Malikarjun Avula, Emil Jovanov Electrical and Computer Engineering Department University of Alabama in Huntsville CPE 495 September 03, 2009

Transcription

1 Malikarjun Avula, Emil Jovanov Electrical and Computer Engineering Department University of Alabama in Huntsville CPE 495 September 03, 2009 Agenda PCB Design Process General Guidelines Express SCH Getting Started Express PCB Getting Started Circuit Board Design Tips PCB Design Tutorial on ExpressPCB

2 PCB Design Process General Guidelines Before the design Preparing the design Draw the board Import the net Place the parts Route the traces Final work PCB Design Process(Cont.) Before the design You will need (as a minimum): The required dimensions of the board A list of parts to be used on the board The data sheets for each part A schematic diagram of the board

3 PCB Design Process(Cont.) Preparing the Design Generating the Netlist The netlist is a file that has the device names of the parts used on the board. It also shows the nets interconnections between the pins of the parts. PCB Design Process(Cont.) Preparing the Design Creating the Parts DataBase Most ECAD systems define the parts to be placed on the board as a special type of file called the PDB file. PDB is built in a hierarchy: Devices Packages Pads Pads: Entities that interface the part pins to the copper traces of the board. Packages: Entities that represent the part. Consist of lines and text. Devices: Entities that link the value or part number of a part with the ECAD package.

4 PCB Design Process(Cont.) Draw the Board First draw outline of the board May need to set up placement keep out areas: if board fits into a card rack need to allow for amount of board that sits in the slides a keep out of around board edge to allow for manufacturing tolerances PCB Design Process(Cont.) Place the Parts Placement factors Electrical function Physical size Temperature factors Routability

5 PCB Design Process(Cont.) Import the Netlist The netlist is then read in and associated with any pre existing fixed parts on the board Any errors made will usually be apparent here Automatic error detection is one of the big advantages of using ECAD systems PCB Design Process(Cont.) Routing the Board Routing is the connecting of the parts that have been placed on the board. Even if adopt automatic routing it is good practice to lay out power and ground traces. Good routing strategy normally means trace should be as short and direct as possible. Vias allow better routing of the board as vertical/horizontal strategy is maintained as well as minimizing signal path.

6 Express SCH Express PCB Express SCH Drawing a Schematic Very easy to use Saves time designing when linked with Express PCB Makes Express PCB easy to learn Component Manager Easiest way to place components in the board layout.

7 Express SCH(Cont.) Custom Components Facilitates building of our own components. Sheets and Ports Two or more sheets can be used in case of over populated schematics. Ports can be used to make the electrical connections between the sheets. Check for Errors Check for errors by running Check schematic for netlist errors option from the File menu. Express PCB Used to design the circuit board. 2 or 4 layer board can be designed. Schematic file can be linked to the circuit board layout. Snap to grid option helps align traces and components. Filled Planes are used to add ground or power planes to a circuit, usually on double sided boards.

8 Express PCB (Cont.) Component Manager The easiest way to place components on board is to use Component Manager Custom Components Easy to build new components while designing PCB. Ordering Boards Compute Board Cost can be used to estimate the cost. Boards can be ordered online. Circuit Board Design Tips

9 Placing Components Minimize trace lengths. Arrange ICs in only one or two orientations: up and down, or, right and left. Polarized parts (i.e. diodes, and electrolytic caps) with the positive leads having same orientation. Leave 0.350" 0.500" between ICs, for large ICs allow even more. Print the layout and ensure enough spacing by placing the parts on it. Placing Power and Ground Traces ICs need to have solid power and ground lines. Use wide traces that connect to common rails for each power supply. Avoid snaking or daisy chaining the power lines from part to part.

10 Bottom layer of the PC board includes a "filled ground plane. Large traces feeding from a single rail are used for the positive supply. Placing Signal Traces Make them as short and direct as possible. Use vias to move signals from one layer to the other. A good trace width for low current digital and analog signals is 0.010". Think about the space between the trace and any adjacent traces or pads. Place text on the top layer of your board.

11 Checking Your Work Double check the routing of every signal. Check for traces that cross each other. Check for missing vias. Inspect your layout to insure that the gap between every item is 0.007" or greater. Inspect for these shorts by placing all the metal components on a printout of the top layer.