PCB Design for IoT Devices
The Constraints of an IoT Board
An IoT device usually has three constraints at once: it must be small, it must run for a long time on a limited energy source, and it must be cheap. The three conflict. A small board makes the radio’s keep-out harder to satisfy and the thermal path shorter; a long battery life requires a low quiescent current, which limits the components that can be used; and a low cost limits the layer count, the testing and the components. The design is therefore a set of compromises, and the important thing is to make them deliberately rather than to discover them at the end.
Power Budget and Sleep Current
The power budget should be built before the components are chosen. The average current is what determines the battery life, and it is dominated not by the active current but by the sleep current and the duty cycle of the radio. A regulator with a low quiescent current, a radio with a short wake time and a microcontroller that can sleep deeply are the three parts that decide the budget. The design should also account for the leakage paths: a pull up resistor that is always connected, an LED that is always on or a sensor that is powered when it should not be all consume more than the sleep current of the processor.
Radio Layout and Certification
The radio is usually a module, and its layout follows the rules described for any radio: a clear keep-out for the antenna, a continuous ground plane, a well filtered supply and digital lines kept away. For an IoT device the additional consideration is the enclosure, which is often small and may be metal or may contain a battery, both of which affect the antenna. The module’s certification conditions have to be met, and where the product requires its own approval, a pre-compliance measurement is the practical way to confirm the layout before the formal test.
Sensor Interfaces
The sensors are where the analogue design matters. A sensor with a small signal needs a short, well referenced connection to the converter, with the analogue and the digital grounds arranged so that the digital return currents do not flow through the analogue reference. A sensor on a long cable needs protection against the environment and against the transients that the cable picks up, and the interface should be filtered and, where necessary, isolated. The reference for the converter should be as quiet as the application requires, and a reference that shares a supply with the radio will move whenever the radio transmits.
Battery and Power Path
The battery’s characteristics decide the design. A primary cell has a high energy density but a rising internal resistance as it discharges, so the regulator must work down to a low voltage and the design must not rely on a stable supply. A rechargeable cell needs a charging circuit whose layout is a switching design in miniature, with a tight loop for the switching current and a thermal path for the dissipation. The power path from the battery to the load should be short and wide, and where the device has a connector for charging, the protection against a reverse connection should be in place.
Manufacturing and Test for a Low Cost Product
A low cost product has a low budget for testing, so the design should make the test cheap. Test points on the critical nets, a programming and debug header, a way to measure the supply current and a way to exercise the radio without a full functional test all reduce the test cost. The board should be designed for the panel and the assembly process that the volume justifies, and the component count and the layer count should be the minimum that meets the requirement. Where the production volume is high, a design that is slightly more expensive per board but much easier to test and assemble is often the cheaper product.
Reliability and the Field
An IoT device is often installed in a location that is hard to reach, which makes the field failure cost much higher than the unit cost. The design should therefore consider the environment, the thermal range, the humidity and the mechanical stress of the installation, and the components should be rated for the conditions rather than for the bench. The firmware’s ability to recover from a fault, the watchdog and the brown out behaviour are part of the reliability, since a device that needs a physical power cycle is a device that will be visited. The board’s design and its firmware are a single system.
Provisioning and Identity
An IoT product usually needs a unique identity, such as a MAC address, a certificate or a key, and the board has to carry it in a way that survives production. The options are a serialised secure element, a microcontroller with a factory programmed identity or a label that is read during test. Whichever is used, the identity has to be linked to the unit’s test record so that a device in the field can be traced to its build data. The provisioning step is part of the production process and should be designed with the board rather than added to the line later.
FAQ
What dominates the battery life? The sleep current and the radio’s duty cycle, not the active current of the processor.
Why is a low quiescent regulator important? Because it draws current continuously, so its quiescent current becomes the floor of the average consumption.
What affects the antenna in a small device? The enclosure, the battery and any metal near the keep-out, all of which detune it.
How should a sensor be connected? With a short, well referenced path and a ground arrangement that keeps the digital return currents away from the analogue reference.
Why spend on testability in a cheap product? Because the field failure cost is high and the test cost is recurring, so a board that is easy to test is cheaper overall.
Conclusion
An IoT board is a set of compromises between size, energy and cost, and the design should make them deliberately from a power budget and a radio layout that respects the certification. Plan the power first. Low power design is part of PCB design and layout, the assembly sits in PCB assembly, and the verification is described under PCBA testing. IoT designs are first built during prototype PCB assembly in 2026.
