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Making Printed Circuits With PCB

June 12, 2000
Forrest Cook

PCB is a printed circuit board design program that was written by Thomas Nau from the University of Ulm in Germany. The program is currently being maintained by Harry Eaton at the Johns Hopkins University Applied Physics Laboratory. PCB is distributed under the GNU General Public License (GPL). This article will demonstrate the use of PCB by making a circuit board from the microphone preamp circuit shown in the LWN feature article on Xcircuit . Pcb allows you to produce custom prototype quality circuit boards in a few hours with inexpensive equipment. Short development cycles have previously been restricted to the domain of software projects. Hardware designers can now benefit from improved, if still somewhat slow, cycle times.

Installation

To install PCB on your system, download the gzipped source code file, (Netscape users should press shift before clicking), and compile the program. The following steps were performed on a Red Hat 6.2 system with all packages installed:

$tar -xzvf pcb-wip.tgz
$cd pcb-1.7
$xmkmf -a
$make
become root and install the program:
#make install

Exit the root shell and run the program:
#exit
$rehash
$pcb

A complete manual and reference card written in PostScript are included in the doc directory. To view the manual, either print the files on a PostScript printer or run gv on the file to view it on the screen. PCB currently has no included man pages.

Working with PCB

NetLists

Sample Netlist File

Gnd J1-2 J2-3 U1-4 R7-2 C5-2 C6-2 Vgnd U1-3 U1-5 R6-2 R7-1 C5-1 VR1-3 Vcc J2-2 C6-1 U1-8 R6-1 R1-2 Shld J2-4 In J1-1 R1-1 C1-1 Cp1 C1-2 R2-1 Cp2 R2-2 R3-1 C2-1 U1-2 Cp3 R3-2 C2-2 VR1-1 U1-1 Cp4 VR1-2 R4-1 Cp5 R4-2 R5-1 C3-1 U1-6 Cp6 C3-2 R5-2 U1-7 C4-1 Out C4-2 J2-1

The first step in the process of making a PC board layout involves creating a netlist file. A netlist file describes how the individual components are connected together. Node names are on the left side of the netlist file and are arbitrarily named. Part names and pin numbers follow on the right. A node is simply a point in the circuit where part pins connect. For simpler circuits, it is possible to skip the netlist stage and enter the parts free-hand. A netlist file may be created manually with a text editor or output from a schematic capture program. The netlist file should be created and pulled into PCB with the "file" menu and the "load netlist file" selection. Once the netlist file is loaded, corresponding parts should be pulled in from the parts libraries and named. To access library parts, click on the "window" button at the top of the screen and select "library". Click on the appropriate section on the left side of the library window and double-click the desired part on the right side of the window. The part should be moved to the main window and placed with a single click. The parts should be given the same names they have in the netlist file. To name parts, place the cross hairs over the desired part, type 'n', and type in the element name.

Rat's Nest

A rat's nest diagram shows which pins are connect together. Connection lines are drawn with the shortest possible path, so the appearence is somewhat jumbled looking, hence the name. After you have labeled all of the parts and loaded the netlist file, plot a rat's nest diagram by typing 'w'. The rat's nest can be erased with 'e'. At this stage, parts should be moved around the page and rotated to minimize line crossings. The rat's nest should be created and erased often during this part of the process. If there are problems with the netlist file or parts are missing, the rat's nest operation will pop up useful diagnostic messages.

Entering Traces

The next step involves running traces between pins on the same node. Prior to laying any traces, you should click on "sizes" at the top of the window and pick "adjust" and "use" for at least one type of trace. The image transfer method shown below requires fairly fat traces, a 0.035" trace width is a good starting point. The rat's nest shows which pins connect together, traces should connect the same pins together without overlapping other traces. In the example PC board, it was possible to make a single-layer board without using jumpers. In a single-layer board, jumper wires will be required if traces must cross one another. PCB supports multi-layer boards, but the prototyping method shown here works best with single layer boards. As the traces are added, the rat's nest should be erased and redrawn to show where traces are still needed. When no more rat's nest wires show up, the trace wiring is complete.

