PCB design best practices for wearable devices

Designing printed circuit boards (PCBs) for wearable devices brings together constraints that a larger product can often treat one at a time. The board has to fit an enclosure shaped around the body, draw as little current as possible from a small battery, and keep sensitive signals clean while a radio, a processor and a switching regulator sit only a few millimeters away.

None of these requirements can be optimized on its own. A smaller board saves space but crowds noisy and sensitive circuits together. A thinner device leaves less room for the battery. Extra copper layers help routing and noise but add cost. Good wearable boards come from settling these trade-offs early and deliberately. The sections below follow the order in which they usually come up.

Size and layout constraints

In a wearable, the enclosure usually defines the board, not the other way around. The outline, the mounting points, the position of every connector, button, light and sensor window, and the height available above and below the board all follow from the product's shape and from how it is worn. Agreeing on them with the mechanical design before layout starts avoids the most expensive kind of rework: a board that is electrically correct but does not fit.

Once the outline is fixed, the space inside it shapes the rest of the design:

• Layer count. Small, dense boards often need four or more copper layers: enough for routing, plus continuous ground and power planes. The extra layers often cost less than the problems that come from forcing a crowded design onto two.
• Package size. Smaller packages save area but tighten the design rules. Fine-pitch and chip-scale packages can require narrower traces, smaller vias or vias placed in the pads, and some need microvias and high-density interconnect (HDI) fabrication.
• Fabrication rules. Every reduction in trace width, spacing or via size narrows the choice of fabricators and can raise the cost of each board. Confirm the stack-up and the design rules with the intended fabricator before routing, not after.

Flexible and rigid-flex boards

Flexible circuits, and rigid-flex boards that join rigid sections with flexible layers, let a design follow a curved enclosure or fold several board sections into a small volume, replacing connectors and cables. They bring rules of their own:

• Keep components and vias out of the areas that bend, and add stiffeners where parts sit on flexible material.
• Route traces straight across a bend, at right angles to the fold line, and use curves instead of sharp corners.
• Respect the minimum bend radius, which depends on the material, its thickness and the number of copper layers. A flexible section that is bent once during assembly can accept a tighter radius than one that flexes every time the device is worn.

Power consumption

A wearable's battery is limited by the size and weight the wearer will accept, so current consumption is a design input from the start, not a measurement taken at the end.

Build a power budget

List the device's operating states, such as measuring, transmitting, idle and asleep, and estimate the current drawn in each and the share of time spent in it. The weighted average of those currents is the device's average current, the basis of a first estimate of battery life.

Design for sleep

Many wearables spend most of their time asleep, so the sleep current often decides battery life. Small, constant drains that would not matter in an active device become significant:

• the quiescent current of regulators and other circuits that stay powered;
• pull-up resistors held low, and resistor dividers that measure the battery voltage, which can be switched off between readings;
• sensors, memory and other peripherals left powered when they are not needed;
• indicator lights.

Load switches, or regulators with enable pins, let firmware cut power to whole sections of the board, and sensors with interrupt outputs let the processor sleep until there is something to do. These decisions are made in the schematic: firmware can only switch off what the hardware lets it control.

Choose regulators for the load

Switching regulators convert voltage efficiently, even when the input and output voltages differ widely, but they produce ripple and switching noise. Linear regulators are simpler and quieter, but they turn the difference between input and output voltage into heat, so their efficiency falls as that difference grows. A common arrangement is a switching regulator for the main supply and a low-noise linear regulator for sensitive analog circuits. At very light loads, compare quiescent current as well as efficiency: the quiescent current of a regulator that is efficient at full load can account for much of the current drawn while the device sleeps.

Rechargeable lithium cells need a charging circuit suited to their chemistry, and protection against overcharge, over-discharge and short circuits, either in the battery pack or on the board. Plan these circuits, and their place on the board, from the start.

Signal integrity and noise

As board area shrinks, circuits that would sit centimeters apart on a larger board end up side by side, and the coupling between them grows. Most noise problems on compact boards come from a few causes, and many of them are settled by the stack-up and the placement before a single trace is routed.

• Give every signal a continuous return path. A solid ground plane on the layer next to a signal layer lets the return current flow directly beneath each trace. Avoid routing fast signals across gaps or splits in that plane: the return current has to detour, and the larger loop radiates and picks up noise.
• Keep the switching loop small. In a switching regulator, the current changes abruptly in one loop. In a step-down (buck) regulator, it is the loop through the input capacitor and the two switches, or the switch and the diode; in a step-up (boost) regulator, the loop through the switch, the rectifier and the output capacitor. The inductor carries a smoother current and is not part of that loop. Place the loop's parts tightly together, with the capacitor right beside the switches and a ground plane directly beneath them, to keep the loop small and reduce the noise it radiates. Keep the switch node, which joins the switches to the inductor, short as well: its voltage swings quickly and couples into nearby traces.
• Decouple at the pin. Place decoupling capacitors as close as possible to the power pins they serve, with short connections to the ground plane.
• Control impedance where it matters. The radio's antenna feed, and some fast digital interfaces, need traces of a defined impedance. The trace width that achieves it depends on the stack-up, so specify controlled impedance with the fabricator and let them confirm the dimensions.
• Slow down edges that need not be fast. Clock and data edges faster than the interface requires add noise without benefit; a series resistor at the driver can soften them.

