The PCB fabrication and PCB assembly (PCBA) is not possible without the generation of PCB Gerber files. In general, the Gerber file comes in the category of PCB fabrication outputs. There are many fabrication outputs that are available in PCB CAD software. Some of them are GERBER X2, Gerber, Drill Drawing, NC drill File, Board Stack Reports and composite drill guide. Moreover the PCB assembly output files include Assembly drawings, test point reports and pick and place files.
Why Gerber Data is Important?
Now these data are critical and any mistake in generation of these data sets can lead to time delays between design stage and fabrication output of PCB. So if the designer wants avoid delays / dissatisfaction between fabrication house and design lab, then he must be very careful while designing the fabrication and assembly output files. We will be discussing about the one parameter here that is Gerber Files.
PCBs are design in various types of CAD software like Altium, Cadence Allegro, PCAD, KiCAD, and EAGLE. These are also called EDA (Electronic Design Automation) software. The Gerber File contains the technical details, reference and guidelines of PCB design that is used by board manufacturers to commence PCB fabrication. The Gerber file provides information regarding the image of circuit of both PCBA and bare PCB
For bare PCBs, image data are used by photo plotters and AOI while for PCBA the stencil layer is also included in Gerber to give information of components location SMT, THT and hybrid circuits.
What is Gerber File?
The Gerber File generation is the standard used in almost every PCB fabrication shop. The Gerber files are the translators between PCB design engineer and PCB manufacturers. The PCB design engineer should be aware of the PCB fabrication shop limits and constraints about the various parameters used In PCB design and also he must be familiar with the CAD software that is being used in fabrication shop to convert PCB designs to PCB Gerber. So it is recommended that the designer at his end generates the Gerber file and rectify problem if found.
The Gerber file is the ASCII digital representation of X,Y coordinates, shapes and apertures. They have the vital information about the PCB layout design in it. Like copper layer, solder mask layer, legends, silk screen printing. These data are used by photo plotters and data imagers
Types of Gerber:
- Gerber X2 - the latest Gerber type including stack up data and attributes
- RS-274-X - The expanded version of Gerber
- RS-274-D - The oldest version of Gerber format substituted by RS-274-X.
Reason Why Design Engineer Should Generate Gerber File:
The two main reasons why the design engineer should generate his own Gerber File are
- The design engineer is not sure about the CAD software that is being used at the end of fabrication shop. So there will be possibility that the Gerber file generated at one software would be slightly different from that generated by other software. So the design engineer should learn to generate Gerber so as to eliminate this slight or major difference what may lead to production delays
- Even if the software used at designer and fabrication end is same, but there still remains a chance that errors occur in the design at the designer end. So the design engineer should generate the Gerber and check for any design errors at his end
Different software have different method to generate Gerber. Here we discuss Altium, Cadence Allegro and EAGLE.
Open the PCB layout file. Then do this
File >> Fabrication Outputs >> Gerber Files
It has five options. General, layers, drill drawings, aperture and advanced
The General has the options to select units of measurement either mm or inches. Second it has three options for resolution. 2:3, 2:4 and 2:5
Here the layers that are needed to be plot and mirrored are determined by marking checks. The mechanical layers are neglected and not checked.
Check mark the embedded apertures (RS274X). No further action required.
Film size should be set as Altium default. Too small size can give error. The aperture matching tolerance should be 0.005 mil. Leading / trailing zeros should be checked as per design engineer requirements. Batch Mode should be separate file per layer
Position on film in Gerber should be determined by NC drill file
Optimized change location commands and Generate DRC rules export file (.RUL) are suggested to be checked.
After all settings click OK to generate Gerber File
The PCB file should be opened in PCB layout mode in Cadence Allegro
Now determine the board outline. Click on the top folder and click Add manual
Write film name say “OUTLINE” click OK.
After this in Subclass Selection window you find BOARD GEOMETRY expand it and check mark OUTLINE Now Click OK
Now in Artwork Control Form window, check mark OUTLINE in “Available films.”
After completing these steps, click Select all with all layers output. Click the Create Artwork to generate the Gerber Files.
The PCB layout design file should be opened in EAGLE.
File>>CAM Processor. A window will open
Now do this
File>>Open>>Job Open your design file in the new window as given below
Now in the above window, you must determine the basic parameters of Gerber files like Component side, Solder side, Silk screen CMP, and Solder stop mask CMP, Solder stop mask SOL, Border, Paste CMP, Paste SOL and Silk Screen SOL. Now click “Process Job” button to generate Gerber Files. Be sure to create a separate Gerber file for EACH layer of your PCB along with PCB edge.
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