Title: Perforated Circuit Board Design Electrical Engineering 317

Authors: Chris Schrage and Tom Grady

Editor: A. John Boye, Professor

ABSTRACT

An introduction to the layout of a perforated circuit board (PCB) is examined. The design specification and layout guidelines are particular only to the equipment in UNL's College of Engineering and Technology Electronics Shop and do not necessarily apply to other processes.

INTRODUCTION AND THEORY

Perforated circuit boards (PCBs) are crucial in today’s electronics market. They provide a compact way for manufacturers to efficiently connect components of a particular product. Designing the layout for an individual PCB is not always an easy task. Considerations may include spacing between traces (high frequency applications), trace length matching (timing between components), and trace thickness (high current). These must then be applied to a PCB that has a set area, and can easily have thousands of traces and components.

Producing a PCB

One way to manufacture a PCB is by using a device called the LPKF ProtoMat^® C60. This is the device used in UNL's College of Engineering and Technology Electronics Shop. This hardware communicates with the computer via a software package called Protel^®. Depending upon which version of Protel^® is being used, the file that is generated by this program is called *.pcb, or *.ddb. (To avoid confusion make this file name unique.) (This file, along with a one-to-one (1:1) printout of the layout is required by the Electronics Shop before production of a single PCB can begin. Depending on their workload, it may take the shop anywhere from one to five days to manufacture a single PCB.)

After a PCB is returned from the shop it will have a shiny clear coat on it. This is a solder flux that protects the copper board from oxidizing. It should not be removed, and will assist in soldering components.

General PCB Guidelines (UNL's College of Engineering and Technology Electronics Shop Only)

• Do not make the board dimensions too small. The larger the PCB, the fewer problems that will occur when trying to solder on components. (Note: The size of a PCB is proportional to the overall cost. Maximum board dimensions are 9” x 12”.)
• Use 60 mil pads on all components. (1 mil = .001 inch)
• Use traces that are greater than 12 mils for routing components.
• The hole size will usually be preset by the selected component.
• Use a spacing of at least 10 mils between traces.
• Always draw a line to designate the “keepout” layer. This line should transverse around the entire board. It denotes the overall PCB dimensions.
• To save on cost, try to stay with one trace size and one hole size.
• If a PCB is to be mounted using standoffs with 4-40 screw holes, the appropriate pad size is 250 mils and hole size is 3mm.

The cost of a particular PCB will depend solely on the layout of the board. The overall size of the PCB, number of traces and holes, amount of different trace and hole sizes, and if the board is single or double sided are all included in the price.

EQUIPMENT

Computer (Protel^® software package) 3.5” Floppy disk

LPKF ProtoMat^® C60

PRE-LAB PREPARATION

1. Select a circuit that you have used before (in this course or any previous one) or design a new one. The circuit needs to have at least one integrated circuit and two discrete components.

2. Design a PCB layout for your circuit. Label all the components and interconnections for the circuit, as well as the trace, pad, and hole sizes. Make an additional copy to hand in.

3. Locate a 3.5” floppy disk to bring to class.

EXPERIMENT

1. Use Protel^® to draw the PCB layout that was designed in Step 2 of the pre-lab portion of the lab. Save this file to your 3.5" floppy disk.

2. Submit your floppy disk and a 1:1 printout of the layout to the shop for construction of your PCB.

3. When you have received your PCB from the shop, install the components and verify the operation of your circuit.

RESULTS

1. Comment on the process you used to construct your PCB. Did your circuit work as expected? Why or why not?

2. Keep in a safe place your initial PCB layout done in the pre-lab portion of the lab, a copy of the printout from using Protel^®, and your circuit. These will be useful for future reference, especially when working on your senior project.

3. Any ideas as to how this experiment could be improved?

Revised 8/02