PCB Design Output Data that a Manufacturer Requires

This article was originally published on the Sierra Circuits blog.

PCB is the backbone of all major electronic devices and is also responsible for making devices compact and efficient. A PCB manufacturer can achieve perfection only if the PCB designer provides a design data file or artwork, that completely describes the board specifications. So, let’s check out what are the different PCB design output data that a manufacturer requires.

Design files, otherwise known as artwork, are the prime communication bridge between fabricators and designers. A manufacturer can check the manufacturability of a PCB by running some advanced design for manufacturability (DFM) checks. For PCB manufacturing, the fabricator requires detailed Gerber or ODB++ files, or IPC-2581 imports/exports.

A PCB designer’s prime goal is to provide detailed documentation to ensure that the PCB manufacturer avoids any guesswork that could lead to undesirable outcomes in the board functionality. Hence, design data files are the bridge between manufacturers and designers.

PCB Design Output Files for Fabrication

Design files are the prime communication channels between fabricators and designers. Initially, Gerber files ruled the industry since its introduction in 1980. Towards the middle of the 90’s, the intelligent format ODB++ came into existence, soon followed by the open and neutral IPC-2581.

Here is a list of the different files you can send your manufacturer for fabrication:

• Gerber files
• ODB++ files
• IPC-2581
• Netlist
• Drill files 
• Component placement file (pick and place)
• PDF files

What is Gerber?

Gerber is an ASCII text file that has a ‘.brd’ extension. When you design a board in a CAD system, once the board is about to be assembled, the manufacturer scans the specifications and transforms them into a format that can be used by the photo plotters and computer numerical controlled (CNC) machines. These files are known as the Gerber files. 

The Gerber files are created for each of the etch and mask layers. The PCB manufacturer produces as many as 30 or more layer files to define different aspects of manufacturing.

What does a Gerber file contain?

• Configuration parameter
• Aperture definitions
• XY coordinate locations for draw and flash command
• TOP – Top layer
• SMT – Solder Mask Top
• SPT – Solder Paste Top
• BOT – Bottom layer
• SMB – Solder Mask Bottom
• SPB – Solder Paste Bottom
• SST – Silkscreen top
• SSB – Silkscreen bottom
• AST – Assembly Top
• ASB – Assembly Bottom
• Read me text files

The modern Gerber files can also contain metadata such as solder mask, legend/silk, number of copper layers, and other related details about printing.

Gerber file formats

The number and types of files created by the Gerber depend on the complexity of the board and the manufacturer’s specifications. The main type of Gerber files is generated for each layer in the PCB design, while there are files to describe procedures across different layers. For example, files are generated for etching, silkscreen, drill files, etc…

The RS-274D and RS-274X (also known as Excellon), are the two most common types of output formats. And now there is the latest X2 version.

Standard Gerber (RS 274-D)

The first form of Gerber file representation was designed as numerical control (NC) format. The RS 274-D has provided simple plotting functionality such as truck drawing. It did not describe coordinates and drawing margins separately.

The standard Gerber file is responsible for the success of Gerber file formats. The extension used is “.gbr” text. 

Extended Gerber (RS-274-X)

It uses human-readable ASCII commands that are combined to create a graphical 2D map. X-Gerber has introduced the use of metadata for the graphics description. This version incorporates all four components of the Gerber details in one file (configuration parameters, apertures, XY coordinates, draw and flash commands). This is associated with extensions of the “.gbr” and the “.gbx” formats.

Gerber X2

This is the latest version of Gerber files which supports additional data. Gerber X2 files can contain details such as the layer function, entity functions like pad types, trace positions regulated with impedance, and much more. 

This file format supports extensions like “.top” and “.bot” and it is compatible with all types of modern CAM software. 

What are the Gerber file extensions?

Using PCB designing tools, a designer can create many files to describe various manufacturing aspects of PCB. Some example of such files are listed below:

What is ODB ++?

ODB++ is an intelligent format. A single ODB++ file or directory contains all the information required to define a PCB layer. 

This file format offers a stable framework for the required data. An ODB++ file doesn’t ensure that the given data is sufficient to manufacture the design but it allows the designer to combine all the data and perform the required checks for manufacturability and reliability.

The ODB++ translator inputs an ODB++ archive or a compressed file (.tgz, .tar, .gz, .zip, or .tar) or a directory of files from a circuit layout program and converts the files in the archive to a Sonnet project.

The ODB++ file is submitted to the manufacturer as a single file with .gzip extension. All required files are stored in this main file.

Why ODB++?

• The ODB++ hierarchy framework enables programmers and organizations to transfer more than just standard artwork and drill data.
• It can allow additional data in a single file. For example, it can consist of material stack-up, bill of materials, component placement, as well as dimension and fabrication data. 
• ODB++ can be accessed via most PCB design programs (Expedition, PADS, Allegro), making it almost a universal format.
• It allows simple and effective production of the component without the complexity.
• This intelligent file is supported by all major vendors of CAD, CAM, and DFM tools.

The difference between ODB++ and Gerber files

Gerber RS274X is the most widely used format for PCB design. It is a single PCB file that includes all the layer data, pad shape, and drawings providing an accurate design.

