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Not all Wearables PCBs are Created Equal

Don’t be fooled into thinking that designing a wearable PCB is always the same. There are differences. As they become more popular, newer PCB design and manufacturing challenges will surface.

Wearables and today’s associated technologies are gaining greater traction. Everywhere you turn, there’s a new wearable device with applications ranging from health and fitness monitoring onto advanced pain relieving devices, and there are more on the way in all shapes and sizes.

When dealing with a wearable PCB design, a number of factors have to be taken into consideration. Among the areas that demand close attention are board surface materials, RF/microwave design, and RF transmission lines.

Fig 1 - Stack Up FR4 & Rogers

Fig 1 – Stack Up FR4 & Rogers

FR4 material has been around forever with excellent performance. Everybody knows about it and is well known in PCB design circles. But now, some wearable devices are calling for more advanced Rogers’ materials. So, it’s best to understand a wearable PCB design requirements before assuming FR4 is ok to use.

In some wearable PCB applications, a combination of FR4 and Rogers’ material can be used as shown in the stack up, Fig. 1.

Also, RF/microwave design, once solely in the high-end mil/aero domain, has migrated to the consumer wearable sector. Now, the PCB designer working on wearables is tasked with this more challenging design aspect, plus getting a better handle on the right PCB materials.

For more details on these critical wearable PCB design issues, read our recent EE Times PCB DesignLines article.

In the meantime, here are some tips and hints that’ll help to better understand these areas of concern.

  • Keep in mind wearables require a high degree of reliability. The issue is using conventional FR4 for cost purposes or more expensive Rogers’ material for higher reliability.
  • Rogers 4350 with dielectric constant (Dk) of 3.66, which is more desirable for high frequency circuits versus FR4 with a Dk of 4.5.
  • However, it’s important to know high performance FR4 has good reliability traits like higher Tg, relatively lower cost, and can be used in plenty of applications.
  • At PCB layout, the ground plane is solid next to power distribution layer, creating a low ripple effect so that system noise is reduced to virtually zero.
  • Wearables PCBs require tighter impedance control for a cleaner signal propagation.
  • RF requires closer considerations as far as routing, keeping signals separated, and keeping high-frequency generating traces away from ground.

Other considerations for successful RF designs include providing a bypass filter, plenty of decoupling capacitors, grounding, plus having transmission and return lines virtually equal to each other. The bypass filter suppresses noise content and the ripple effect of crosstalk. Decoupling capacitors need to be placed closer to a device’s pins that carry power signals.