Indicating slots, milling, contour and rout-outs in your PCB design

When designing a printed circuit board (PCB), there are often times when you need to create cutouts, holes, contours or other non-copper regions in the board. These openings serve various purposes - allowing access to components, creating mounting holes, milling out spaces for connectors, etc. Indicating these areas accurately in your PCB design software is crucial to get the right physical board fabricated. This article provides a detailed guide on the various options to indicate routing, milling, slots and cutout contours on a PCB design.

Outline Pad vs Mechanical Layer

There are two main ways in PCB design software to indicate routing and milling regions:

Outline Pad

An outline pad is a copper pad on the signal layers that has the outline shape of the required cutout region. So for example, if you need a 20mm x 10mm slot to be milled out, you would draw a rectangular pad with those dimensions on the signal layer at the appropriate location.

The fabricator recognizes this outline pad and uses its shape to create the cutout, slot or hole in the PCB. Anything within the outline pad shape gets milled away during fabrication.

Mechanical Layer

The mechanical layer in PCB design software allows you to specifically draw board outlines, milling contours, routing keepouts etc. This is a non-copper layer meant to guide PCB fabrication.

You can draw any required cutouts, slots, holes as polygons filled with a mechanical layer hatch pattern. The fabricator recognizes this layer and creates the indicated cutouts on the final board.

When to use Outline Pads vs Mechanical Layer

As a thumb rule:

• Use Outline Pad for simple slots/holes or any copper edge routing that needs to happen
• Use Mechanical Layer for complex board outlines and milling contours

Outline pads integrate well with your copper pours and signal routing. So use them when you want tight integration with copper edges.

For everything else like complex board shape, component keepouts etc. the mechanical layer gives you more flexibility and control over fabrication.

Next, we look at some specific examples of common slotting/routing requirements and how to indicate them.

Indicating Slots

Slots on PCBs serve functions like allowing access to trim pots, connectors etc mounted on the underside. Or simply saving board space.

To indicate slots for milling:

1. Using Outline Pads

• Place a rectangular copper pad on top or bottom layer
• Set pad shape and dimensions equal to needed slot
• This pad outline signals slot to fabricator

2. Using Mechanical Layer

• Draw required slot geometry on the Mechanical Layer
• Fill with dense hatch pattern for fabrication

Note that slots should not cut into any ground plane layers as that would break the return path. So choose layer stack-up wisely.

Indicating Round/Oval Holes

Round holes serve mounting purposes or allow cables to pass through the board.

To indicate holes:

1. Using Outline Pads

• Place a round copper pad equal to hole diameter on any signal layer
• This pad outline signals hole to fabricator

2. Using Mechanical Layer

• Draw round polygons equal to hole diameter on Mechanical Layer
• Fill with dense hatch pattern for fabrication

Oval holes can be indicated similarly using oval pad outlines or mechanical layer polygons.

Routing Edges

Routing along the board edge is needed to create custom non-rectangular board profiles. Steps to indicate edge routing:

1. Using Outline Pads

• Place track or pad outlines along board edges on top/bottom layers
• Connect them to create the profile path
• Fabricator will route copper along the indicated path

2. Using Mechanical Layer

• Draw board profile on the Mechanical Layer along edges
• Fill with dense hatch pattern for fabrication
• Useful for complex non-standard profiles

Note that any required copper edges get defined through outline pads, while non-copper geometry gets indicated on the Mechanical Layer.

Component Keepouts

Components may sometimes require specific areas under them to be milled away to a certain depth. Common examples are tall connectors and capacitors. Steps to indicate component keepouts:

1. Using Mechanical Layer

• Draw keepout contour around component footprint on Mechanical Layer
• Set desired milling depth in fabrication notes
• Fabricator will mill away material under component to needed depth.

So in summary, use the Mechanical Layer to define all complex board geometry, non-copper regions and component spaces that need special milling.

Common Pad Shapes to Indicate Cutouts

Apart from regular rectangular/circular pads, here are some other outline pad shapes that have special meanings for PCB fabrication:

So pick appropriate outline shapes to tell the fabricator your exact routing/milling needs.

Contour Milling

Contour milling refers to a slot/cavity with an irregular non-rectangular profile. This is typically needed along board edges or around certain components.

To indicate any complex routing path:

Using Mechanical Layer

• Draw milling contour profile on Mechanical Layer
• Fill with dense hatch pattern
• Fabricator will mill custom cavity profile

The Mechanical Layer allows flexibility to create any required contour shape.

Fabrication Notes

Always include any special instructions for PCB fabrication in the manufacturing notes. For example:

• Milling depths for cavities
• Tooling holes if needed
• Special base material if required e.g. FR4 vs Aluminium PCB
• Thicknesses of any attached plates
• Surface finish specifications

Providing the right instructions makes sure you get boards fabricated to your exact requirements.

Summary

Indicating PCB slots, holes and milling contours accurately is an important step before sending your design to fabrication. To summarize:

• Use rectangular outline pads to indicate slots and simple holes
• Use mechanical layer polygons and paths for complex board geometry
• Define any component keepouts on mechanical layer
• Provide complete fabrication instructions

Following these PCB design guidelines will ensure all your milling, routing and contour needs get implemented correctly on the final boards.

FAQs

Q1. Can slots cut across ground or power planes?

A. It is best to avoid slots crossing any ground/power plane layers. This would break the return paths and disturb signal integrity. Where needed, use bypass capacitors around the slot or run a continuity trace across the layer.

Q2. What are the typical milling depths available?

A. Most fabricators can mill cavities up to about 6mm deep. Some support more than 10mm depths. Please check with your fabricator on their capabilities. Specify your needed depth in the manufacturing instructions.

Q3. Is edge routing possible on inner PCB layers?

A. Usually edge routing can only happen on outer layers of a PCB. However, some fabricators specialize in processes that allow selective depth edge routing into inner layers as well.

Q4. What is the benefit of indicating holes/slots with pads vs polygons?

A. Pads integrate with your signal layers and let you route traces to the edges easily. Polygons help define complex non-copper geometry. Use both approaches per your specific requirements.

Q5. How are rounded corners specified in routing?

A. Use rounded rectangle pads or arc/curve polygons on mechanical layer to indicate rounded corners for fabrication. Many fabricators require minimum radius sizes too.