Custom Number Pad PCB: From Key Map to Tested Design

What Is a Custom Number Pad PCB?

A Custom Number Pad PCB is defined by the specified keys and functions implemented within one combined electrical system, mechanical system, and firmware system. The host connection, method of programming, and physical stack are also defined along with verification criteria; thus, a replacement board, kit, or finished numpad has a different intent than a custom numpad PCB.

Custom products can be very vague, usually evident at the time the potential purchaser requests a quotation. For example, the request may be for “custom numpads”; however, because the word “custom” may encompass many different items, it’s important to clarify the request and what ‘custom’ actually means. What is defined as ‘custom’ can mean something as simple as the same product that will have a different color PCB and a logo on the product compared with a product that will have a different key layout or added rotary encoder, which will also be quoted, tooled, and scheduled for manufacture in a completely different way.

With only the PCB replacement and no design files, while the PCB may fit inside an existing case, the surrounding clearances of the PCB may not match due to the connector location, switch system, and so on.

To accurately compare price and lead time, each thing that must be procured should be clearly defined prior to the quotation process. This applies even to a nominal product like a 20-key USB numpad, where wider-key geometry, remapping, recovery access, connector openings, simultaneous-key behavior, and enclosure clearances still need to be defined.

Define the Numpad Before You Choose the Circuit

Define the application and which kind of key map must be built before choosing which type of controller to use, what type of matrix to use, how many connectors to provide, or whether to use a radio module. The process of selecting the circuit should begin with defining the behavior of the application, as well as defining the geometry of the enclosure, as these two things will drive how the circuit is laid out. Just because a designer finds an attractive MCU or module does not mean that the selected part will not later limit the number of GPIO connections, limit access to programming, limit available features, or limit placement of the rest of the components.

Once the feature list is defined, reserve controller pins for programming, communication, indicators, encoders, lighting, and all other functions before assigning the switch matrix. Verify whether you want to use USB or wireless, and how your technician will access recovery on both the bare board and the enclosed version of the product.

Marketing claims (e.g., “wireless”, “hot-swap”, or “VIA-compatible”) are not test criteria. A feature will only be considered designed once its circuit, firmware behavior, mechanical impact, and pass condition are known.

Host Interface Design for Custom Number Pad PCBs

The host interface will impact the PCB shape, power path, firmware, enclosure opening, and acceptance testing. A wired USB numpad and wireless auxiliary pad may share the same key mapping, yet require different electrical and mechanical design decisions.

USB-C Numpads

For USB 2.0 devices using a USB-C receptacle:

Define the connector, CC configuration, D+/D- routes, ESD protection, power entry, and connector mechanics as part of one interface. Follow your chosen controller’s or USB interface’s reference circuit to define the connector, CC configuration, D+/D- route, and ESD placement. A generic Type-C footprint copied to the schematic will correctly map the pinout, but may leave out controller-specific track length/protection placement requirements.

Maintain the USB differential route with the approved stack-up, and locate any required interface protection close to the connector in the intended signal route. When using USB interface protection guidelines, consult the documentation for the controller selected, rather than relying on these documents as universal schematics for every numpad.

The receptacle can be electrically correct; however, the product may fail if the receptacle shell, anchor pads, plug body, or cable bend intersects with the enclosure. Verify power-up success, USB enumeration success, unplug/replug function, recovery method, and accurate HID output on the intended host before accepting the interface with the intended cable.

Wireless Numpads

Wireless numpads require additional design considerations such as radio placement, antenna clearance, battery/charging, sleep/wake behavior, pairing/reconnection, and enclosure interaction. The chosen radio module or SoC reference design must provide antenna and RF keepout requirements. Generic radio placements do not suffice for device-specific guidance.

For example, Nordic Semiconductor states keepout requirements associated with antennas in its reference radio circuitry; a different radio may have different vendor specifications. Final verification must include connection stability, successful pairing/reconnection, charging behavior when applicable, wake from the intended low-power state, and operation inside the actual enclosure.

RP2040 Design Example

The RP2040 illustrates why controller checks remain device-specific. Raspberry Pi has published documents describing the RP2040 as having 30 multifunction GPIOs, USB 1.1 host/device capability, BOOTSEL USB mass-storage programming, and a reference USB circuit based on Hardware design with RP2040.

The reference circuit uses 27-ohm series resistor values close to the chip, targeting approximately 90 ohms differential for USB signaling. These values are applicable to this controller context and must be considered separately for another microcontroller or USB interface designed from its own documentation instead of inheriting RP2040 values.

How Does Each Key Map from the Matrix to Firmware?