Protect small analog signals

Many wearable sensors produce small analog signals that are easily corrupted. Keep the analog front end away from the switching regulator, the radio and fast digital lines. Give it a quiet supply, for example from a low-noise linear regulator or a filtered rail, and keep its input traces short. Route differential inputs as a matched pair. When analog and digital circuits share one ground plane, placement does most of the work: keep digital return currents from flowing under the analog section.

Respect the antenna

A radio's antenna needs clear space: no copper, components or ground plane inside the keep-out area that its documentation defines. Place it at the edge of the board, away from the battery and other metal, and leave room for a matching network. The human body absorbs radio energy and detunes antennas placed close to it, so an antenna that performs well on the bench can behave differently on the wrist or the head. Plan to check it, and retune the matching network if necessary, in the finished enclosure, worn as intended.

Component placement

Placement decides most of what routing can achieve. A practical order:

1. Fixed parts first. Connectors, buttons, lights, sensors that face a window or touch the skin, the antenna and the battery contacts are positioned by the enclosure and by the wearer. Place them before anything else.
2. Zones by function. Group the power circuits, the processor and memory, the radio and the analog front end, and arrange the zones so that noisy and sensitive circuits do not share space or return paths.
3. Critical parts beside their partners. Decoupling capacitors next to their pins, the crystal next to its processor with short traces kept away from noisy lines, and the regulator's loop components close together.
4. Test and programming access. On prototypes, keep test points and the programming and debug interface reachable on the assembled board, and ideally inside the enclosure too, so that boards can be measured and reprogrammed during development. For the units that ship, decide how firmware is loaded and updated, and whether the debug interface is disabled or locked, as part of the device's security design: an interface left open lets anyone with physical access read or replace the firmware.

Placing parts on both sides of the board saves area but adds an assembly step and can make inspection and rework harder. It is a trade-off to decide early, with the assembly process in mind.

Heat matters more in a wearable than in most products, because the device sits against the skin. Regulators, the processor, the radio while it transmits and the charging circuit all produce some heat. Keep them away from the areas that touch the skin, spread their heat into copper planes, and check surface temperatures on the prototype, including while it charges.

Designing for the body

A wearable board is bent, pressed, dropped, exposed to sweat and touched constantly. Several design decisions follow from that:

• Thickness and comfort. The board's thickness and its tallest components set the device's thickness at that point. Place tall parts such as inductors, connectors and large capacitors where the enclosure has room, and consider a thinner board where the design allows it.
• Mechanical stress. Ceramic capacitors and fine-pitch solder joints can crack when the board flexes. Keep them away from mounting holes, connectors that take force and edges that may bend, and orient ceramic capacitors so that the board's likely bending does not strain them between their terminals.
• Moisture and sweat. Sweat is conductive and corrosive. Sealing is mainly the enclosure's job, but the board can help: a protective conformal coating, no exposed copper near openings, and a plating suited to any contacts that stay exposed, such as charging contacts. If the board is coated, decide early which areas, such as connectors, test points and contacts, must stay uncoated.
• Electrostatic discharge. People touch wearables all day. Protect buttons, exposed contacts and connectors with discharge protection placed close to where the discharge enters, with a short path to ground.
• The battery. The battery is often the largest part of the device. Decide its size and position early, keep it away from heat sources and from the antenna, and protect it from pressure and sharp edges.

Design verification and prototyping

Wearable boards are dense, and problems are expensive to find once the board is assembled and enclosed. Checks before fabrication and a planned bring-up save board revisions.

Before ordering boards

• Review the schematic against each part's datasheet: supply ranges, pin functions, required external parts and power-up sequencing.
• Run the electrical rules check, and the design rules check with the fabricator's rules.
• Check the board inside the enclosure's 3D model: the outline, mounting points, component heights and the clearance around connectors and buttons.
• Confirm the stack-up, the controlled impedances and any flexible sections with the fabricator.
• Give every power rail a test point and a way to measure its current, such as a solder jumper or a zero-ohm resistor in series.

Bringing up the first boards

Then measure what the design assumed. Compare the current in each operating state with the power budget, check the noise on the analog supply and inputs, test the radio in the enclosure while the device is worn, and record the surface temperature of the parts near the skin. Each measurement either confirms the design or shows what to change in the next revision.

Plan for more than one board revision. A first prototype can be larger than the final board, with extra test access, to prove the circuits before they are miniaturized.

A checklist for wearable boards

• The board outline, mounting points and component heights are agreed with the enclosure design.
• The stack-up and design rules are confirmed with the fabricator.
• A power budget covers every operating state, including sleep.
• Firmware can switch off every section that does not need to stay powered.
• Noisy and sensitive circuits sit in separate zones, over a continuous ground plane.
• The antenna has its keep-out area and room for a matching network.
• Parts that produce heat are kept away from the areas that touch the skin.
• Buttons, connectors and exposed contacts have discharge protection.
• Every power rail has a test point and a way to measure its current.
• A bring-up plan exists before the first boards arrive.