Wearable Device PCB Design: Materials and Antennas

A wearable is a radio, a battery, a sensor and a processor inside something the size of a wristband, and the board has to survive being worn. There is no established standard for that combination, so the design decisions come from the physics of a small enclosure rather than from a template.

What Makes Wearable Design Different

Space is the obvious constraint, but the harder ones are the antenna, the battery and the body. The radio has to work next to a lossy dielectric, the battery has to be charged without a connector in some products, and the board has to tolerate continuous small movement.

Reliability expectations are also high, because the product is worn against the skin and cannot be opened for repair. A joint that would be acceptable in a set top box becomes a return when the same board is flexed several times a day.

Choosing the Laminate

The standard material, a glass reinforced epoxy with a dielectric constant near 4.5, is the cheapest and the easiest to source. It is also the lossiest of the common options, which matters as soon as the radio or the data rate rises.

A lower constant material with a lower loss tangent improves the antenna efficiency and the trace loss, at a higher cost and a longer lead time. The dielectric constant is not a single number either; it varies with frequency and with the resin content, which is why the value used in a simulation has to come from a measured curve.

Materials and the Stack

Where the product needs to bend, a polyimide core takes the place of the epoxy, because it withstands repeated flexing and a higher process temperature. A rigid flex construction then allows the radio section to be rigid and the interconnect to fold.

The stack for a wearable is usually thin, which reduces the bend radius but also reduces the impedance for a given trace width. The designer has to choose between a narrow trace on a thin dielectric and a wider one with more copper, and both change the loss.

The Antenna in a Small Enclosure

An antenna in a wearable is surrounded by the battery, the display and the body itself, all of which absorb or detune it. The ground plane it works against is small, so its bandwidth is limited and its efficiency is lower than the same antenna on a larger board.

The feed has to be kept clear of metal parts, which is a placement decision as much as a layout one. Every mechanical part near the radiating element should be reviewed, because a metal bezel or a battery shield can move the resonant frequency enough to lose the band.

Designing for Flexing

Where the board bends, the copper should run across the fold rather than along it, and the bend radius should be generous relative to the stack thickness. Rolled copper is used in the flexible layers because its grain structure tolerates the strain.

Vias and large copper features are avoided in the bend area. Both are rigid points in a flexible structure, and a crack that begins at one of them propagates along the conductor until the net opens.

Connectors and Charging

Pogo pins, spring contacts and magnetic connectors are common in wearables because they allow the product to be sealed. Each of them applies a force to the board, so the pad and the stiffener behind it have to be designed for that load rather than for electrical connection alone.

Wireless charging removes the connector but adds a coil, which occupies area and produces a field that must not couple into the radio. The coil and the antenna should be placed as far apart as the enclosure allows, with the ground plane arranged to limit the coupling.

Power and Thermal Limits

A wearable cannot dissipate much heat. The power budget is set by the battery, and the thermal path is limited by the enclosure, which is usually designed to feel comfortable rather than to conduct.

That combination pushes the design toward low duty cycle operation and toward components with a low quiescent current. The layout can help by keeping the charging path short and by giving the power components copper to spread into, but it cannot change the physics of a sealed plastic case.

Sensor Placement

A sensor that measures the body, such as an optical or a bio potential sensor, has its own layout requirements. Its analogue front end is a high impedance node, and it should be short, guarded and kept away from the switching supplies that the rest of the product uses.

The mechanical interface is part of the electrical design. A sensor that does not make reliable contact with the skin will produce a signal that no amount of filtering can recover, and the board outline and the window in the housing are what decide that contact.

Testing a Wearable Board

Test access is limited by the size, so a wearable is often tested through a fixture on a panel before it is separated. Test points have to be planned into the layout, because there is no room to add them afterwards.

The tests should include the radio in the presence of the enclosure, the charging function at the extremes of the battery voltage and a flex test on a sample of the flexible area. Each of these addresses a failure mode that a functional test on the bench will not reveal.

Documentation and Traceability

Because the product is worn, traceability matters more than in many other applications. The board should carry an identification that survives the assembly and the enclosure, and the records should link the unit to its material batch and its test results.

Where the product has a medical or a safety function, the documentation requirements grow again, and the medical wearable case adds requirements that a consumer product does not carry.

Design Rules That Save a Revision

Keep the radio section rigid, keep the bend area free of vias, keep the antenna clear of metal, keep the sensor front end short, and plan the test points before the layout is finished. Each of these is cheaper to follow than to correct.

The remaining decisions are about the material and the stack, which the high frequency routing guidance and the flexible multilayer construction describe in more detail. Together they cover most of what a wearable board has to get right.

Process Control and Verification

On a design of this kind, wearable is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

FAQ

Does a wearable need a high frequency laminate? Only if the radio or the data rate requires it. A lower loss material improves efficiency and link margin, at a cost that has to be justified by the product.

Why does the antenna work less well on a wearable? Because the ground plane is small and the body absorbs energy. Both reduce the bandwidth and the efficiency compared with a larger product.

Can a standard stack be used for the flexible part? No. The bend area needs a polyimide core with rolled copper; the rigid sections can use a conventional laminate.