But a Gerber cannot define the layer stack-up and the drill files cannot be included, while ODB++ can include a huge amount of data.

ODB++ is a more intelligent file format since a single file could include a large amount of data. Most of the fabricators now prefer ODB++ because it helps reduce human errors. Using ODB++, a PCB manufacturer could remove the necessity of working with a lot of low-level files. 

What is IPC-2581?

IPC-2581 is a universal PCB assembly and manufacturing standard for data definition and transition methodology. IPC-2581 can include a large number of files within a single XML file.

Also recognized as “Standard Specifications for Manufacturing Description Data and Transfer Methodology for Printed Board Assembly Products,” is the IPC-2581 standard.

IPC 2581 includes the following data:

• Copper structures per layer
• Order of build-up
• Layer stack information
• Netlist in-circuit testing
• Material information
• Drill data
• Test points
• Assembly notes
• Bill of materials for purchase

Drill Files

PCB drilling is the basis for the vias and the connectivity between various layers. Drilling is the most expensive, irreversible, and time-consuming process in the manufacturing process of PCBs. A small misposition may require refabrication of the whole board from scratch.

A drill file is a secondary file sent to the manufacturer with the Gerber file. The drill file refers to the position, size, and the number of holes in the desired PCB. The NC drill file can be used to correctly determine where all the drill holes are located on your board and the size they require. The number of files created for the drill varies depending on the complexity of PCB wiring. 

Types of drill holes

There are two types of holes to be drilled in the PCB, plated and non-plated holes. Plated holes are drilled with an intent to fix electronic components to the PCB and non-plated holes to fix the PCB to the respective electronic device. If a board design has both plated and non-plated holes, you can merge the drill details in one NC drill file for both types of holes or have different NC drill files created by the PCB design tool.

Usually, two separate files are created for plated and non-plated holes. This is because the plated through holes are drilled before the plating process happens, whereas, the non-plated holes are drilled after the plating process.

Drill file extensions

The regular, plated drill file uses ‘.drl’ file extension and non-plated hole drill files have ‘np’ in the file name; eg: ‘np.drl’.

PCB drill file format

What is a PCB netlist?

The netlist (formatted as IPC-356) is a list of networks that determine a bare circuit board’s conductivity interconnection scheme. In general, the netlist is an ASCII text file that includes the PCB CAM system instructions such as net names, pins, and XY coordinates of the start and endpoint for each net or node. 

Why do we use a netlist?

Gerber 274x does not support the netlist. In certain instances, Gerber files exported from your PCB CAD software can contain an unnoticed error, because there is no way to verify whether the files are in accordance with your design intent. You can prevent this by loading your manufacturing data kit with an IPC-356 format netlist file. The IPC netlist aims to find defects in the pre-production process.

The customer’s CAD netlist will be compared by the fabricator with the list derived from the Gerber data provided by the customer. The customer will be notified about any errors during the analysis. 

What do we do with the PCB netlist?

• We check if the designer’s netlist is the same as the manufacturer’s netlist created on the basis of their graphic data.
• We check the netlist data each time we edit the board so it doesn’t cause any netlist violation.
• Fabricators use the customer-supplied netlist during the electrical validation of the PCB.

PCB Dimensions for Design Data

Standard panel sizes

The size of the panel should be mentioned in the design under XY coordinates. The size of the PCB is one of the major factors that affects the cost of the product.  

Finished PCB thickness

Final thickness is the combined thickness of all layers, including core and prepreg thickness. 

Core thickness

Core is one of the main parts of the PCB layers. There are different ranges of core thickness that are plated with copper on either side, top and bottom.

Prepreg thickness

Prepreg is used to join two different cores or a copper foil in a stack-up. The prepreg thickness depends on its adjacent layers, like the signal layer or plane layer. 

Copper thickness for PTHs and vias

Copper is plated inside the component holes drilled in the PCB. The thickness of the plating is usually 20 to 100 microinches, depending on the capabilities of the fabricator.

Copper cladding thickness

The thickness of copper-clad is an important parameter that will be requested by the manufacturer. The thickness depends on the strength of current passing through traces and the width of traces. 

Hole dimension

Holes are drilled on to a PCB for various functions. Standard drill bit sizes are specified in ANSI standard B19.11M. There are a number of techniques to reduce the size of the hole in order to reduce the size of the entire board. Mostly, the hole dimension mentioned in the files will be actual drill size, not the finished hole size.

Copper trace and etching tolerance

While the process of etching the copper near the etch resist begins to be removed under the mask. This is called etch-back or undercutting. The width tolerance for etch-back is defined by IPC-2221A, which varies from 4 mils to 0.6 mils for 1 1?2 oz copper, depending on the degree of reducibility and plating.

With the introduction of Gerber X2 in 2013, Gerber is still in the play. However, modern versions and innovations in the area of design file formats are emerging in order to simplify the exchange of data between designers and manufacturers, while PCB designs are getting more complex. In the end, the manufacturers will expect a single file format that is sufficient to meet all their needs.