Design Keyboard PCB: A Comprehensive Guide

September 16, 2023

Introduction

Creating a custom keyboard PCB (printed circuit board) can be an incredibly rewarding project for engineers, hobbyists, and keyboards enthusiasts. A well-designed keyboard PCB serves as the foundation of a fully functional keyboard, enabling customized layouts, switch options, and added features like RGB lighting.

This comprehensive guide will walk you through the key aspects of designing a keyboard PCB from start to finish using best practices and considerations for optimal performance.

A keyboard PCB is the printed circuit board containing the electronics and circuitry that allow a keyboard to work. It serves several core functions:

Houses the switches that register key presses
Connects the switches in a keyboard matrix to the microcontroller
Provides mount points for components like controllers, connectors, LEDs, etc.
Creates conductive pathways between components using copper traces

The PCB design determines factors like the layout, number of keys supported, available switch options, and inclusion of components like backlighting.

Let’s look at some key components that make up a typical keyboard PCB:

Switches

These are mounted onto the PCB and get activated when pressed to register input. There are several types of keyboard switches to choose from:

Mechanical switches like Cherry MX are the most popular. They have separate moving parts and use a spring mechanism to provide precise tactile feedback and audible click-on activation. Different mechanical switch types have varying actuation forces and feedback characteristics.
Rubber dome switches use a rubber membrane sheet that gets pressed down to close the electrical contacts. They provide a soft springy feel when pressed but lack the crisp tactile feedback of mechanical switches. Rubber dome keyboards are common in budget keyboards due to their lower cost.
Optical switches detect actuation using an infrared light beam that gets interrupted when the switch is pressed. They allow very fast switching but don’t provide the tactile feedback of mechanical switches. Optical keyboards need to be kept clean as dust can interfere with the light sensors.
Topre switches use a combination of mechanical and electrostatic capacitive mechanisms for switching. This provides a smooth tactile feedback when pressed. Topre switches are considered high-end but can be expensive.
Magnetic switches use magnets and magnetic coils to detect actuation. They are highly durable with contact-less operation but the switching feel is very different from conventional mechanical switches.
Low-profile switches have a shorter travel distance and reduced height compared to regular switches. These allow slimmer keyboard designs. However tactile feedback can be lacking on some low-profile switches.

The switch choice depends on the desired tactile feedback, operating noise, force required, etc. The PCB design must properly accommodate the physical and electrical characteristics of the selected switches.

Diodes

Small electronic components are placed between each switch and the microcontroller. They play a vital role in matrix-scanning keyboard PCBs.

Diodes allow current to flow only in one direction. This prevents “ghosting” issues where multiple pressed keys erroneously register as one input.
A keyboard matrix uses rows and columns wired to the microcontroller. The diode makes sure only the key at the intersection gets registered.
1N4148 signal diodes are commonly used due to their fast switching speed. Other diode types can also be used.
Keyboard PCB design must incorporate a diode mounting pad with polarity marking for each switch position.

Microcontroller

The microcontroller is the brain of the keyboard PCB, processing all inputs and outputs.

It scans the switch matrix, detects which keys are pressed, and sends the data to the connected computer using a protocol like USB.
Some popular microcontroller options are ATmega32U4, STM32, Elite-C, Teensy, etc. based on needs.
The microcontroller should have sufficient I/O pins to handle the entire switch matrix scanning.
Suitable connector footprint and mounting pads must be included on the PCB for programming and debugging.

Traces

The copper traces on the PCB act like wires, forming connections between components.

Traces connect the switches in a matrix to the microcontroller, allowing scanning for pressed keys.
LED strips for backlighting also get connected to the controller or LED driver using these traces.
The PCB layout involves carefully routing traces on different layers without collisions.
Wider trace widths are used for high-current paths. Thinner traces for simpler signal routing.

Connectors

Connectors allow plugging in external components like cables.

For connecting to computers, keyboards predominantly use a USB connector. The PCB must include the corresponding USB port footprint.
If using a detachable cable, optional connectors may be added to connect to the cable.
For easy programming, an ICSP or SWD connector can be added to flash the microcontroller firmware.
Reserving space on the PCB edge for connectors facilitates board mounting and access.

Design Considerations for Keyboard PCBs

Several critical factors have to be evaluated when designing a keyboard PCB to meet functional requirements and performance goals:

Size and Layout

The overall size and layout are primary considerations:
Determine the number of keys needed based on the form factor – full size, TKL, 60%, etc.
Consider the spacing between switches and keycap sizes to be used.
Incorporate any additional keys like function keys, macro keys, numpad, etc. per layout.
Plan intelligent component placement conducive to the desired layout.
Ensure adequate routing room between tightly clustered components or switches.

Switch Support

Most DIY keyboards need support for both 3-pin (plate mount) and 5-pin (PCB mount) mechanical switches.
The PCB must incorporate the required copper pad configurations and hole sizing for each switch type.
Consider adding hot swap sockets for tool-less switch swapping without soldering.

Layers

Simple keyboard PCBs can be designed using a single copper layer.
More advanced keyboards with LEDs, complex switch matrices, etc. require 2 or more layers for intricate routing.
Additional layers increase layout flexibility but also add to complexity and fabrication costs.

