The Internet of Things market is growing at a tremendous speed. Among them, wearables represent a sizeable portion. However, there are no standards governing the small size PCBs or Printed Circuit Boards for these wearables. The unique challenges emerging in these areas require newer board level development and manufacturing experiences. Of these, three areas demand specific attention – surface material of the boards, RF or microwave design and RF transmission lines.
Surface material of the boards
PCB materials are typically composed of laminates. These can be made of FR4, which is actually fiber-reinforced epoxy, of polyamide, Rogers’s materials of laminates, with pre-preg as the insulation between different layers.
It is usual for wearables to demand a high degree of reliability. Although FR4 is the most cost-effective material for fabricating PCBs, reliability is one issue the PCB designer must confront when going for a more expensive or advanced material.
For example, with applications requiring high-speed and high frequency operation, FR4 may not be the best answer. While FR4 has a Dk or dielectric constant of 4.5, the more advanced Rogers series materials can have a Dk of 3.55-3.66. The designer may opt for a stack of multilayer board with FR4 material making up the inner cores and Rogers material on the outer periphery.
You can think of the Dk of a laminate as the capacitance between a pair of conductors on the laminate, as against the same pair of conductors in a vacuum. Since there must be very little loss at high frequencies, the lower Dk of 3.66 for a Rogers’s material is more desirable for high frequency circuits, when compared to FR4, which has a Dk of 4.5.
Typical wearable devices have a layer count between four and eight. With eight layer PCBs, the layer structuring offers enough ground and power planes to sandwich the routing layers. That reduces the ripple effect in crosstalk to a minimum, while significantly lowering the EMI or electromagnetic interference. For RF subsystems, the solid ground plane is necessarily placed right next to the power distribution layer. This arrangement reduces crosstalk and system noise generation to a minimum.
Issues related to fabrication
Tighter impedance control is an important factor for wearable PCBs. This results in cleaner signal propagation. With today’s high frequency, high-speed circuitry, the older standard of +/-10% tolerance no longer holds good and signal-carrying traces are now built to tolerances of +/-7%, +/-5% or even lower. This influences the fabrication of wearable PCBs negatively, as only a limited number of fabrication shops can build such PCBs.
High-frequency material such as Rogers require to have a +/-2% of Dk tolerance and +/-1% is also a common figure. In contrast, for FR4 laminates it is customary to have Dk tolerances of +/-10%. Therefore, Rogers’s material presents far lower insertion losses when compared to FR4 laminates.
In most cases, low cost is an essential factor. Although Rogers’s material offers low-losses with high-frequency performance at reasonable costs, commercial applications commonly use hybrid PCBs with FR4 layers sandwiched between Rogers’s material. For RF/microwave circuits, designers tend to favor the Rogers’s material over FR4 laminates, because of their better high-frequency performance.
