms of connection,
mechanical and electrical, used in modern electronic circuits are the
traditional wire leads, threaded through holes in a printed circuit
board and the more modern surface mounting devices (SMDs) that are
soldered directly on to the tracks of a board. Both passive and
active components can use either type of connection and mounting.
Components for surface mounting use
flat tabs in place of wire leads, and because these tabs can be short
the inductance of the leads is greatly reduced. The tabs are soldered
directly to pads formed onto the board, so that there are always
tracks on the component side of the board as well as on the opposite
side.
Most SMD boards are two sided, so that
tracks and components are also placed on the other side of the board.
Multilayer boards are also commonly used, particularly for mobile
phones (4 to 6 layers) and computer motherboards.
The use of SMDs results in
manufacturers being able to provide components that are physically
much smaller, but with connections that dissipate heat more readily,
are mechanically stronger and have lower electrical resistance and
lower self-inductance. Some components can be made so small that it
is impossible to mark a value or a code number onto them.
This presents no problems for automated
assembly, since the tape or reel need only be inserted into the
correct position in the assembly machine, but considerable care needs
to be taken when replacing such components manually, and they should
be kept in their packing until they are soldered into place.
Machine assembly of SMD components is
followed by automatic soldering processes, which nowadays usually
involve the use of solder paste or cream (which also retains
components in place until they are soldered) and heating by blowing
hot nitrogen gas over the board. Packaging of SMD components is
nowadays normally on tapes or in reels.
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