PCB LAYOUT DESIGN USING CAD
To design a printed circuit board layout for a given circuit.
1.Personal computer with windows os.
2.PCB layout software such as ORCAD.
General rules for designing PCBs:
The PCB designer follows few rules of thumb that can be used when laying out PCBs. Here they are,
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.
2.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.
3.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.
When routing traces, it is best to have the snap to grid turned on. Setting the snap grid spacing to 0.050” works well. Changing to a value of 0.025” can be helpful when trying to work as densely as possible. Turning off the snap feature may be necessary when connecting to parts that have unusual pin spacing.
It is a commo0n practice to restrict the direction that traces run to horizontal, vertical or at 45 degrees angles.
When placing narrow traces, use 0.015” or less. Avoid sharp right angle turns. The problem here is that , in the board manufacturing process the outside corner can be etched a little more narrow. The solution is to use two 45-degree bends with a short leg in between.
It is a good idea to place text on the top layer of the board, such as the product or company name.
4.CHECKING YOUR WORK:
After all the traces are placed, it is best to double-check the routing of every signal to verify that nothing is missing or incorrectly wired. Do this by running through the schematic, one wire at a time. Carefully follow the path of each trace. After each trace is confirmed, mark the signal on the schematic with a yellow highlighter.
Inspect the layout, both top and bottom to ensure that the gap between every item is 0.007” or greater. Use the pad information tool to determine the diameters of pads that make up a component.
Check for missing vias. The CAD software will automatically insert a via when changing layers as a series of traces are placed. The user often forget that vias are not automatically inserted otherwise. For example, when beginning a new trace, a via is to first print a top layer , then print the bottom. Visually inspect each side for traces that doesn’t connect to anything. When a missing via is found, insert one. Do this by clicking on the pad in the side tool bar from the down list box and click on the layout.
Check for the traces that cross each other. Inspecting a printout of each layer easily does this.
Metal components such as heat sinks, crystals, switches, batteries and connectors can cause shorts, if they are placed over traces on the top layer. Inspect for these shorts by placing all the metal components on a printout of the top layer. Then look for traces that run below the metal components.
Thus the PCB is designed using CAD.