Meeting the challenges of wearable PCB design

By their very nature wearables are tiny. What’s more they’re packed full of functionality. This can easily become a major headache for a design engineer. Even if you’re working with pre-built modules, what you gain in design time you can often lose in board routing flexibility. On top of size comes shape: it’s incredibly rare that a wearables board is a rigid rectangle, much more common are thin multi-layer boards with curved outlines, tightly packed to fit within beautifully designed product casing.

Use your board space as efficiently as possible by reducing the size of the peripheral components wherever you can. By using clever design principles tracks can often be made to navigate the most efficient route, but be careful with intricate routing over multiple board layers. Check out these useful PCB design guidelines.

Above all, do not depend on the auto-router system for this type of work. PCB Design software is good but nothing beats getting an engineer’s eye on the problem.

Added to the complexity of designing small, intricate boards, some wearables require board designers to work with unusual materials including, fabric, plastic, flexi-rigid material and mesh. These materials require specialist knowledge and in some instances original research in order to find the best way of working with them. Many groups including the likes of Google and Toshiba are currently conducting research into truly wearable circuits, the technology behind which would allow fabric-based wearables to penetrate the mass-consumer market.

If your application calls for an unusual material board, but you have no experience with designing these types of boards, it is highly recommended to seek specialist advice. There are several research groups working within these areas, many of whom are active in publishing their latest findings and this research can be incredibly useful when planning a route forward.

Here are some useful links to current wearables research groups around the world:
http://www.holstcentre.com/
http://www.sussex.ac.uk/strc/
https://www.wearable-technologies.com/

It’s inherent in the product type: wearables usually rely on batteries or energy harvesting to power them through the day. This means that wearable designs come with strict power consumption budgets and engineers need to think twice before implementing even the smallest of power extravagances.

Plan for power. You can approach the problem with an energy budget, broken into tasks or power-consumption allowances allocated to each circuit block. By basing these allowances on each function within the system, you can often find more flexibility when it comes to specifying the power consumption of the components you need. Speak to your component supplier’s sales team and explain the problems you have. Often they will have recommendations for alternative parts you could use to help with the power-consumption problem.

Connectivity is at the core of most wearables products and so modules that provide connectivity through Bluetooth, wifi, and other protocols are essential parts of the wearables design. This means the engineer needs to find space on their board to include these modules or RF circuits.

Thankfully with the IoT just around the corner more and more off-the-shelf connectivity products are becoming available that are small enough to be suitable for wearable products. Tiny modules and RF components, designed specifically for wearable products are now being offered that cover almost all connectivity protocols.

The real knotty problem with wearable PCB design comes in the form of designing antennas for use so close to the human body. The human body is extremely lossy and this poses a big problem for design engineers who need to preserve signal strength when transmitting and receiving near to the body.

In order to preserve signal strength electromagnetic fields should be concentrated away from the body to the maximum possible extent. This will often require specialist design and the use of components such as low-noise-amplifiers (LNAs) that can help to mitigate the problem by amplifying signals near to the body.

As well as being exceptionally lossy the human body also provides a level of humidity that can wreak havoc with sensitive systems. In particular high impedance circuits (for example circuits involving tuning-fork crystals) are not tolerant of humidity. This poses a protection issue for the engineer: how to ensure your lovingly-crafted design doesn’t fail due to the environment it has to work in.

Ideally the product casing or packaging needs to be hermetically sealed in order to resist humidity and prevent it penetrating to the product at all. In circumstances where hermetically sealed casing is impossible to fully achieve, a conformal coating can be applied to the board. This coating acts in a similar way to varnish to stop moisture penetrating to delicate components. Prevention of leakage currents is especially important in wearable designs where the operating current can be down in the nA region.