Aperture Lists

 

Increasingly, vector photo-plotters are being replaced by the laser photo-plotter which emulates the older machine in a raster (bit - map) fashion. While use of the term “aperture” to describe a pad or a trace shape persists, the term “aperture wheel” is now being replaced by “aperture list”, which implies the greater flexibility now available to the designer.

 

Remark: For improving the PCB, the aperture list must be checked because sometimes one or more aperture has wrong shape or dimension.

 

Advantages:

-         Laser plotters operate much quicker. A complete plot (30dm2) can be performed in 8 minutes.

-         Aperture shape can be easily generated in software, thus eliminating the need to design a physical wheel.

-         The aperture shapes can be described in the GERBER file, if certain extended- GERBER formats are supported (“RS274X”, “MDA FIRE AutoPlot”, etc). This reduces chances of getting the wrong list.

-         More apertures can be defined on a list.

-         Allowable apertures sizes are typically (but not always) grater than those imposed by the physical dimensions of an aperture wheel.

 

Flash and Draw Apertures

 

No distinction has to be made between Flash and Draw aperture types since the light source intensity is constant.

 

 

 

Disadvantages:

-         Any graphic element (flash, draw) is done by successive passing Þ the plotted articles are never perfect.

 

The laser emission is modulated with GERBER information.

Laser photo-plotter has a defined set of apertures that can not be changed and for some forms the photo-plotter can realize any dimensions. LASERGRAPH (Hungary) – doesn’t accept directly the GERBER file because it has to make first a screening for transforming the command files of the photo-plotter.

 

Design requirements:

Board:

            - Alignment elements (usually in 3 points with minimum thickness of 6-8 mils)

Outside the board:

-         Firm, laboratory, section, miscellaneous information.