Challenge of PCB from Wearable electronics

Because of their small size and dimensions, few printed circuit board standards suit for the growing wearable electronics market. Until they emerge, we'll have to depend on what we have learned about board level development and manufacturing experience and carefully consider how they apply to the unique challenges emerging there. Three areas we should be paying particularly close attention to are: board surface materials, RF/microwave design, and RF transmission lines.
FR4 has a high dissipation factor (Df) compared to Rogers' material, especially at high frequencies. Df values for higher performance FR4 laminates are in the range of 0.002, an order of magnitude better than regular FR4. However, Rogers' laminates are 0.001 or less. A meaningful difference in insertion loss is thus created when FR4 material is subjected to high frequencies. Insertion loss is defined as a loss of signal power in transmission from point A to B resulting from using a laminate such as FR4, Rogers, or other materials.

PCB layers are composed of laminates, which can be made of FR4 (fiber reinforced epoxy), polyimide, or Rogers materials or laminates. Insulation between different layers is called pre-preg.

Wearables demand a high degree of reliability, which becomes an issue when the PCB designer is confronted with the choice of using FR4, which is the most cost-effective PCB fabrication material, or a more advanced, more expensive material.
If the wearable PCB application requires high-speed, high frequency materials, FR4 may not be the best answer. FR4 has a dielectric constant (Dk) of 4.5, whereas the more advanced Rogers 4003 Series materials have a Dk of 3.55, while its companion series Rogers 4350 has a Dk of 3.66.
The Dk of a laminate refers to the capacitance or energy between a pair of conductors in a vicinity of the laminate compared to that pair of conductors in a vacuum. At high frequencies, it's desirable to have a very small loss, thus a Dk of 3.66 in Rogers 4350 would be more desirable for higher frequency circuits versus FR4, which has a Dk of 4.5
Normally, layer count varies from four to eight layers for wearable devices. Layer structuring is such that if it's an eight layer PCB, it provides enough ground and power plane to sandwich the routing layers. Thus, the ripple effect in crosstalk is kept to a minimum and electromagnetic interference or EMI is significantly reduced.
At board layout stage, the layout schedule is such that the ground plane is solid next to the power distribution layer. This creates a low ripple effect and system noise is reduced to virtually zero. This is especially important for RF sub-systems.
The miniaturization of electronic products continues to drive printed circuit board manufacturing towards smaller and more densely packed boards with increased electronic capabilities. PCB designers and manufacturers are employing new technology to keep survive in the market. These and other challenges will keep the manufacture of printed circuit boards a dynamic field for many years.