A single mismatch can break an otherwise fully functional system. A board that passes power and continuity can have the correct BOM and function correctly by using the right firmware compiled from the correct source, and still send the wrong keystroke when the operator presses the key marked correctly on the silkscreen because the matrix-to-keycode mapping has changed as a result of a revision late in the layout process and was not re-verified against the traceability row.

The QMK firmware uses ‘rows’ and ‘columns’ as one method of reducing the need to connect directly to each switch. Multiple switch closures can create ghosting effects through unintended paths in the matrix. Ghosting caused by multiple switches being pressed at the same time can be prevented by using per-key diodes to create a one-way path for current to flow, when the diode orientation matches the scan direction of the firmware (How a Keyboard Matrix Works).

In QMK, there are row pins, column pins, matrix dimensions, and DIODE_DIRECTION. Once the hardware has been created, if there is a need to change the way that the hardware is electrically connected to the controller, the firmware must also be changed to match.

To help avoid mistakes, a traceability table should be created before the final schematic is made, and this table should be kept until prototype acceptance. If at some future time the traceability table is needed, there will not be any need to recreate the relationship from disconnected CAD files, firmware, or test files.

In the illustration, we show one hypothetical key, not a standard matrix coordinate. The additional positions for wider keys also require recording the type of stabilizer used with wider keys and its geometry, since the electrical mapping cannot confirm how the stabilizer, PCB, and plate were aligned.

While the VIA specification requires both a matrix definition and a description of the physical arrangements of the key switches in a keyboard layout, the instructions regarding the QMK configuration provided by VIA also require compatible firmware settings. The physical arrangement of the switch, its matrix coordinate, and the configurable action corresponding to that location cannot be validated by either a JSON definition or the firmware binary alone.

Testing to verify which keycodes are received at the expected host output converts an overall “this key feels wrong” statement into a specific error that can be fixed (i.e., stuck keys, swapped rows or columns, or firmware mapping that wasn’t updated for the last layout revision).

Lock the PCB to the Plate, Case, and Connector

To create a manufacturable key layout, all components must share the same geometry. If a connector has been moved too far away from the edge or a wider-key layout is selected, it can cause defects in the PCB outline, routing space, cable accessibility, stabilizing features, and firmware key mapping.

The PCB layout process will identify one dimensionally approved reference point (datum) and catalog every 1U and wider-key center, stabilizer orientation, mounting hole, plate cut-out, enclosure wall, and connector opening against that reference point. The choice of mounting stabilizers on top of a plate versus attaching them directly to the PCB will also result in different mounting hole and keepout configurations on the PCB.

The height on both sides of the PCB is important in the assembly process because a hot-swap socket, programming header, connector shell, or similar component may clear the PCB outline but hit the sidewall of a case; check the assembled stack directly with the intended switch, plate, stabilizer, case, plug body, and cable instead of assuming that 2D clearance is adequate.

For example, one of the most common design errors occurs when you have a connector that lines up correctly according to the PCB assembly drawings but does not line up properly with the opening in the case once the actual PCB has been assembled with the assembly plate on it. Correcting this design error may require you to change the enclosure or make a change to where the connector or assembly stack is located; therefore, the assembled geometry must be approved before generating the production files.

For all manufacturer drawings for selected switches, sockets, stabilizers, connectors, or encoders, you must use the drawings as references for your own production geometry. Using photos and generic footprints for parts, or substituting any part for another part, will require another mechanical review.

When the envelope of the product is final, the PCB layout process will complete the PCB’s placement, routing, return paths, and design rule checks without having to estimate the case or plate dimensions.

Should You Use Soldered or Hot-Swap Switches?

Selecting whether to use soldered or hot-swap switches depends on each application’s specifications; neither is superior to the other.

A hot-swap label does not provide information about the socket part number, footprint, or orientation, nor does it provide information about retention or clearance, so compatibility cannot be determined by looking at a category-level description of a hot-swap switch. To determine compatibility, all elements (switches, sockets, plates, stabilizers, enclosures) must be physically compared to each other.

PCB Construction and Assembly Choices

Although a four-layer, ENIG PCB can be suitable for use in a wireless RGB numpad, it can be over-specified when used in conjunction with a simple wired matrix keyboard, but under-specified when used in conjunction with a denser matrix keyboard with the same key layout.

Select the least complex (yet fully valid for routing, USB or RF, component assembly, mechanical geometry, test access, and expected production quantity) way to build the keyboard instead of choosing four layers, ENIG, or a similar option because it seemed like a premium numpad specification.

What Must Work Before the Prototype Is Accepted?

A prototype is acceptable only after proving out the entire input chain. The prototype must recover and program properly, connect with the target host, produce all outputs assigned to it, handle required simultaneous key actions, operate according to any optional functions, and fit in the assigned enclosure.

