How to generate Gerber file from AutoCAD
Gerber files are the industry standard file format used for manufacturing printed circuit boards (PCBs). They provide the photoplotter and PCB fabrication house with precise image data of copper layers, solder mask, legend, drill holes etc. in individual files essential for photographically producing intricate multi-layer circuit boards.
Since PCB layouts are designed using CAD software, being able to accurately output manufacturing-ready Gerber files from the native CAD design files is a crucial step in correctly realizing complex multilayer board assemblies. This ability allows designers to hand-off electronic drawings directly to board shops.
While several PCB layout applications contain inbuilt Gerber file generation tools, a majority of designers use more universally adopted CAD platforms like AutoCAD for schematic capture and board geometry authoring. AutoCAD offers extensive drafting functionality however lacks innate support for generating industry-standard Gerber images in the various required layers.
This article provides PCB designers using AutoCAD, detailed step-by-step instructions on reliably creating manufacturing-compliant Gerber files from their board layout drawings correctly prepped with all essential PCB fabrication and assembly information.
Gerber File Format Basics
Before diving into the methodology of constructing PCB fabrication data from AutoCAD, it helps to understand basics of the ubiquitous Gerber file format widely used to convey PCB image information.
What is a Gerber File?
Gerber files are vector image file types developed by a company called Gerber Systems, who pioneered photoplotters for printing PCB conductor images on film sheets for eventually transferring onto circuit boards using chemical etching and plating techniques. Hence Gerber format became the industry norm for representing PCB copper layers and fabrication data.
Key attributes:
Gerber File Types
There are two distinct gerber file versions - Gerber RS-274X and Extended Gerber RS-274X. The extended format includes additional commands defining regions, netlist attributes etc. beyond just basic imagery. We focus on traditional RS-274X.
RS-274X Defines basic PCB images for fabrication purpose in terms of graphical primitives like line, arc, polygons etc.
Extended Gerber RS-274X Supports netlist metadata through Attribute commands and electrical test instructions via Region statements beyond just graphical shapes of base RS-274X
We focus on ubiquitous RS-274X files - containing all fabrication imagery - that still dominate PCB manufacturing information exchange.
Why Gerber Format?
Gerber files standardized graphical information exchange eliminating ambiguity and content errors to ensure accurate PCB imaging. Even 50 years since its inception, Gerbers survive as de-facto file transfer medium between designers and fabricators due to:
Attempts to supplant the venerable Gerber such as IPC-2581 or ODB++ have faced challenges getting traction indicating we remain stuck with Gerbers for a long time!
Contents of PCB Gerber Packages
Complete set of gerber files conveying PCB fabrication and assembly data is formally termed a ‘gerber package’. This comprises:
Layer Images
Attributes Data
Documentation
This aggregate gerber package conveys the layered construction blueprint essential for PCB production process.
Native AutoCAD Limitations
While AutoCAD includes powerful functionality for electrical schematic layout and drafting intricate PCB traces geometries, natively it lacks built-in utilities generating manufacturing formats like Gerbers for fabrication documentation purposes.
Layers structure equipped to recognize conductor layers is also missing. Without additional Gerber processing software, fabrication documentation aspects need to be manually implemented making integration suboptimal. Key gaps are:
Documentation
Layers Setup
Design Rule Checking
CAM Processing
This necessitates additional processing outside AutoCAD before design files become manufacturing-ready.
Constructing Gerber Package
Generating Gerber fabrication dataset from AutoCAD PCB drawings involves methodically orchestrating and translating native layer images into multi-file RS-274 photoplotting format throughscripted automation rather than manual procedures.
The workflow encompasses two phases:
We examine key steps to achieve both correctly.
1. Prep Design Layers
With inherent software limitations, systematically arranging data intelligently right from layout stage is imperative.
Layer prep
Allocate Layers
Reserve separate drawing layers each mapped to eventual Gerber image role - conductor, solder mask, keepout etc.
Ensure layer names match numbering conventions. We suggest:
1,2 - Signal Layers
15,16 - Solder Mask Layers
17,18 - Silk Layers
3 to 14 - Unused signal layers
19 - Board Edge Layer
20 - Keepout Layer
21 - Drills Layer
Assign Properties
This enables disciplined authoring recognizable images destined for specific photoplots.
