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Should You Use Mechanical Layers or Etch Layers for Switch Pads?

pcb etch layers

Some types of switches and buttons require multiple layers to properly display where a component will belong in the assembly. Switches and buttons obviously need electrical contacts, which demands a portion of the component footprint appear in a copper layer, or etch layer. This shows exactly where mechanical contact points appear when the switch is closed and current can flow to a load. High power switches, tactile switches, and push buttons only need a copper layer and a 3D body to fully model, but there are other switches that may not appear directly on the PCB.

One example is film switches or carbon film buttons, such as you might see on a TV remote. These film switches are not always off-the-shelf components that are soldered onto the PCB, instead they may touch conductors through a contact film that bridges exposed pads. Those pads then make some electrical connection back to a processor or ASIC. If this is how you want to add HMI to your product, how should you create these components in your CAD data?

Layer Sets For Most Components

The vast majority of components used in a PCB layout will have a small set of layers used to define the component. Most components will be a simple passive device, active device, or integrated circuit, all of which have component leads that bond to the PCB. A package size and shape can be specified, as well as the 3D model of the component. In total, following sets of layers would be needed:

Copper or etch layers

Defines the copper pads for soldering

Assembly layers

Shows component outlines to verify assembly and placements

Mask layers

Shows stencil openings for solder mask and paste mask

Silk layers

Shows the silkscreen markings for a component footprint

This set of layers details the bare minimum requirements to define a component in a PCB library. The role of your PCB layout editor is to ensure these layers are included in your fabrication and assembly exports.

There are other components that rely on additional mechanical information to aid placement: panel-mount switches, film buttons, wire or cable runs in the board space, or switches with large protrusions are all examples of parts with relevant mechanical data. Some of these parts may have a mechanical element that makes contact with etch layers on the PCB, but may not mount directly to the PCB (no soldering). So this all begs the question, what data should be put into etch layers versus mechanical layers?

Etch Layer Definitions

In general, the best practice for etch layer definitions is to only include the portion of the component that is required on a copper layer to make the necessary electrical connections. This is the case even if nothing is soldered to the board, which is definitely the case for film buttons mounted to the product enclosure.

pcb etch layers

These kinds of buttons sometimes make contact with pads on a PCB to create a closed circuit, and they may not directly mount onto the PCB.

Because these kinds of parts are essentially just pads, there will be a solder mask opening and paste mask opening defined automatically in the PCB layout editor. Make sure to close the paste mask opening but maintain the solder mask opening so that the button contacts can make contact to the pads on the PCB.

Here, there is no PCB assembly layer data for the enclosure-mount button array because there is nothing to mount directly to the PCB. Therefore, this component should be marked as a mechanical type component and should not appear in the BOM for the PCB assembly. It would appear in the product’s assembly BOM, however.

Mechanical Layer Definitions

The other side of defining these kinds of components is the mechanical layer definition. The mechanical layer should include the outlines of the mating components that do not mount to the PCB. For example, in the film type buttons above, the film should appear in its own layer, and the electrical contacts on the film should not appear in an etch layer. The convention is to do all of this in a mechanical layer so as not to create any confusion for a fabrication house.

These film definitions should not be done in another etch layer. This gets confusing because, once the fabrication data is generated, there will be an additional etch layer that does not actually appear in the PCB. Unfortunately, if the component is not created correctly, the data may not be mirrored over to a mechanical layer and would have to be drawn by hand. For array components, such as a switch array or keypad, the array can be defined as a single mechanical layer, while each element in the array can be defined in an etch layer as its own component.

Final Checking in 3D

At the end of the design, the entire placement and layout will still need to be checked in 3D. This is largely done with an export to an MCAD format, which can then be used in a mechanical application to determine interferences in 3D. This portion of verification would not use your etch layers as the basis for verification. Instead, your component data needs to include 3D models, such as STEP files that illustrate the size and shape of the component.

3D PCB verification

Today, most component manufacturers have realized the importance of these models and they provide these models on their websites. This may not be the case for all integrated circuit manufacturers, but models for mechanical components like connectors and switches can usually be found on the manufacturer's website. Open-source libraries and third-party resources are also including more of these models for mechanical verification. Make sure your mechanical parts include the required models so that interferences can be checked.

When you’re ready to streamline your documentation for enclosure mount switches and film pads, use the best PCB design features in OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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