Secrets of a Fabrication Drawing
Essential to the success of any printed circuit board (PCB) manufacturing project are the detailed notes included in the fabrication drawing. While the design files (typically Gerber or ODB++) contain the most basic structure and outline of the board design, it is the fabrication drawing that tells the manufacturer the specific details about a board that make it unique.
Without a fabrication drawing, engineers are subject to the default specifications of their manufacturer. When a fabrication drawing is included, it is regarded as the blue print of the project and makes clear to the manufacturer exactly what quality and specification requirements are needed to create the boards precisely as envisioned by the engineer. A fabrication drawing allows engineers to customize beyond the simple dimensions and layout of their board designs.
The main items included on a fabrication drawing are specifications and tolerances. Specifications include any information that is not readily available from the Gerber data. Tolerances are any requirements that could differ from the manufacturer’s standard tolerances or what is established as the minimum quality tolerances. Most often, the typical tolerances are standard practice; so, if there are requirements that differ from the standard tolerances, it’s important to make sure it is noted during the quoting process (tighter tolerances may increase costs, as the manufacturer considers risk associated with lower yields).
Below are the key elements that *should* be included on every fabrication drawing:
- Special fabrication notes (cutouts, unusual shapes, etc.)
- View of board – Component or Solder Side
- Layer stack-up
- Board thickness and tolerance
- Finish (including thickness for hard gold, soft gold, and gold fingers)
- Solder mask requirements, color, type, tolerances
- Hole chart with tolerances
- Part number details with revision #
- IPC class or other quality standards
Other elements that *could* be included:
- Copper thickness
- Tolerance for annular ring
- Tolerances of conductor widths
- Material specifications and dielectric thicknesses
- Detail blow up if needed
- Markings on the board
- Bow and twist tolerances
- Silkscreen, color, type, tolerances
- Electrical test tolerances and standards to be used
- ITAR requirement
- Bevel detail
- Countersink and/or counterbore requirements
- Array requirements
- Impedance requirements
Many PCB design software tools will allow the creation of a fabrication drawing. Start with just the board outline and use text to fill in the details outside of the outline.
When the fabrication drawing is complete, simply include the drawing as part of the data package provided to the PCB manufacturer. This helps to ensure that the information does not get lost or forgotten as it travels through the ordering process.
One important note about the relationship between fabrication drawings and purchase orders is that, occasionally, a purchase order will include specific details and requirements for the job. If a purchase order has details that conflict with the fabrication drawing, the purchase order details will be followed and will override the fabrication drawing.
If your PCB design software does not include functionality to create a fabrication drawing, there are alternatives that work just as well. Use a basic graphic design or presentation software tool, as simple as Microsoft PowerPoint, to create the drawing and save it as a PDF file. Or, even a README text file can be helpful to include the relevant details.
While fabrication drawings are not required by manufacturers to fabricate a PCB, they’re certainly very useful and will help ensure that the PCB you receive meets all of your required specifications.