1. Prepower Inspection – Inspect all elements prior to applying power to the board, including but not limited to polarity, orientation, shorts, missing components, connector condition, solder joints, and the intended power path.
2. Programming and Recovery Verification – Program the approved firmware image onto the board, and then verify programming (load) and recovery after assembly using only the means of access available in the final enclosure.
3. Host Interface Verification – For wired USB keyboards, verify enumeration and reconnection when unplugged and plugged back into a computer; for wireless keyboards, verify pairing, reconnection, and intended operating mode on the target host computer.
4. Key Testing – When a switch event fires but the output sent to the host computer is incorrect, log all such incidents immediately against the traceability row that corresponds to each key; i.e., key ID, matrix coordinate reported by the keyboard firmware, and keycode sent, before proceeding to test the next key. Determining which key drifted after the final full pass may be difficult, as the underlying cause could be due to a design flaw, firmware error, or other issues.
5. Ensure proper use of defined key combinations. Hold the specified keys and document any missed events, unintended outputs, or any additional matrix behavior that is inconsistent with the designated acceptance conditions.
6. When utilizing optional functions, test each function separately, as encoders, RGB lighting, displays, battery charging, indicators, etc., all have unique pass conditions associated with them.
7. Validate all assembled geometries. Assemble the stabilizers, plates, switches, cases, and connectors with their associated cables, and ensure no interference, excessive force, blocked access, or incorrect connector alignment exists.
8. After a power cycle, confirm that critical checks have been performed. Document hardware revisions, firmware versions, host status, and results to allow for product comparisons to the original prototype after future developments take place.

CAD rule checks cannot determine whether the K07 key sends the expected command or whether recovery remains possible after enclosure closure. The only way to establish whether these functions work is to physically connect and test the electrical, firmware, host, and mechanical elements.

For production, develop a repeatable FTP or fixture that transforms the per-key map and optional-function criteria into manufacturing documentation that provides verification of expected outputs based on hardware revision and firmware revision.

Files Required for Custom Number Pad PCB Production

Gerber files do not store key coordinates or the geometry of stabilizers, recovery access, compatible firmware, or anticipated host outputs. Production requires one approved revision with the files for each area (mechanical, circuit, PCB, firmware, programming instructions, and test criteria) that accurately reflect the same device.

The complete input package includes the physical layout; the key mapping; plate and enclosure drawings; the selected switch and socket part numbers; any connection or radio requirements; and the approved feature list. Design sources will provide the editable schematic, a schematic PDF, PCB files, approved libraries, and any design constraints necessary for review.

Producers will require all the following to produce an assembled PCB: Gerber files or ODB++, drill files, BOM, component placement file, fabrication drawing, assembly drawing, and any necessary process instructions. Any firmware files should also include source or project files as required; requirements for programming, compilation, or release; the binary file programmed on the PCB; QMK configuration; any applicable VIA definitions; version identifiers; and required programming dependencies.

Programming documentation should describe the regularly used method for flashing the firmware and the recovery method available after enclosure assembly. The following items should accompany programming documentation: the per-key traceability table; the target host procedure; combination cases; optional-function checklists; and pass criteria per key.

The usual assembly order is to mount and solder SMT devices such as RGB components, diodes, MCUs, USB or ESD devices; follow-up will include the inspection of devices and the installation of hot-swap sockets or through-hole hardware. A fully assembled PCB needs to be programmed and full-key functional testing must be executed. However, the precise order of assembly is contingent upon the components used and individual assembly processes.

Design services for custom boards are available for PCBs still open in the design stage. Custom Board Design Solutions will start with the same physical key map and mechanical inputs. After approval, Prototype Boards can be assembled using the same firmware and functional-test criteria as future Custom Assemblies, thus no redefinition of the product is needed.

Applications

It is not the industrial nomenclature, but rather the key, interface, service, and testing criteria that define the differences amongst these types of PCBs.

Numeric Data Entry Terminal

When a terminal for data entry is constructed using a number pad, the characteristics most valued include deterministic numeric key behavior; Enter and larger-key geometry that provides intuitive access to keyboard functions; a layout that does not make the operator second-guess which function is assigned to which key; reliable enumeration of the host; and confirmation of actual host output, including whether the software distinguishes numeric-keypad keycodes from top-row number keycodes, by way of the firmware map and acceptance tests.

As such, the PCB provides a fixed relationship between physical key centers; stabilizers; functions of the keypad; connector locations; and behaviors associated with the target host. Utilizing the per-key table during production testing allows for the verification of every numeric and function key prior to the board being placed in an enclosure.

Many POS and warehouse/inventory systems use the Enter key on a numeric keypad for verifying quantity fields, while the Enter key on the main keyboard submits complete sales transactions. If a numeric keypad is wired to output the wrong keycode for its physical Enter key, the device will be able to pass both continuity and enumeration tests, but when used in conjunction with application software, incomplete sales transactions may result even if a generic keyboard tester shows no error.

