Summary of Castellated PCB Edges
This article explains how to design and fabricate PCBs with castellated edges, commonly used for modules like the ESP8266-12E. It outlines design considerations such as adding fallback holes for rework and specific rules for indicating vias in design files, including pad shape and size limits. The text also details how to create rectangular castellations using overlapping SMD pads and advises on cleaning up final boards by filing off support tabs if necessary.
Parts used in the Castellated Edges Project:
- Vias
- Round pads for copper
- Stop mask
- Board outline line (10 mil wide)
- Rectangular SMD pads
- Support tabs
Castellated Edges
Castellations are small plated edges, typically used for making circuit boards into small pcb modules. These are often seen on wireless modules, such as the ESP8266-12E.
While we don’t offer full support for castellations, they can be fabricated if you don’t mind a few minutes of rework and verification of the PCBs.
Design Considerations
It’s helpful to include a fallback hole near the edge. The ESP8266-12E boards, are a great example here. The extra via allows easy connecting of wires for rework, and makes it easy to salvage a module if the fabricated PCB doesn’t turn out perfectly.
Indicating in the design file
Castellations are simple to call out in most design tools. Simply include a via on the PCB, so the board outline goes through it.
However, due to our panelization process, the castellated vias must be indicated with round pads for copper and stop mask. The pads must also not extend more than 40 mil from the board edge. Square pads or pads that extend far beyond the edge will be trimmed, and the via will not be plated.
It’s also helpful to use a 10 mil wide line for the board outline. With our milling tolerance of 5 mil, this provides a good visual indicator of where the physical board edge might be. The fabricated edge can be anywhere within that line. This is very helpful for fine-pitch castellations with smaller holes.
Rectangular castellations can be made by using vias with round pads as noted above, and adding overlapping rectangular SMD pads. Since these pads are inside the board outline, they will not be trimmed, and will provide additional area for soldering (see below for example).
Cleaning up the final boards
We make a best-effort to minimize support tabs on castellated edges, but it sometimes happens. In these cases, you’ll need to file the tab off of your edge.
Read more: Castellated PCB Edges
- What are castellations typically used for?
They are small plated edges typically used for making circuit boards into small pcb modules. - How should vias be indicated in the design file?
Include a via so the board outline goes through it, using round pads for copper and stop mask. - What is the maximum distance pads can extend from the board edge?
The pads must not extend more than 40 mil from the board edge. - Why is a 10 mil wide line helpful for the board outline?
It provides a visual indicator of where the physical board edge might be given the milling tolerance. - How can rectangular castellations be made?
By using vias with round pads and adding overlapping rectangular SMD pads inside the board outline. - What should you do if support tabs remain on the final boards?
You will need to file the tab off of your edge. - Why is a fallback hole recommended near the edge?
It allows easy connecting of wires for rework and makes it easy to salvage a module. - What happens if square pads are used instead of round ones?
They will be trimmed, and the via will not be plated.
