General Engineering Terms
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Editor note; PWB terms are listed below.
The alphabetic links above will switch to the main dictionary.
This graphic show four different methods of mounting a metal can, used by transistors, FETs, Op amps and some regulators.
The first and most common is Flush mounting [plug-in mounting]. In high vibration environments a bead of Adhesive might be applied to one or more sides of the can to insure the device stays in place.
In some cases the component is left above the circuit board. Normally components are lifted above the board for one of two reasons; the package is conductive or high power. Conductive packages or metal cans are raised above the surface of the board so the metal package does not short traces running underneath the bottom of the component. Components that generate a large amount of heat are raised off the PCB so the board does not get burnt and to allow air flow under the component.
TO-5 metal Can Board Mounting Options
A plastic spacer might be used beneath the component to make the component more secure.
The final component mounting method is the Dead-Bug approach. Although this view shows the component body going through the board [which would imply that it was designed to mount that way]. The normal method is just to turn the device upside down and wire to the leads as shown in the picture to the right.
In most cases when a part is dead-bug it's due to a mistake. Either the board was laid out wrong or the part was changed at the last minute and had a different pin-out. Turning the component upside down solves the wiring problems but creates a new one. Now the part has to be glued to the board and the board needs to be re-worked with 'white-wire', [mag wires] or what ever term is used to indicate discrete wiring during re-work.
Worse yet some boards may not be allowed to have this type of rework, meaning either the board will have to be scraped or just used for circuit debug.
The resistor dictionary, which is separate from this PWB dictionary, covers PWB Resistor Mounting.
Four different mounting methods are shown using a TO-5 metal Can. However any component could have been used as the example and the TO-5 is just one of a number of Transistor packaging options. Transistor or IC Sockets would add two additional concerns; vibration and power dissipation. In most cases a component is held in a socket by pressure, only a few component sockets depend on a solder connection. Because the component is no longer attached to the printed wiring board, but instead attached to the socket instead, the actual component may be susceptible to system vibration. Power dissipation only becomes a concern because the lead length of the component was increased, increasing the resistance to heat flow out the component leads. Additional effects like ground bounce also are increased by using a IC socket.
