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GERB20

How to make a PCB without using messy and dangerous chemicals

Instead of etching a pcb, run Gerb20.exe and convert a Newton fleXYcad circuit into Extended Gerber format.
Extended Gerber is an industry standard (EIA RS-274X) and an electronic circuit in this format can be sent to a pcb-maker for the manufacture of a top-quality pcb.

Gerb20.exe (download here) runs on a PC or Linux box and very quickly creates 4 files from one fleXYcad circuit text file.

The design process:
1. Layout the circuit components using fleXYcad on the Newton
2. Save the circuit in the Newton Notes store
3. Transfer the circuit text-file from the Newton to a PC (try using the utility slump.exe)
4. Run gerb20.exe and process the circuit text-file into Extended Gerber format
5. Open the Extended Gerber files in a Gerber Viewer and check the design looks correct
6. Zip up the 4 files with some extra information, and send it to a pcb-maker.

These further instructions supplied in gerb20.ZIP include detailed notes about using Gerber Viewers.

These pictures show details of a new circuit designed on the Newton, then converted into Extended Gerber format by gerb20, and then created by a milling-machine (model: LPKF-Protomat 91S). This latest project (2 small circuit boards) sends heartbeat signals in real-time direct to a PC or Linux box at 19.2k (or 9600) speed, without needing digital storage.

Notes:

Gerb20.exe has been a complicated utility to develop and there are limitations in its capability:
1. Gerb20 does not yet create silk-screen information such as component outlines, text, or component ID numbers.
2. Drill holes are limited to 3 sizes: 0.036", 0.048", 0.126" (= 0.9mm, 1.2mm, 3.2 mm). These correspond to small-holes for component legs, medium-holes for switch or trimmer legs, and larger-holes for the D9 serial connector.
3. Design rules are relaxed in order to get gerb20 working e.g. the amount of copper around a drill hole is user-selectable by changing the pad-size, and it may be less-than-optimum around the medium-size holes.

Drill sizes provided by different pcb-makers may vary e.g. some will be Metric, some Imperial, and their sizes might not exactly match the 3 Imperial drill sizes required for gerb20. Some drills may be smaller, some larger.
Determine the drill sizes offered by a pcb-maker beforehand - such information should be on their www pages and it might be called a drill-rack.
If the pcb-maker cannot exactly match the drill sizes required for gerb20, and they offer a slightly larger equivalent drill size, then there may be less copper around the drill hole on the finished pcb. In this event, try using a larger pad-size in gerb20 before sending the board to the pcb-maker. Be sure to use a Gerber Viewer to check the tracks and pads.

Alternatively, it is also very easy to change a gerb20 drill size in the output drill file.
In the drd.txt file, just edit the appropriate Imperial drill size in this list:
T01 0.036
T02 0.126
T03 0.048
e.g. depending on the pcb-maker, 0.044 or 0.046 might be a better choice than 0.048".
Be sure to review any changes to the file by using a Gerber Viewer.

Ideally, these drill sizes need to be in a preferred dimension e.g. some drill size ranges will only jump in 4 mil steps (4 mil = 0.004").

Some milling-machine software may generate an error with this drill format and require instead:
T01C00.036
T02C00.126
T03C00.048
where the space has been replaced with C0.

The first two trial boards for the new Heartbeat project used drill-holes of 0.040" and 0.052", with a pad-size of 8x6.
There was insufficient copper around some holes, and with hindsight, a better choice of pad-size would have been 8x7 or 9x7. The latest boards have smaller drill holes of 0.036" and 0.048", with a track size of 0.025" and a pad-size of 9x7, and this combination provides a better area for soldering.

To run gerb20.exe on a Linux PC, install dosbox (a DOS emulation utility).
To use a Gerber Viewer on a Linux PC install wine (a Windows emulation utility). ViewPlot will run, but not ViewMate.

NB:
gerb20.exe has been tested on half-a-dozen circuits and appears to work, but there may still be some bugs in the code.
Three pairs of milled-boards have been made in a first trial, followed by 5 boards in a second test.
The term 'pcb' on these www pages means both printed or milled boards.

Resources:
Comprehensive list of pcb information and www sites here.
Useful account of PCB design rules here.

The milled circuit-boards for this project were supplied in the UK by: tswelectronics_at_googlemail.com, to whom enquiries may be made for the production of other single or double-sided boards.

Updated: 17 November 2008.



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