08-01-2013, 03:01 PM
PCB LAYOUT DESIGN USING CAD
AIM:
To design a printed circuit board layout for a given circuit.
APPARATUS REQUIRED:
1.Personal computer with windows os.
2.PCB layout software such as ORCAD.
THEORY:
General rules for designing PCBs:
The PCB designer follows few rules of thumb that can be used when laying out PCBs. Here they are,
PLACING COMPONENTS:
Generally, it is best to place parts only on the topside of the board. Firstly place all the components in specific locations. This includes connectors, switches, LED mounting holes, heat sinks or any other item that mounts to an external location.
Give careful thought when placing components to minimize trace lengths. Doing a good job here will make laying the traces much easier.
Arrange ICs in only one or two orientations (up and down or right and left). Align each IC so that pin 1 is in the same place for each orientation, usually on the top or left sides. Position polarized parts with the positive leads, all having the same orientation. Also use a square pad to mark the positive leads of these components.
Frequently, the beginners run out of room when routing traces. Leave 0.35 to 0.5 between ICs. For large ICs allow even more space.
Parts not found in the component library can be made by placing a series of individual pads and then group them together. Place one pad for each lead of the component. It is very important to measure the pin spacing and pin diameters as accurately as possible.
After placing all the components, print out a copy of the layout. Place each component on the top of the layout. Check to insure that you have allowed enough space for every part to rest without touching each other.
PLACING POWER AND GROUND TRACES:
After the components are placed, the next step is to lay the power and ground traces.A power rail is run along the front edge of the board and a ground rail along the rear edge.From these rails attach traces that run in between the ICs. The ground rail should be very wide, 0.100” and all the supply lines should be 0.50”. When using this configuration the remaining of the bottom layer is then reserved for the vertical signal traces.
PLACING SIGNAL TRACES:
When placing traces, it is always a good practice to make them as short and direct as possible. Use vias to move signals from one layer to the other. A via is a pad-through hole. Generally the best strategy is to lay out a board with vertical trace on one side and horizontal traces on the opposite side. A good trace width for low current digital and analog signals is 0.010”.
Traces that carry significant current should be wider than signal traces. The table below gives rough guidelines of how wide should a trace be for a given amount of current.
RESULT:
Thus the PCB is designed using CAD.