As a board designer, you need to communicate all your fab and assembly requirements to the manufacturer through standard PCB file formats. All the standard formats are discussed below.
What files do PCB manufacturers need?
Board fabricators require the following files:
- NC drill files:
- XNC format
- Excellon drill file
- IPC netlist
- Bill of materials (BOM)
- Pick and place files (centroid files)
- Fab drawings
Gerber files are the most widely used file format for PCB manufacturing. These files drive the photoplotter that creates the film to expose each conductor layer. A Gerber file does not convey any details regarding design rules, net connectivity, or component libraries. It is a 2D representation that assists the manufacturing equipment to place copper, solder mask, or silkscreen layers.
There are two primary versions of the Gerber format:
1. Standard Gerber (RS-274-D) is the first format that is now obsolete.
2. Extended Gerber, (RS-274X) is the Gerber format currently in use. This has been updated to include meta-data about the files such as file attributes, and this format is known as X2.
What is the file extension used for Gerber files?
The standard file extension of Gerber files is “.GBR” or “.GB”. Other extensions are also used based on the manufacturer’s PCB design software.
ODB++ format that deals with CAD to CAM data exchange. When ODB++ is used, its data is stored in multiple files and the folders are arranged hierarchically. The designers will use common operating system commands that retain the hierarchy structure by fusing all of the data into one compressed file.
ODB++ allows designers and manufacturers to transfer more than just the standard layer artwork and drill data. The hierarchical nature allows this. This format permits the storage of volumes of data in a single file that comprises material stack-up, BOM, component placement coordinates, and dimension & fabrication details. ODB++ can be accessed through EDA tools (Expedition, PADS, Allegro), making it almost a universal format throughout board manufacturing (except Eagle).
Difference between Gerber and ODB++
Gerber RS274X is a frequently used format for board design. It is a single file with layer information, pad shapes, drawings, etc. A Gerber cannot define the layer stack-up and the drill files, whereas the ODB++ format can define such kind of data.
IPC-2581 refers to the generic standard for the description data and transfer methodology of PCB assembly and manufacturing. It is meant for transferring information between a PCB designer and a fabricator. IPC-2581 is a guideline that companies follow to achieve manufacturing efficiency, reliability, process consistency, and quality.
NC drill files
PCB NC (numerically controlled) drill files are used to define board drilling and routing data. The drill file refers to the position, size, and the number of holes in a circuit board.
IPC-NC-349 standard refers to the machine-readable input format used by computer numerical control (CNC) drilling and routing tools. It can be used directly to transfer drilling and routing information among board designers, manufacturers, and users. This is the only standard that defines the drill and routing formats related to a PCB.
The exchange NC (XNC) format is a part of the IPC-NC-349 standard and is highly compatible with existing software for NC drill files. It serves as a bridge between different drill formats to serve as a common standard.
The Excellon drill format includes necessary instructions to operate CNC drilling and routing machines. It is the default standard for drill files. There are two types of Excellon drill files – Excellon 1 (older version) and Excellon 2.
Fabrication or fab drawings include board dimensions, drill details, class of fabrication (class 2, class 3), stack-up details, etc. These drawings are sent in a PDF format to manufacturers.
The fab drawing includes various design-related information such as:
- Class of the PCB (class 1, class 2, or class 3)
- Stack-up details
- Overall board thickness
- Solder mask details
- Silkscreen specifications
- Layer-wise impedance details
- Cut-out details
- Drill chart
- Version number and date
The output of PCB design software is a CAD file format and cannot be used by manufacturers directly. These CAD files are converted into computer-aided manufacturing (CAM) format to physically realize the design.
The IPC netlist (IPC-356) contains a list of networks that forms the conductivity interconnection scheme of the board. It is an ASCII text file with the PCB CAM system instructions including the net names, pins, and X-Y coordinates of the start and end point for each net/node.
Bill of materials (BOM)
BOM or bill of materials contains the list of all items and their specifications required for manufacturing a product. This file includes the manufacturing part number (MPN), vendor part number (VPN), part quantity, value, do not install (DNI) parts, reference designator, footprint, etc.
Pick and place files (centroid files)
PCB manufacturers use pick and place machines to pick the right component and place it at the right position on the board during the assembly process. These machines need the correct data to do so. Pick and place files store the data in ASCII format and can be generated by any design software. The main purpose of these files is to convey information about the position and orientation of all surface mount devices. The X-Y coordinates and rotation for each component to be assembled on the board are mentioned in this file.
PCB manufacturing requires efficient design communication that can be achieved using the appropriate file formats. Designers should have a clear idea about each file and the format to achieve accurate board manufacturing.