3.5 Preparing for orcad layout  (Page 2/9)

Creating footprint libraries

We are now ready to begin working in layout to proceed with our design. The first thing we need to do is create a library of footprints to be used in our PCB layout. Footprints are a representation of the physical area that a part occupies on a PCB.

Start Layout Engineer’s Edition to begin working with footprints. Libraries for Footprints are very similar to libraries for schematic parts. Layout has a separate tool for working with footprint libraries, though. To start this tool, select Tools-->Library Manager . You will get a new window that looks like this.

You will notice that there are already several libraries available for use. OrCAD has many existing footprints that you can use in your own design. As with the schematic symbols, be very careful to check that these footprints for correctness before using them. Often, you will have to make footprints for partsthat don’t already have one. Most datasheets for parts will contain the mechanical information necessary to make a correct footprint. However, before making a footprint it is necessary to understand a little bit about how PCBs are constructed.

Let’s take a look at a padstack definition for an existing part. In the Library Manager , select the library DIP100T and highlight the first part DIP.100/14/W.300/L.700 . You will see the part footprint in the Library Manager .

Layout uses a series of spreadsheets to store information about your design. Padstacks are stored in the padstack spreadsheet. To access this spreadsheet,click the View Spreadsheet icon and choose Padstacks . This footprint is composed of two padstacks, one for pin 1, which is square, and another padstack for the other pins. When you open the spreadsheet, you will first see a padstack called T1 . Padstacks T1 to T7 are default padstacks and can be modified for your own use. The padstacks we want to look at are at the bottom of the list; scroll down until you see DIP100T.llb_pad1 or DIP100T.llb_pad2 . These are the two padstacks for this footprint. You will notice that there are numbers on some of the layers that define how the padstack looks physically on that particular layer. We will come back to this in aminute.

Close the padstack spreadsheet and open up the footprints spreadsheet. The name is confusing; it should really be called something like the pins spreadsheet because this spreadsheet defines the locations of the pins and also which padstack they use. You will see each pin for the part in thisspreadsheet, its x and y locations, and the padstack used for each pin. Notice that pin 1 uses the square padstack, while the others use the round one.