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Photoplotting Concepts
High quality and precision of artwork films can be ensured by creating them on Photoplotters. A photoplotter is a numerically controlled part that creates images on photosensitive films using light. Photoplotter is like any pen plotter, except that it has a light source in place of the pen. Just like pen size is chosen for plotting different line widths, a Photoplotter uses specially shaped apertures through which light falls on the film in order to create a given shape on film. An aperture can be used without movement to make a shape (a “flash”) or with movement to make a line or an arc.
The de facto standard for photoplotter data is Gerber format. Gerber data is a simple, generic means of transferring printed circuit board information to a variety of parts that convert the electronic PCB data to artwork produced by a photoplotter. The term Gerber refers to the Gerber Scientific Instrument Company, a pioneer and leader in photoplotter manufacturing. It is a software structure consisting of X,Y coordinates supplemented by commands that define where the PCB image starts, what shape it will take, and where it ends. In addition to the coordinates, Gerber data contains aperture information, which defines the shapes and sizes of lines, holes, and other features. These Gerber files are generated using the aperture sizes available in the Aperture Table. These apertures are defined in terms of a format recognized by the photoplotter. These are called DCodes. All the available D-codes and the sizes they represent are listed in Aperture Table. The interpretation and repertoire of D-codes may vary depending on the make and model of the photoplotter. For example D-code D100* may result in one machine as a .062" line and on another as a .100" line. There are photoplotters which allow flexible D-code-to-aperture assignment. For certain others the repertoire of D-codes may be fixed. Latest photoplotters available in the market may recognize various D-codes for creating different shapes. There are three DCodes for each available aperture size, namely for plotting lines, flashing round pads and flashing square pads which are recognized by almost all standard photoplotters.
When using extended Gerber format, the user does not have to bother about these Aperture Tables as the system automatically generates the required aperture parameters in the Gerber file.
Gerber file format
Gerber Format, which is the format in which Gerber data is expressed, actually is a family of data formats that are subsets of EIA Standard RS-274D. Extended Gerber Format, which is also called RS-274X, provides enhancements that handle polygon fill codes, positive/negative image composting, and custom apertures, and other features. RS-274X produces a variety of Gerber data called X data. RS-274X is a superset of the EIA Standard RS-274D format. RS-274X supports some of the parameter data codes (G codes) and aperture codes (D codes) contained in RS-274D, as well as codes referred to as mass parameters. Mass parameters are plot parameters that define characteristics that can affect an entire plot, or only specific parts of the plot, called layers. Mass parameters extend the capabilities of Gerber Format. Their presence makes the Gerber data X data.
The Gerber output (.GBR file) is similar to the one shown below. The data is organized in “blocks”. A block consists of a combination of codes for : aperture size selection, movement pattern and an end of block (EOB) character which only follows a combination of the above codes. The EOB character in this case is an asterisk (*).
For more details contact the web site at http://www.gerbersystemscorp.com
DCodes
D codes have multiple purposes. D-codes control the aperture size or rather the Aperture’s position on the list or wheel. D code with values of 10 or greater represent aperture size of the photoplotter the Aperture’s position on the list or wheel. D codes also control the state of the light being On or Off. There are three standard DCodes provided for this purpose:
D02* instructs the photoplotter to move to the given X-Y position with Light Off (Move Command).
D01* instructs the photoplotter to move to the given X-Y position with Light On. (Draw Command) thus plotting traces.
D03* instructs the photoplotter to create a flash at the given X-Y position i.e. Light On, Light Off at the given position thus plotting pads.
D10 and higher value refer to corresponding aperture shapes and dimensions assigned to them by each individual user based on the photoplotter he intends to use.
X & Y Codes
The X & Y values in the Gerber file determine where the aperture shapes and dimensions will be positioned and drawn. The X & Y values are used as co-ordinate pairs to determine where the light will be exposed using the D codes.
Example:
D100*
//allows to select aperture size corresponding to DCode D122
X100Y100D02*
//moves the photoplotter to the (100,100) coordinate position with Light Off
X150Y150D01*
//moves the photoplotter from (100,100) to the (150,150) coordinate position with Light On thus plotting a trace of size corresponding to D100.
D159*
//allows to change aperture size to that corresponding to DCode D159
X150Y200D02*D03*
//moves the photoplotter to the (150,200) coordinate position and creates a flash on the film thus plotting a pad with aperture size corresponding to D159.
Aperture Table
All the available D-codes and the sizes they represent are listed in Aperture Table. Aperture table must contain only those D-codes which the currently used photoplotter can understand. This Aperture Table dialog box may be got by clicking the SELECT button under Aperture Table in Project Explorer and in Gerber/ Setup dialog box. This dialog box displays the current Aperture Table and a list of Aperture Tables used by certain standard photoplotters from which any Aperture Table may be selected as current. The apertures defined in the selected aperture table are displayed alongside. Custom.apt provides apertures of all sizes from 1 mil to 330 mils with a difference of 1mil but it may happen that the photoplotter used cannot plot these apertures. The user may edit these aperture tables to suit his requirements.
Note:
System uses existing apertures to generate plotting data even for sizes not available in the aperture table. In this case, the system will use an aperture of the next lower size and then apertures of very small sizes to bring about the required shape and size (multistroking technique). The resulting files will be larger and the time required for generation of Gerber ASCII files and photoplotting may be considerably longer. Hence it is advised that as far as possible the sizes of pads and line widths should be limited to those covered by the aperture table.
Customizing the Aperture Table
Adding new sizes
Select INCH or MM button, to specify the unit of measurement. An input box appears to enter new size. Enter the new size. Click OK. Enter aperture code (D-codes) for Line, Round Pad and Square Pad in for this new size and save the changes. Note that the D-code must be terminated by asterisk (*) for ex: D200*.
Modifying DCodes
Select the required size from the Aperture Table by clicking on it. Redefine the DCode of the selected aperture size and save the changes made.
Deleting Sizes
Select the required size from the Aperture Table by clicking on it. Click DELETE SIZES. Aperture tables may be updated and saved under a different name. For this, give the name in the Named As: text box and click SAVE APERTURE TABLE. In this way, the user can retain the existing aperture tables while editing a copy of it.
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