Developing a PCB Board In PADS Layout Micajah Worden 11-15-10 Executive Summary Pads is a powerful tool to develop PCBs. This document will provide a look into some of the basic functions of this software, which will allow you to develop your own designs and have them fabricated. Key Words: Layer, Via, Net, PCB, Gerber, NetlistIntroduction: Printed circuit boards, or PCBs, are used in a wide variety of applications. From cell phones to microwave ovens, virtually every electrical device utilizes PCBs to expedite the development of their projects with minimal human assembly of the final product. PCBs allow a large amount of circuitry to be placed in a small area, with many different options for the number of layers on the board and thus how densely you can populate the board. They are milled, machined, and assembled by machines and can be produced in great quantities. Though the initial cost of development is high, with a large quantity for production, the cost per unit goes down as well as the likelihood of error inherent in human assembly. For this reason it is a valuable skill to be able to design your own boards. While at first tricky, with the help of this document, the help files included in the PADS Logic software package, and a little practice you will be able to develop your design, export Gerber files, and have a company fabricate your board for production. Objective: At the completion of this document, the user should have the basic tools to utilize this software to develop a two layer PCB board, as well as check the project for errors, and export the necessary files to have the board fabricated. It is assumed that the user has a schematic developed for their project, and a basic understanding of electrical principles. Developing new components for the PCB board, as well as developing schematics in PADS Layout are outside of the scope of the project. For each of these operations the user can view the help files and tutorials included in the software package.Starting a Project: Start a new project by going to File>new. A black screen marked with a hashing of dots should appear. To modify the layer definitions of your project go to Setup>Layer Definition. A layer Setup dialog box will appear. The default number of layers is two. That is all you will need for this project. There are many options in this dialog box for layers. The first two layers labeled Top and Bottom are the electrical layers of the PCB. This is where all the traces and components will be located. The CM under Type specifies this as a component layer. Further down the layer list, in the nonelectrical layers, the silkscreens, drill drawings, assembly drawings, and soldermasks for the top and bottom layer of the board are located. These are necessary layers for production of the board. Figure 1: Layer Setup dialog box.Creating a Board: Now that the layers are specified a board must be made to construct your PCB design. This can be done by selecting the Drafting Toolbar on the Toolbar. Next select Board Outline and Cutout. There are three options for general board shape; Polygon, Circle, and Rectangle. Select rectangle. Left click on the workspace and create a rectangular workspace. Now a board outline has been created. > Figure 2: Drafting Toolbar icon, Board Outline and Cutout icon, example of board outline in the workspace Design Rules: Design Rules specify the minimum distance between traces, Vias, and pins. For this you must know the capability of the PCB fabricator you will be using. To modify the design rules for the PCB simply select the Setup Menu, and click on Design Rules. A Design Rules dialog box will appear. Click on Default rules. By selecting Clearance you will be able to adjust minimumspacing between many different types of components. A matrix is located within this dialog box, with eight different categories for design rules. Adjusting these will help when fabricating your board if you set them greater than the minimum resolution of the PCB fabricating machine. Once you’ve checked the level of precision of the machines that will be fabricating your boards you can safely enter these values, to ensure that you have created a design that the machines can handle. Figure 3: Clearance Rules Dialog box.Adding Components: So far the board outline has been created, the layers have been specified, and design rules now govern the PCB design. Now it is time to add components to your design. Keep in mind that components can be mounted on both the top and the bottom of this two layer board, so there is no reason to crowd one side or the other. Components mounted on the Top layer will appear as red and components on the Bottom layer will appear as blue. All components will appear on the primary side of the board at the origin, and must be moved to their desired location. PADS has a large library of standard components, including resistors, inductors, capacitors, and many ICs. If your part is not found in the parts library you must create the part. This function is outside the scope of this application note, but many resources can be found in the help file of the PADS software package to aid you in this endeavor. To place a part Press the ECO button on the Toolbar. A dialog box will appear. Press the OK button. In the ECO Toolbar select ADD Component. There are many libraries to select parts from including ICs and common components, but to view the parts you must type the first letter of the part name followed by an asterix e.g. R* to search for resistors. Select the desired component from the library and you will see an example in the Preview Area. Add the desired part to your design and place it in the desired location. The part name should appear in the component list in the Project Explorer bar. If you would like to place the component on the other side, select the component in ECO Mode, right click the component and select Flip Side. If you are importing a Netlist from a schematic parts in the library will be uploaded to the workspace automatically. You can change the name of components in ECO mode by selecting Rename Component in the ECO Toolbar. Components can also be deleted in by selecting Delete Component from the ECO Toolbar.> or Figure 4: ECO Toolbar Icon, Add Component Icon, Rename Component icon, Get Part From Library Dialog box as well as part placed in the Board Outline. Creating Net Lists: In PADS there are numerous ways to
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