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Until now you've probably only used your CAD program to design your boards and print the final artwork on a laser or inkjet printer. Maybe you've used the heat transfer film processes to place the artwork on a piece of copperclad board or used photographic techniques to expose presensitized boards. Once prepared, you etched the board in a chemical bath with ferric chloride or ammonium persulfate to remove the undesirable copper. Later, you drilled the board with a small drill press or handheld tool and then cut the board to size with a saw or knife.
For crude boards, this may have worked fine. However, new technology (in particular surface mount components) calls for more exacting means of board fabrication to ensure that none of the pads and tracks are damaged during etching.
During the past few years, low cost CAD programs have added the capability of producing files which can be submitted to a professional PC service bureau in order to generate photoplots and drill programs. The two most common industry standards used for this application are Gerber photoplotter files and Excellon drill files. Gerber photoplot information is defined in the EIA specification known as RS-274D (and RS-274X). The Excellon format is defined in other industry standards. Using these files, the service bureau would generate the photoplots used to expose presensitized panels for etching. The Excellon files would be used to program a drilling machine to drill the plated and unplated holes for the thru-hole pads and vias.
With milling machines, boards can be fabricated with the same level of precision as professionally etched and drilled boards for a fraction of the cost. Also, precision interior cutouts and countour routing are possible.
Your probably saying to yourself, "OK, sounds great. But I don't know anything about Gerber and NC drill files. I've never worked with them before." Well, don't be intimidated! This brief tutorial will help you "get your feet wet" and maybe even teach you something along the way.
First, let's take a look at a very simple board. It consists of one thru-hole part, one surface mount part, one thru-hole via, top and bottom tracks, and a board outline. One the screen, this is what you would typically see:
The green tracks and pads represent the component (top) side of the board while the red tracks represent the bottom (solder) side. The yellow line is the board outline. Dimension data is placed outside of the board. This data is provided for use by the production facility's CAD operator and is deleted prior to board fabrication. The board outline is drawn with a 1 mil line width to ensure that the final dimensions of the contour rout are as close to the intended dimensions as possible (1 mil is usually the minimum track width that can be generated by PCB layout programs). The thru-hole component pads use 28 mil-holes and the via uses a 32 mil-hole.
So far, so good! Now, let's generate the Gerber and NC drill files using the PCB layout program. Please follow the hyperlinks to the other pages in this tutorial.
Now that the Gerber and NC drill files have been generated, let's preview them one at a time using Lavenir's ViewMate, a freeware Gerber/Drill viewer program.
Preview the top and
bottom layer data.
Preview the
top, bottom, and outline layer data.
Preview
the top, bottom, outline and drill data.
View sample Gerber
and Excellon NC drill data.
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