Plotting PCB files to film on a budget
Problem:
Generating PCB artwork for small business/hobbyist applications.
Techniques used include budget plotters, photo copiers and overhead transparency film, iron on artwork and even 9 pin dot matrix printers. Having generated a "plot", a negative usually has to be made.
Solution:
Use a desktop publishing bureau to print the job direct to photographic film. This saves several steps that are difficult to achieve in the small workshop with any great degree of success.
Why use a desktop publishing bureau? Because we can get a first generation photographic negative for about $15 for an A4 page and we don't need to run a plotter and make contact negatives. Tracks down to several mil will also turnout.
The image setting machines that the bureaus use are raster based rather than the vector based machines such as plotters. As these raster based machines are capable of resolutions higher than 2000 dots per inch, very acceptable results are obtainable for all but the most intricate PCBs.
The most common file format handled by the desktop publishing bureau is the .EPS (Encapsulated Post Script) format.
Encapsulated Post Script is a page description language invented by Adobe, the makers of Pagemaker. It is intended to allow the easy exchange of complex pages that incorporate text, vector and bitmap graphics.
All we need to do to have a PCB file printed by a bureau, is to create an EPS file from the PCB file, send it to the bureau and collect it later that day.
Creating EPS files
Protel's PCB doesn't create EPS files directly, but it can be done by printing to a postscript printer driver and saving the output to a file.
I used the Linotronic 530 as the postscript printer. Linotronic is a brand of image setter used by desktop publishing bureaus, and no, I don't have one. The Linotronic 530 driver was suggested by a bureau as a driver that works well for creating EPS files from applications that don't have EPS export capabilities. I grabbed the latest Linotronic 530 driver and version 3.58 of the Window's Postscript Driver from Microsoft via the Internet. There is a Postscript Driver included with the Linotronic 530 driver, but version 3.58 is more recent.
Once the printer and new postscript drivers have been installed, setup Protel's PCB (using Print, Final artwork option) to print a positive (black tracks on a white background) on your local printer. Check that the bottom layer is correctly mirrored and that all else is correct. This print out can be faxed to the bureau along with an order.
Change to the Linotronic 530 driver and change its settings as detailed below ...
Under Options ...
Print to ... Encapsulated Post Script File (Give your filename an .EPS extension)
Scaling ... 100%
Choose "Send Header with Each Job"
Margins ... Default
Under Options, Advanced ...
True Type Fonts ... Go with the defaults as True Type Fonts aren't used by PCB
Virtual Memory (KB) ... 1026 (default)
Graphics resolution ... 1693 (default). The final resolution is setup by the bureau, not this option.
Make sure "Negative Image" is not checked. This is done by the bureau during printing, and is what they are used to doing.
Choose "All Colors to Black"
Choose "Conform to Adobe Document Structuring Convention"
If it's not listed above, just use the defaults.
Press "Print" and create the EPS file.
Find a Bureau
Desktop publishing bureaus are listed in the yellow pages under Desktop Publishing Services. Selection of a bureau should be based on user friendliness rather than best price. Most bureaus are Apple Mac based (the desktop publishing standard) but can easily and painlessly accept files from IBM machines.
One major bureau in Adelaide hides its operators behind "a corporate front office veneer". This makes it hard to speak to the actual operators and to work through any problems. They also told me that I'd need special Windows software (which wasn't available yet) and a password to be able to use their modem to transfer files. Not real handy for after hours work. They were also aggressive in their manner.
Another bureau was friendly and happy to spend time discussing my requirements. Their modem access required no password, had a Windows communications program that I could download if I wished, and worked great with my favourite communications program (Procomm Plus for Windows). The shareware communications program Telix, also worked flawlessly. I was also able to sit beside the operator while the job was runout (to use their jargon). This would be very handy if there were any problems to be ironed out.
Once you've chosen a bureau, have them FAX you a Run Out Request form.
This form does the job of a purchase order, but there is less chance of leaving out vital information.
A sample Run Out Request form is shown below.
The critical parts of this form are FILM, NEGATIVE and RIGHT READ EMULSION DOWN.
The bureau also needs to be told that the job is LINEART (i.e. does not contain grey scale graphics) and it is to be runout at the highest possible resolution. There is a limitation in the Windows Driver that restricts us to a maximum output of 32,000 dots. (i.e. 20" page at 3000 dpi = 60,000 dots = won't print) [CHECK PREVIOUS SENTENCE WITH CRAIG ROSE FOR ACCURACY]
Fill out the runout request and return it via FAX along with the positive page(s) printed earlier on your local printer. This lets them know what the finished product should look like. Next, send in the EPS files via modem and wait for the finished negatives to be returned via courier later that day (you'll need to arrange an account if you don't want to go and pay at the time of collection). All this, and you haven't even left your desk!
This is all fine, except that a few obstacles may be placed in your way.
1.The version of printer driver used by you and the bureau has to be current.
2.raster based printers do suffer from linearity problems.
3.Print to file doesn't support multiple pages.
As I have found, all of the above have fairly simple solutions.
Item 1. Make sure the bureau is using version 3.58 of the Windows Postscript Driver. The version is determined by doing the following:- Open "Control Panel, Printers". Choose any postscript printer. Goto "Setup, About" and there it is.
If you don't have a postscript printer installed, install the Linotronic 530 driver and then install Windows Postscript driver version 3.58.
If the bureau doesn't have the latest drivers, give them a copy.
Item 2. The bureau's image setting machine may suffer from non linearity problems. The fix is to put X and Y axis scales on our PCB and monitor the results. If the bureau's result is unacceptable, adjust the X and Y scaling in Protel's Print Dialog box before creating the EPS file again. Keep a record of the X and Y scaling and use it on subsequent jobs. In real life, I've found that the errors aren't a big problem as we only need to worry about the error over the length of the largest component on the PCB. The overall error from one end of the PCB to the other often isn't important. The bureau may not have the facility to scale in one direction and not the other, so it's up to us to work around that one.
Item 3.Watch the print job as the EPS files are created and note if more then one page is created. If the print job is too big to fit on one page of the size you've chosen, the second page will be written to the same EPS file, overwriting the first page. The fix is to print one layer at a time (to a differently named EPS file), or use a larger page size.
The low cost and quality of finished artwork is worth the amount of time and effort invested to get the first PCB printed. After the first one, it's a lot easier.
I hope that my experiences will be beneficial and time saving to others who wish to walk the same path.