Wearable PCB: Design, Materials and Manufacturing

What Makes a Wearable Board Different

A wearable board is a small electronic product that is worn on the body, and the constraint that dominates it is not the circuit. It is the combination of a very small volume, a very small battery, a moving surface to attach to, and a user who expects the device to survive sweat, rain, temperature change and the occasional drop without becoming uncomfortable or unsafe.

The result is a board that is denser than a phone board, quieter than a phone board because it usually contains a radio, and mechanically far more heavily qualified because the body moves and the skin is a thermal and chemical environment.

The Types of Board

Rigid. A small rigid board is still used in watches, wristbands and clip on devices, where the electronics sit in a case that does not bend. The advantage is cost and the availability of the standard surface mount process; the disadvantage is that the board cannot follow a curved shell and needs connectors or cables to reach the display, the battery and the sensor.

Flexible. A polyimide flex circuit bends to follow the wrist, the ear or the chest, and removes the connectors and cables that would otherwise be the least reliable parts of the product. It is the usual choice for a sensor patch, a heart rate band or a device that has to wrap around a limb.

Rigid flex. Rigid islands for the components and the connectors joined by flexible sections combine the assembly convenience of a rigid board with the shape freedom of a flex. It is common in smart glasses, high end trackers and medical wearables, where several subsystems have to be connected in three dimensions.

Stretchable and textile integrated. Conductive fibres, conductive inks and elastomer substrates allow a circuit to be integrated into a garment, but the electrical performance and the reliability are different from those of a conventional board, and the design rules come from the textile industry as much as from electronics.

Design Rules That Matter

Miniaturisation. The board carries fine pitch components, often the smallest passive sizes in use, and it may use high density interconnect construction with microvias and fine lines to route the necessary connections in the available area. The stack must be planned with the assembly house because the pad geometry and the solder paste printing become the limiting factor, not the routing. Embedded components and package on package are used where the density demand justifies them.

Low power from the first line of the schematic. The battery is small, so the average current decides the product. The quiescent current of the regulator, the leakage of the pull-ups, the sleep current of the radio and the duty cycle of the sensor are the variables, and they have to be chosen deliberately rather than inherited from a development kit. A wearable that runs for two days instead of five usually has a power architecture problem rather than a battery problem.

The antenna is the hardest part. A wearable has almost no ground plane, is surrounded by a display, a battery and a metal case, and is worn against a lossy body that detunes the antenna and absorbs its power. The antenna has to be designed and measured with the product and the body present, not evaluated on a bare board. Keep the feed clear of copper and metal, use the enclosure as part of the design rather than fighting it, and verify the performance in the intended position.

Sensor integration. An optical heart rate sensor needs a controlled optical path and a tight seal against ambient light; a bio impedance or ECG electrode needs a conductive path to the skin with a defined contact impedance; an accelerometer needs mechanical coupling to the body. Each of these is an electromechanical requirement that the board and the mechanical design have to satisfy together.

Skin temperature. A device worn against the skin has a much tighter temperature limit than a handheld product, and the surfaces that touch the body have to stay within a comfortable range even when the device is charging or the radio is transmitting. The thermal design is therefore a safety and comfort requirement, and the placement of the hot components and the charging coil has to respect it.

Mechanical durability. The board is bent, twisted, compressed and knocked against the arm, the door frame and the gym equipment. The strain relief at the flex sections, the support under the connectors and the positioning of the components relative to the bend areas decide whether the product survives a year of use. Our notes on PCB design and layout cover the practices involved.

Body Contact, Sealing and Materials

The materials that touch the skin have to be safe for prolonged contact, and the choice is a regulatory question as well as an engineering one. The board itself is normally encapsulated or coated so that no copper, flux residue or bare laminate is exposed, and the enclosure material, the adhesive and the coating are all part of the biocompatibility assessment for a medical device.

Sweat is the reason the sealing has to be taken seriously. It is a mildly aggressive salt solution that corrodes copper and can carry current between exposed conductors, and it is present exactly where the seals are most stressed, at the wrist or the ear. Conformal coating protects the board, a sealed enclosure with a defined gasket protects the assembly, and the ingress protection rating the product claims has to be verified by test rather than assumed. Our notes on conformal coating describe how the coating is selected and applied.

Charging brings its own constraints. A sealed product is usually charged inductively, which puts a coil on the board and a wireless power receiver alongside the radio, the display and the battery, in a space where every one of them interferes with the others. The coil placement, the ferrite shielding and the separation from the radio antenna are part of the layout, not an accessory.

Manufacturing and Assembly

A wearable board is usually a fine pitch assembly on a thin substrate, and both aspects make the process more demanding. Thin boards and flex sections need support through printing, placement and reflow, the paste volume has to be controlled for the small pad geometry, and the placement machines have to be capable of the fine pitch devices and the thin components. The thermal profile has to be gentle enough for the flex substrate and the battery, which is usually assembled afterwards.

Encapsulation or overmolding follows the assembly in many products, and it has to be done without trapping flux residue or moisture and without stressing the solder joints. Because the product is small and sealed, a defect that occurs during encapsulation cannot be repaired, so the process control and the inspection before sealing carry the whole quality burden. Our PCB assembly group builds these assemblies, and our notes on PCB manufacturing describe the substrate and stack options.

Testing

The test plan for a wearable has to cover more than the electronics. The functional test verifies the sensors, the radio and the power behaviour, including the current in every sleep mode, because that is where the battery life is won or lost. The radio performance is measured in the product and in the intended wearing position, because a link budget measured on a bare board does not describe a device worn on a wrist.

The mechanical and environmental qualification covers the flex life, the drop and the wear, the ingress protection, the sweat exposure and, for a medical device, the biocompatibility of the materials in contact with the skin. Our notes on PCBA testing describe how these checks are structured.

What Drives the Cost

The component density and the fine pitch assembly dominate. A small board with many small components costs more to assemble than a large board with few, because the placement time, the paste printing and the inspection are all per feature rather than per unit of area. The flexible or rigid flex substrate adds its own premium, and the encapsulation, the sealing and the environmental testing add process steps that a conventional product does not need.

Volume helps, because the tooling and the process development are amortised, but a wearable is rarely a low cost product to assemble. The design decisions that reduce the cost are integration, a smaller component count and a stack that can be built on a standard process rather than a bespoke one.

FAQ

What board type is used in a smartwatch? A combination is typical: a small rigid or rigid flex main board for the electronics, with flexible sections or a separate flex for the display, the sensors and the charging coil.

Why is the antenna so difficult in a wearable? Because there is almost no ground plane, the space is full of metal and lossy materials, and the body detunes the antenna and absorbs its power. It has to be designed and measured with the product and the wearer present.

How is the board protected from sweat? With a conformal coating on the board and a sealed enclosure with a defined gasket, qualified by an ingress protection and a sweat exposure test rather than by a material claim alone.

Can a wearable board use a standard FR-4 process? The rigid sections can, but the flex and rigid flex sections need a flexible substrate process and the assembly has to handle a thin, fragile board.

What limits battery life in a wearable? The average current rather than the peak. The regulator quiescent current, the radio sleep current, the sensor duty cycle and the pull-up leakage are the usual causes of a product that falls short of its target.

Conclusion

A wearable PCB is a dense, low power, mechanically qualified board wrapped in a sealed product that sits on the skin. Miniaturisation and the fine pitch process set the cost, the power architecture sets the battery life, the antenna design decides whether the radio works when the device is worn, and the sealing and the materials decide whether the product survives the environment it lives in. Treat the mechanical, thermal and radio requirements as design inputs from the start, because none of them can be fixed at the end.