CERN Open Hardware Licence Guide for Schematics and PCBs

This article explores how the CERN Open Hardware Licence (CERN-OHL) establishes a robust legal framework specifically designed for sharing, modifying, and distributing schematic capture and printed circuit board (PCB) layout files in open-source electronics. It details the shortcomings of using traditional software licenses for physical hardware, explains how the CERN-OHL defines editable design source files, and breaks down the three distinct variants of the license—permissive, weakly reciprocal, and strongly reciprocal—so hardware developers can confidently protect and share their electronic designs.

The Challenge of Licensing Open-Source Electronics

Historically, hardware developers attempted to use open-source software licenses (like the MIT License or GNU General Public License) or Creative Commons licenses to share their electronic designs. However, these licenses rely heavily on copyright law, which presents a major problem for electronics.

While artistic elements and documentation are covered by copyright, functional physical objects and the utilitarian aspects of circuit layouts often fall outside the scope of copyright protection. Using a software license for a PCB layout creates legal ambiguity regarding what constitutes "source code" versus an "executable," making it difficult to enforce sharing requirements when a third party manufactures and sells a physical board based on open designs. The CERN-OHL was written specifically to bridge this gap by addressing the unique legal realities of physical hardware and electronic design automation (EDA) files.

Defining "Complete Source" for Schematics and PCBs

The core mechanism of the CERN-OHL is its precise definition of hardware documentation and "Complete Source." In the context of open-source electronics, viewing a static PDF of a schematic or inspecting raw Gerber files used for manufacturing is the equivalent of looking at compiled binary code in software. While you can manufacture a board from Gerbers, you cannot easily modify the design.

The CERN-OHL solves this by legally differentiating between manufacturing outputs and the actual modifiable design files. Under the CERN-OHL, if a license variant requires you to share the source, you must provide the files in the preferred format for making modifications. For electronics, this explicitly covers:

  • Schematic Capture Files: The native, editable files generated by EDA software (such as KiCad, Altium, or Eagle) showing the logical connections between components.
  • PCB Layout Files: The modifiable board routing files, including copper layers, footprints, and design rules.
  • Supporting Libraries and BOM: Custom schematic symbols, PCB footprints, and a complete Bill of Materials (BOM) required to successfully reproduce and modify the assembly.

By strictly defining these editable files as the "source," the CERN-OHL ensures that downstream users receive the actual building blocks of the project, not just the uneditable manufacturing outputs.

The Three Variants of CERN-OHL v2

Version 2 of the CERN-OHL recognizes that different hardware projects have different commercial and community goals. To accommodate this, the framework is split into three distinct variants:

1. CERN-OHL-P (Permissive)

This variant allows anyone to use, modify, and manufacture the schematics and PCBs for any purpose, including proprietary commercial products. The user is not required to share their modified schematic or PCB layout files, provided they retain the original copyright notices and attribution.

2. CERN-OHL-W (Weakly Reciprocal)

This variant acts as a middle ground. If a user modifies the provided schematic or PCB layout files and distributes a product based on them, they must release those modified design files under the same CERN-OHL-W license. However, if they integrate this open-source PCB as a sub-module into a larger, more complex electronic system, they do not have to open-source the schematics of the surrounding proprietary system. Furthermore, they can use proprietary components or closed-source firmware on the board without violating the license.

3. CERN-OHL-S (Strongly Reciprocal)

This is the strictest "copyleft" version. If anyone distributes a physical product or modified design based on CERN-OHL-S schematics and layouts, they must make the complete, modifiable design files of the entire resulting hardware product publicly available. This requires sharing all native EDA files and ensures that any improvements to the circuitry or routing are returned to the open-source community, preventing third parties from taking open designs and locking future iterations behind closed doors.

Patent Protection and Liability

Beyond copyright and file-sharing rules, the CERN-OHL framework provides critical patent clauses. Schematic designs often involve novel electronic circuits that may be subject to patent law rather than copyright. The CERN-OHL includes an express patent license from the original designer to the user, granting permission to make, use, and sell the patented circuitry contained within the shared schematics. It also includes a patent retaliation clause: if a user sues anyone alleging that the open-source design infringes on their patents, their rights to use the CERN-OHL licensed designs are immediately terminated.

Finally, the license includes strict "no warranty" and limitation of liability clauses. If a third party downloads a PCB layout, manufactures the board, and the circuit fails or causes damage, the original designer is legally insulated from liability.

Practical Implementation for Designers

To properly apply the CERN-OHL to an electronics project, the designer must include the full text of the chosen license variant inside the project repository. Additionally, the license notice should be placed directly inside the schematic capture sheets (usually in the title block) and on the silkscreen or copper layer of the PCB layout itself. By embedding the notice into the physical and digital files, the legal framework remains attached to the hardware throughout its entire lifecycle, from digital modification in an EDA tool to physical fabrication.