In order to begin the learning of PCB post-processing process a basic understanding of Gerber is helpful in understanding the elements of a PCB design.The present tutorial is intended to be a guide to understanding the fundamentals of Gerber, and how it relates to the use of CAM software system (for example: GerbTool, CAM350, GC CAM, CAM Plan, PC Gerber, etc).For a more thorough explanation of the Gerber 274-X format, the "Gerber RS-274X Format User's Guide" by Barco Graphics N.V is recommended.When the user imports Gerber or any other data file into GerbTool, GerbTool reads the information and stores it in its own internal database format. When the user  manipulates the data,he is not manipulating his original files.The user can save his data in a GerbTool .gtd file, which is a single file that contains all the data, or he can export his files as Gerber or any of the supported file formats listed in the File|Export menu.

Apertures and D-codes

Two types of Gerber data (274-D, and 274-X) can be used to create individual, photoplotted layers of PCB artwork. The original Gerber formats were designed for use by vector photoplotters. These photoplotters selected individual aperture shapes from an aperture wheel, exposed the aperture to film by opening and closing a shutter. The Gerber file provided the instructions to the photoplotter, telling it which shape to place and where to place it. When an aperture was "flashed", the shutter merely opened and closed in a single location on the film. When an aperture was "drawn", the shutter opened, the aperture was moved across the film, and then the shutter closed, thus creating a line segment. Thus a Gerber file is essentially an ASCII file with a series of commands instructing the photoplotter where to move, when to open and close the shutter, and which aperture to use. In order for this to occur, the photoplotter needs an aperture list to define the shapes, or D-codes, used. The aperture list cross-references the aperture number with a D-code of the desired shape and size. Each aperture is assigned a number (which represented its position on the wheel). In the 274-D format, the aperture list is a separate file.

Following is an example of an aperture list:

Aper  Dcode  Type  Size-X  Size-Y

1      10   Round    10     0

2      11   Round     5     0

3      12   Round    20     0

4      13   Round    12     0

5      14   Round    75     0

6      15   Round    50     0

7      16   Round    80     0

8      17   Round   100     0

9      18   Round    60     0

10     19   Square   60    60

11     72   Target  200     0

12     73   Thermal  80    60