Aesthetic Options

Determine if LEDs will be used – for per-key backlighting or underflow strips. Select suitable LED footprints.
For advanced RGB lighting, allow for additional components like RGB controllers.
If LED light pipes are needed, transparent overlay layers become essential. This impacts the overall layer stack up.
The PCB fab may impose restrictions regarding overall board shape, coatings, etc. based on capabilities.

Interface and Protocol

Most DIY keyboards use a USB port for connecting to computers. Choose a suitable USB connector footprint.
The communication protocol like USB HID or serial depends on the selected microcontroller and its configuration.

Debugging and Programming

Incorporate test points to validate power and signal connectivity during development.
Allow for ICSP, SWD, or other programmer pins for easy microcontroller reflashing.
A bootloader may be used to reprogram the microcontroller firmware through USB, without needing physical IC pins.

Other Considerations

Additional factors include switch matrix arrangement, stabilization methods, casing constraints, microcontroller choice, etc. Extensive planning is key.

Keyboard PCB Design Process Step-by-Step

Designing a keyboard PCB involves several key stages:

Schematic Design

The schematic design is the first major step where the circuit diagram is created showing components and their interconnectivity.

Select required components like microcontrollers, switches, diodes, connectors, LEDs, etc. based on the keyboard design goals.
Arrange components cleanly and logically on the schematic sheet following best practices. Group-related sections.
Label each component with a unique designator like U1, R5, etc. This aids identification during PCB layout.
Draw wires to connect components and define the overall circuit topology based on functional requirements.
Assign PCB footprints to each component. This maps schematic symbols to physical land patterns.
Add textual annotations to provide explanations and context where needed.
Incorporate peripherals like RGB controllers if needed for advanced lighting effects.
Create multi-page schematics to logically segment and organize more complex keyboard circuits.
Run electrical rules check to validate the schematic for issues with unconnected pins, conflicting designators, etc.
Generate a netlist mapping schematic connectivity to logical net names for PCB layout.

A thorough schematic design forms a solid foundation for proceeding to the PCB layout stage. Iterating on the schematic is easier compared to layout changes.

PCB Layout

The PCB layout involves designing the physical board itself based on the schematic diagram.

Place components on the board according to the schematic, taking into account type, spacing, orientation, and other layout considerations.

Route traces to connect components based on the connectivity defined in the schematic. Use appropriate trace widths based on expected current levels.
Design power delivery circuitry like power and ground planes, decoupling capacitors, ferrite beads, etc.
Create board cutouts for mounted components like USB ports, sockets, etc.
Incorporate board outline and mounting holes for assembly into the case.
Implement applicable design rules specified by the PCB manufacturer like track width/spacing, annular rings, etc. This ensures manufacturability.
Route high-speed or noise-sensitive traces carefully to match impedance and reduce interference.
Add text labels for identifiers, polarity markings, legends, etc. to facilitate assembly and debugging.
Run design rule check (DRC) and use trace spacing tools to verify adherence to all specifications.
Perform manufacturability checks like acute angle avoidance, teardrops, etc. to maximize fabrication yield.

Meticulous PCB layout is key to achieving a high-functioning keyboard PCB free of issues like short circuits, crosstalk, or assembly errors.

Prototyping

The designed PCB layout files are sent to a manufacturer to produce a physical board for testing.

Test board dimensions, mount points, connector alignments, etc.
Verify any component clearance or placement issues.
Check for short circuits, broken traces, or any board defects.
Test basic functionality by connecting switches and microcontrollers.

Iterate fixes based on prototyping feedback for the final board.

Assembly and Testing

Populate the PCB by soldering all required components like switches, controllers, connectors, etc.
Thoroughly test each subsystem – power, USB connection, switch matrix, microcontroller, etc.
Program the microcontroller firmware and test keyboard functionality.
Make any required firmware tweaks for optimal performance.
Debug any issues with connections, soldering, wiring, etc. if present.
Final integration into keyboard case and useability testing.

Conclusion

Designing a custom keyboard PCB requires careful planning, schematic design, layout considerations, prototyping, and extensive testing. Using suggested best practices along with quality design software and fabrication services can help bring keyboard ideas to reality in an efficient, cost-effective manner.

The ability to fully customize the electronics foundation empowers designers to create unique PCBs tailored specifically to their keyboard projects. So dive in, explore the possibilities, and build that perfect keyboard PCB you always wanted!

Frequently Asked Questions (FAQs)

What are the essential components of a keyboard PCB?

The bare minimum components are the microcontroller, switches, diodes, power circuitry, and USB connector. Additional parts like LEDs, RGB controllers, etc. provide added capabilities.

How complex is the PCB design process for a keyboard?

It can range from simple to highly complex depending on factors like size, layout, number of layers, inclusion of advanced features, etc. A basic 60% single-layer keyboard PCB is quite straightforward to design.

What design software is most suitable for keyboard PCBs?

Many options like EasyEDA, KiCad, or Eagle provide the necessary schematic and layout capabilities. Choose one that best fits your skill level and budget.

What are the recommended PCB materials and finishes?

FR-4 fiberglass with 1oz copper thickness is typical. Plated finishes like ENIG, Immersion Gold, or HASL are suitable depending on requirements.

How can I source prototyping and production runs of my keyboard PCB?

Reliable PCB manufacturers like PCBWay support prototyping in small batches as well as production runs in large volumes. Their online ordering system makes procurement easy.

Ready to take your PCB designs to the next level?

Experience the quality and precision of MorePCB today.

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Tag: PCB Design