Programmable Macro Pad

Although it may have fewer keys than a standard number pad, it offers additional features that allow the user to access multiple functions by utilizing layers, remapping the key functions on the macro pad, encoder settings, RGB indicators, and specific application commands. Since you may need to add many additional functions to the macro pad, these increased features add complexity to the design in what will be needed for GPIO, firmware, storage, recovery, and testing, even though the total size of the physical PCB area is considerably smaller than that of a standard number pad.

When creating QMK or VIA projects, it is important to connect the physical location, matrix coordinate, configurable action, and expected output through the same traceability record.

A frequent failure that you may see is when a user binds a layer key or encoder function that works correctly on the first flash of the firmware, but that layer key or encoder function resets back to Layer 0, or loses its custom action after being power cycled. This failure may occur when the intended layer state or encoder configuration was not stored or restored correctly from persistent storage; this failure does not show up during initial bench testing, but instead only appears when the user removes power overnight; at that point, the user may return the device as defective, rather than identify it as a firmware storage failure.

Industrial Control Panel

You can also create a compact control panel using a numpad-style matrix if you need to represent fixed commands, machine parameters, or operator inputs where the labeled functions must remain constant on multiple builds. Design engineers should give priority to connector retention, enclosure geometry, service access, and repeatable functional testing. Repeatability of command output among different units is of greater importance than customization based on users’ preferences for switches.

If the keypad panel has various sizes of keys, an array of indicators, or is encased in a sealed assembly, the board and testing package must account for these electromechanical characteristics from the design phase forward. Safety systems and interlocks at the system level must be developed separately as design tasks and should not be inferred from the keypad PCB.

However, for a machine that employs an emergency stop function, a fail-safe interlock mechanism, or is certified as a complete machine, it is the responsibility of the machine builder to develop the entire safety design; the keypad PCB alone does not establish those safety functions. Clarification of the boundary conditions before layout will prevent the resultant panel from being electrically correct but unusable for installation.

Wireless Auxiliary Numpad

The addition of a wireless auxiliary keypad introduces many new constraints, including the design of batteries, chargers, placement of radios, clearance around the antennas, characteristics of sleep and wake, and connection to the host system. These new constraints will affect both the geometrical and physical design of the PCB and enclosure, as well as firmware. As a result, converting a wired design to a wireless version cannot be achieved by merely adding a radio to an existing design, even late in the layout process.

The acceptance should verify the pairing and reconnection of the device, the key output, its ability to wake the device, charging behavior when applicable, and the availability of a low-battery indication. The antenna area and other metallic or grounded elements surrounding the antenna area must comply with the radio vendor’s requirements for installing the mechanical elements.

Another area of design conflict is retaining a vendor-supplied metal enclosure panel, an RGB heatsink plane, or a battery shield located in the same area as the antenna after the radio’s placement has already been approved. Once the enclosure is closed, the connection range may drop dramatically, and even if a board performed well when paired on an open bench, it may not perform well in an enclosed environment due to the same limitations. Therefore, acceptance testing must occur in the enclosed environment using the actual battery and shielding.

What Changes Custom Number Pad PCB Cost?

The price of a PCB is based on the overall design and production work and not just on the number of keys. Two separate 20-key PCB designs can vary significantly in cost when comparing a simple wired matrix design with a complex design that includes hot-swap sockets, RGB LEDs, a radio, battery charging, remap capability, hardware to assemble the PCB in a housing, programming, and functional testing.

• Specifications of the PCB size, layer count, board thickness, copper requirements, clearances, surface finish, and controlled-impedance characteristics, if applicable.
• Complexity of the BOM, availability of components, MCU or radio choice, USB protection, RGB LEDs, encoders (e.g., rotary encoders), displays, and components used for charging batteries.
• Number of hot-swap sockets required, through-hole versus SMT assembly requirements, hand insertion, rework access, and inspection requirements.
• Firmware setup, QMK configuration, method for programming the firmware, serial number or revision information for the firmware, and recovery plans.
• The extent of functional testing to be completed, which will include every key, key combinations, connections using USB or wireless, optional functions of the hardware, and inspection of the assembled housing itself.
• Prototype quantity, volume for pilot runs, volume for future production, hardware supplied by customers, packaging requirements, and any remaining responsibility associated with the customer.

For this reason, quoting a replacement PCB for comparison purposes does not provide a valid comparison for a new and different design. The quote for a custom design can only be compared when the complete set of requirements for the electrical systems, mechanical systems, firmware systems, assembly, and quality-testing systems for each product has been defined.