Integration Guide

PCB LAYOUT RULES
TRACE CHARACTERISTIC DESIGN
MODULE INTEGRATION GUIDE PROPRIETARY 25
SEQUANS Communications
4.2.3 RF Traces
1. Avoid burying these traces as much as possible, because it increases RF
losses compared with routing on the top.
2. Keep as short as possible to help reduce RF losses.
3. Design the impedance of the trace keeping in mind that the footprint of the
RF components should be of similar width. This help avoid impedance
discontinuities.
4. Ensure the steps provided in Section Controlled Impedance Traces are
taken into account when making the trace width.
4.2.4 Controlled Impedance Traces
• Calculation of traces width and spacing:
Use simple RF design tools to calculate the copper trace thicknesses based
upon:
a) Thickness of the dielectric substrate that is used between the RF copper
trace and the ground plane
b) Spacing between the copper trace and the adjacent ground plane (on
the same layer)
c) Dielectric constant of the substrate material being used for manufac-
ture. It happens that the required trace width is impossible to manufac-
ture. It must be reconsidered until feasible. In this case, consider
implementation of one of the following:
- Thicker substrate
- Moving the ground plane reference to the next layer down by
removing the ground plane under the transmission line of interest
• General good practice guidelines
a) Careful placement is required to keep RF traces short and kink-free.
b) Do not route RF traces on intermediate layers.
c) Ground planes beneath RF traces should be continuous.
d) The ground fill around RF traces should have sufficient clearance to
maintain the desired impedance.
• RF matching component footprints
Depending on the substrate thickness and the size of the components pads
used can deviate the desired transmission impedance from the wanted
(nominally 50 Ohm).
For RF devices, if any copper pad in relation to RF signals is significantly
larger than the transmission line width, then the ground reference could be
moved to the next layer down.