We all are mostly familiar with electronic circuit design files that are also called “Schematics”. These files are the representation of hand drawn circuit diagrams into computer understandable CAD file having extension .sch. These files are then converted in the same CAD environment to PCB “Printed Circuit Board” layout file with extension .pcb. The PCB file shows the actual board layout and component placement on the actual PCB board and routing, layers, vias, PTHs and connectors etc.
However when it comes to convert/fabricate the PCB layout to actual PCB, then these PCB layout files are again converted to special file that is readable by computer and plotters to actually print the layout on board. These files are called Gerber Files.
The Gerber File is based on the ASCII (American Standard Code for Information Interchange) Character representation that is originated from PCB layout file.
History of Gerber File:
The Gerber files are the creation of engineering student named Joseph Gerber who was studying aeronautical engineering in Rensselaer Polytechnic Institute in New York. Gerber marked out a scale on the band and used it to help him with the tedious measurements, calculations and re-plotting that was required to complete his project. This idea he patented and incorporated in Gerber Scientific Instruments Company and sell the devices he designed.
Understanding the Gerber Files:
The Gerber file is actually the skeleton representation of multilayer or double/single sided PCBs. The Gerber files gives complete information about the details of PCB like number of layers, solder mask, solder paste, vias and drill holes, pads size, board outline, silk screen, traces width and clearances.
Usually the Gerber files are the bunch of multiple files and each layer has its separate file. For example there is a separate file for top layer solder mask, bottom layer solder mask, top copper trace, bottom copper trace, silkscreen top layer and silk screen bottom layer and so on.
The Gerber Files are used to give instructions to Photo plotter machine, to expose the piece of photo film by image file and control the light source. Actually the Gerber files are represented as image files that photo plotter sees. The black ink represents the copper area and the transparent area is where there will be no copper. The photo resist film, the substrate, the copper foils, the photo film are all stacked up and are lined up by “registration holes”. The registration holes allow all the layers to properly align with each other when the stack is going to be exposed and developed.
In a typical 2 layer PCB there will be at-least 4 sheets of films. Top copper trace, top solder mask, bottom solder mask and bottom copper trace. In the end the ultraviolet rays are blasted on the stack and the exposed area is etched away hence leaving the copper beneath the photo resist film. Thus creating copper tracks
Issues regarding Gerber Files:
- There may be some issues when generating the Gerber Files for your PCB layout. The Gerber files do not have information about the X,Y coordinates of the holes, so a separate N.C (Numeric Controlled) Drill File must be generated as fabrication output files in CAD software like Altium or ORCAD. Be sure to check the option to differentiate between the blind and buried vias holes and PTH holes separately.
- The Gerber files are not self-explanatory either it is top, bottom or inner layer, hence a README file is auto generated that gives this information.
- The technicians at the fabrication shop must run the DFM (Design for Manufacturing Check) before going to actual PCB fabrication process. The Gerber files do not give information about the silkscreen on component pads, putting PCB features too close to the outline of PCB and incorrect hole drills. These errors need to be re-checked by DFM technicians
- Technician must ensure there is not Gerber file missing and all files are present
RS274D Gerber File:
The RS274D is the older type of Gerber file format that has separate Gerber files for each layers attribute (Solder mask, silkscreen, copper traces) and aperture files are not embedded in Gerber file but need to be manually input by CAM technician and hence prone to errors due to ASCII characters that are difficult to be read by humans. The RS274D version is now obsolete and newer version RS274X is introduced. However the older PCB layout circuits are still available in RS274D version of Gerber and many fabrication shops still use and prefer this RS274D Gerber format.
RS274X Gerber File:
The RS274X is the “Extended” and enhanced version of RS274D in the sense that the aperture file and macro definitions are embedded in Gerber File as standard output. Introduced in 1991 by EIA and set as the standard Gerber File format, this format is highly compatible with today’s advanced laser/raster photo plotter. By using RS274X Gerber File format, the designer has the flexibility to design any image shape as a full pad, a long track or on a plane or polygon.It specifies clearly the pads and planes without vector fill and painting needs. Gerber RS274X can be highly reliable and precise in rendering perfect image files for signal layers, power planes, copper shapes etc.
Disadvantages of RS274D/X:
There are many other parameters of PCB fabrication and assembly that are not included in RS274X Gerber format. Like layer stackup order, materials information, drill data, Pick & Place data, BOM, Netlist and Test Point Report. These data are generated separately by another method and not by RS274X Extended Gerber format. Hence there are some flaws during transferring the PCB fabrication and assembly data from CAD (Computer Aided Design) environment to CAM (Computer Aided Manufacturing). Some of these are
- Copper Layers out of order
- Missing copper layer
- Mie-registered drill holes
- Missing drill holes
- Misinterpretation of fabrication notes causes delays
In order to cut down these issues, the newly introduced Gerber X2 and IPC-2581 Gerber File formats are commonly used nowadays by many PCB fabrication and assembly shops.
The Gerber X2 is the expanded version of Extended Gerber RS274X. This Gerber X2 along with image data of PCB layouts, also includes all the design data like layer stack-up information, drill size, drill locations, layer span, plated and non-plated drill holes information, information about either the board is single PCB or panel, information about the through-holes, SMT pads, vias, fiducials and many other information regarding PCB assembly and fabrication is embedded in Gerber X2.
The RS274X and GerberX2 are back to back compatible meaning that the technician who has expertise over Gerber X2 can easily interpret / understand the format of data of RS274X and vice versa. Also the Gerber X2 fabrication process will support the data files generated by RS274X. But the RS274X fabrication process will use the Gerber X2 layer image data only to create Gerber and other fabrication outputs will still be generated from separate conventional method.
The IPC-2581 Geber data format consist of a single file in XML that contains all the information regarding PCB fabrication and assembly. Layer stack, test procedures, layer material info, Netlist data, and information regarding V-Groove, back drilling, slots. The IPC-2581 also generates the purchasable BOM and assembly drawings
The Gerber Data is the core of PCB manufacturing and assembly. The Gerber Data is the interface between the CAD and CAM environments for PCB production lines. The Gerber X2 is the latest development that ensures high quality and reliability of PCB with minimal errors in the PCB fabrication and assembly process.