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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