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          PRECISION CIRCUIT MANUFACTURERS PTE LTD
           

           No.18 Tuas South Street 3                      Phone: +65 6862-0802
           Jurong Industrial Estate
                         Fax:     +65 6862-0800
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Gerber and NC drill files
 

 

General
In order to manufacture a PCB, universally speaking a PCB manufacturer requires two types of files. The first is a NC drill file. The Second are Gerber files for the copper image layers, the solder mask layers and the silkscreen layers.

NC Drill
The NC drill file defines the X and Y axis locations of the holes, along with the size of the holes. It is a predefined format file which, more or less, is read directly by the CNC drilling machines, used to drill the holes into the PCB laminate material. Most PCB designs have both plated and unplated holes. PCB design software generally caters for this by generating two drill files, one for the plated holes and one for the unplated holes. During the manufacturing process of the PCB panel, the plated holes are drilled during one of the first processes in manufacturing the board. The unplated holes are drilled as one of the final processes, usually during the final routing process. By having two files, the manufacturing process is made easier and removes any possibility of introducing human error into the process.

Gerber files
Gerber files are simply high tolerance plot files used to produce a positive film at a 1:1 scale of the copper patterns, the solder mask patterns and the silkscreen patterns.

The first Gerber plotters consisted of a light source driven on an X/Y axis, which in turn exposed photo sensitive film. The Gerber file format simply defined the start point for the exposure, the end point of the exposure and the shape and size of the exposure head. So basically the plotter picked up an exposure filter and then ran the light source from one point on the film to the next, hence creating a track. Since the system relied on different exposure sizes and shapes, a separate exposure table file was required so that the plotter would pick up the appropriate filter to cover the light source, and hence create the correct shape during the exposure.

Today photo plotting is done on laser plotters, however since the Gerber format has become a defacto standard, modern Gerber plotters still use this format. Typically the modern Gerber plotter inputs the file then processes the data internally converting the data to drive the laser heads.

The major concession to the Gerber format was the revised specification, which embedded the aperture data into the Gerber file. This format is known as "Extended Gerber" or "Embedded Gerber" or "RS274X". All Gerber files should be supplied in this format as separate aperture tables would need to be manually entered, introducing a high opportunity for human error.

The Gerber format also supports a number of other options. One of these options relates to the resolution of the plot. These formats are referred to as "2.3", "2.4" and "2.5". Format "2.3" resolves down to 1Mil, "2.4" resolves down to 0.1Mil and "2.5" resolves down to 0.01Mil.

 

 

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