Filling Empty Space

When all of the parts are connected, it may be useful to fill in unused space on the board with "copper". I recommend filling the large spaces with rectangles and using the polygon fill for the smaller spaces. Filling the "whitespace" with "copper" may improve electrical shielding and will extend the life of the etchant chemicals. The "silk" button on the side can be clicked to toggle the parts silk screen layer, this improves visibility of the copper layer. After the empty space has been filled, the printed circuit artwork may be used to build a prototype circuit board.

Printing

The artwork should be saved to a file by clicking on "file" at the top of the screen and selecting "print layout". A series of PostScript files will be created, one per layer. The output_group1.ps file contains the bottom copper layer that will be used in making the prototoype. The output_componentsilk.ps file contains the silk screen image that may be used to determine where the parts go.

Parts Silk Screen

Optionally, the parts silk screen image may be put onto the component side of the circuit board. To do this, it is necessary to mirror the silk screen image by manually editing the output_componentsilk.ps PostScript file.
Find the lines that say:

%%Page: 1 1
%%BeginPageSetup

and add a line of PostScript as shown below:

%%Page: 1 1
8.5 72 mul 0 translate -1 1 scale
%%BeginPageSetup

The silk screen image should be printed onto the transfer material and that should be ironed on to the etched and drilled circuit board. Packing tape should again be used to remove the excess paint from the board. The example circuit board does not have a silkscreen layer.

Producing a Prototype PC Board

This step involves the use of a commercial circuit board transfer film called Techniks Press-N-Peel blue. Techniks appears to be the only source of this type of transfer film. A PostScript laser printer is required, an ink jet printer won't work. It is possible to print the artwork on an ink jet printer and photocopy the image onto the Press-N-Peel film, but some quality will be lost. The film works by combining its emulsion with the printer's toner to make an adhesive paint-like coating that can be fused to a circuit board with heat. If you are making small boards, you can get several transfers from a single sheet of Press-n-Peel film. Print the image onto the paper to get an alignment area. Cut out an oversized piece of the blue film and tape the leading edge of the film to the paper. Run the paper with film through the printer and print the image onto the film. This technique may not work on all laser printers, proceed with caution as it could result in paper jams. If your printer has a straight-through output path on the back, use it.

The following steps describe the process of producing a prototype board:

Iron the film onto the copper circuit board for several minutes.
Peel the film off of the board while running water over the board.
Peel the residual blue material from the board using clear packing tape.
Apply packing tape to the board to mask the edges and fix minor errors.
Etch the board in Ferric Chloride or equivalent etchant.
Clean the etchant from the board.
Apply paint solvent to the board and wipe off the residual blue mask.
Scrub the board clean.
Drill the holes in the board.
Optionally apply a silk screen image to the component side of the board.
Solder the components into the board.

Voilà, Instant Hardware

Wish List

PCB's built in parts libraries are rather minimal. There are enough parts defined to do many basic circuits, but adding new new parts to the libraries is difficult. The addition of a graphical parts editor would be a big improvement to PCB. PCB lacks many of the fancy features found in commercial circuit board packages such as automatic parts placement and wire routing. Automatic white space filling would also be a big plus for PCB. While those features are time savers for production work, they are not show stoppers, the program is a very useful tool for the hardware designer.

Conclusion

PCB is reliable and effective for basic PC design work. It is a good addition to the Linux hardware-hacker's toolkit. PCB could definitely benefit from the addition of more features, it could be a good project for a grad student to work on.

It is now possible to produce electronic prototypes in a few hours using only free software. The previously long development cycle for making circuit boards has been greatly shortened. Lessening the time delay means that designs can be iterated more, as in software design, and a better final product can be created.

References

PCB home page
Xcircuit LWN feature article
Xcircuit home page
Techniks Press-N-Peel blue film

author: Forrest Cook, cook@eklektix.com.

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