Wearable PCB base material requirements

The PCB typically consists of a laminate that can be made of fiber reinforced epoxy (FR4), polyimide or Rogers material or other laminate. The insulating material between the different layers is called a prepreg.

Wearable devices require high reliability, so this is a problem when PCB designers are faced with the choice of using FR4 (the most cost effective PCB manufacturing material) or higher, more expensive materials.

If wearable PCB applications require high speed, high frequency materials, FR4 may not be the best choice. The dielectric constant (Dk) of FR4 is 4.5, the dielectric constant of the more advanced Rogers 4003 series material is 3.55, and the dielectric constant of the Brothers Rogers 4350 series material is 3.66.

The dielectric constant of a laminate refers to the ratio of the capacitance or energy between a pair of conductors in the vicinity of the stack to the capacitance or energy between a pair of conductors in a vacuum. At high frequencies, it is preferred to have a small loss. Therefore, Roger 4350 with a dielectric constant of 3.66 is more suitable for higher frequency applications than FR4 with a dielectric constant of 4.5.

Typically, wearable devices have 4 to 8 layers of PCB layers. The principle of the layer structure is that if it is an 8-layer PCB, it should provide enough ground and power layers and sandwich it. In this way, the ripple effect in crosstalk can be kept to a minimum and electromagnetic interference (EMI) can be significantly reduced.

In the board layout phase, the layout scheme typically involves placing large chunks close to the power distribution layer. This will produce a very low ripple effect and the system noise can be reduced to almost zero. This is especially important for the RF subsystem.

Compared to Rogers materials, FR4 has a higher dissipation factor (Df), especially at high frequencies. For higher performance FR4 stacks, the Df value is approximately 0.002, which is an order of magnitude better than conventional FR4. However, Rogers has only 0.001 or less in chips. When using FR4 materials in high frequency applications, the insertion loss can vary greatly. Insertion loss is defined as the power loss of a signal transmitted from point A to point B when using FR4, Rogers or other materials.