Setup Workspace
Select PCB template to instantiate suitable plotting area with predefined substrate envelopes through drawing extents capturing board dimensions.
Create custom template if defaults lack frame sizes corresponding to panel requirements.
Construct Images
With layers provisioned, compiling illustrations involves strategically populating appropriate shapes.
Route Traces
Use LINE / POLYLINE to create conductor paths on designated signal layers following electrical connectivity scheme.
Assign trace widths based on current ratings and controlled impedance needs per constraints.
Define Pads
Use PAD shapes with holes to define surface mount lands and thru-hole drill spots.
Set pad diameters matching component termination sizes per IPC-7351 land patterns standards.
Assign Keepouts
Apply REGION areas on Keepout layer to mandate exclusion zones enveloping tightly packed traces or small components. Adds clearance insulation margins.
Draw Silkscreen
Text and LINE elements on Silk layers to annotate components, logos, board signages for identification post assembly.
Outline Board
RECTANGLE, LINE or POLYGON on Mechanical layer to demarcate extents of board dimensions for fabrication.
Identify Drills
All points necessitating drilling for vias or component holes should use a crosshair marker centered on the PAD hole on Drills layer plotting extent areas needing hole drilling.
This comprehensive set of images furnishes necessary visual construction data.cripts.
2. Export Gerber Data
With intelligent layers setup and vital illustrations in place, exporting AutoCAD drawings into multi-layer Gerber photoplotting files involves:
Plot Drawings
Plot separate PDF printouts from AutoCAD for each relevant layer mapped to eventual PCB fabrication imaging role.
Plot in 1:1 scale matching real world PCB dimensions.
Turn off print borders, annotations, other decorations.
Per Layer Plot
Repeat plotting process for every essential layer required for fabrication purpose.
Each plotted PDF sheet then contains graphics shapes destined for a specific Gerber file.
Convert PDF to Gerbers
Next we transform plotted PDF documents into Gerber RS-274X photoplotter files using a CAM tool. This involves:
The script based processing ensures correct output formatting, precision coordinate data, consistent layer mappings while adding essential fabrication information like date stamps, polarity indication etc.
This effects reliable transformation of native AutoCAD illustrations into manufacturing-compliant Gerber layer files essential for PCB production processes.
Submitting Gerber Package
The generated set of Gerber files constituting images for each necessary fabrication layer along with accompanying documentation like drawings, stackup diagrams, panel layouts, netlists and attributes libraries constitutes the final manufacturing package submittable directly to board production houses for realization of designed PCB without errors or delay.
FQA
How are separate design layers mapped to individual gerber files?
By intelligently assigning different drafting elements like traces, keepouts, drills etc. onto specifically designated and named layers purpose mapped to eventual fabrication imaging requirements, exporting each such layered AutoCAD drawing to Gerber files ensures that the original native illustrations get correctly translated into respective fabrication photoplots through systematic post processing.
What are the key advantages of using Gerber over other formats?
Vector graphics based Gerber files avoid potential image distortions or enlargement artifacts associated with pixelated bitmapped images allowing clean rendering at higher resolutions necessary for producing intricate board geometries. Ubiquitous industry tool support for editing, conversion and processing coupled with extensive backward compatibility also keeps format relevant despite over 5 decades of existence.
How are fine pitch BGAs and 0201 passives correctly plotted?
The ability to set adequate image resolution while transforming PDF files into Gerber formats ensures micron-dimensioned packages and routing can cleanly resolve without jagged contours or stair-stepping issues. Calculated image settings matched to smallest feature sizes allows reliable plotting down to even 12 microns details.
What are common reasons for generation of incorrect gerber files?
Failing to methodically prepare individual layers purpose mapped to fabrication images right from layout stage and lack of transparency into whether source drawings have design rule violations before exporting intermediate files leads to error-prone transformations producing imprecise or mismatching images lacking essential details ultimately causing fabrication failures.
Why is script based processing recommended over manual options?
Manual options for generating gerbers leaves too much opportunity for human errors at each transformation stage besides being extremely tedious and time consuming while standardized scripts process intermediate files error-free everytime to produce manufacturing-ready outputs. Tool calculated image settings also optimally render ultra-fine geometries reliably.