BoardSurfers: PCB Design Technique for Designing a Small RF Section in a Digital Board

Are you designing a 5G or radar application that requires RF components? Are cost, size-reduction, and performance improvement major concerns for you? Most probably they are. Here we talk about an innovative solution for mixed-signal RF designs using Cadence layout editors.

One of the key requirements for an RF PCB design is a low-cost PCB optimization of the cost of the board. However, it is easier said than done. For example, a typical mobile handset PCB accommodates thousands of components; most of the components are digital and only a few are RF. At high-frequency RF signals, the normal FR4 (FR-4) dielectric material does not fulfill the requirement of high-speed signals – the dielectric material attenuates and, as a result, the RF circuit will not work. One way out of this situation is to change the dielectric material of the PCB, but that increases the cost manifold.

Mixed-signal designs of RF products also require major performance improvement and size reduction. These products tend to use up many passive components, and most of these products use surface mount discrete parts, which require a sizable percentage of the printed circuit board area, greatly limiting size miniaturization and performance.

The root cause of all the challenges is RF signals. Since designs using RF signal are extremely complicated to build and test, it is best to keep the designs simple. But how do you do that? Material zones for multi stackup for a single design is the ideal option because it provides a flexible yet less expensive time-to-market approach for next-generation RF design, such as 5G or radar application. Here we tell you with an example as to how to go about it.

The above images – 3D view of a 5G design and its layout, respectively – show the RF zone and the digital zone.

Multi Stackup for Mixed Design

You can define multiple cross-sections in Cadence layout editors. Create a new stackup with a high-frequency dielectric material for the layer(s) that will carry 5G or RF signals. You must keep the same thickness for the equivalent layers in the two stackups. Using different materials for the same layer in the two stackups reduces the cost and the board is easily manufacturable.

Material Zones for RF Design

In this example, we have chosen an eight-layer PCB stack-up and defined two separate zones, one for RF and one for digital.

The digital zone has a regular FR-4 eight-layer stackup whereas in the RF zone, the top two dielectric layers are of R5670 material, which is suitable for RF design and has been characterized up to 30GHz. Key requirements of low-loss, high-gain, and improved signal integrity dictated the choice of the dielectric constant for the RF zone.

Here is a graphical description of the size and material definitions of the two zones. The dielectric constant of FR-4 reduces as the frequency of operation increases in the design.

Zones are physical areas in the design that map to one of the available stackups in the cross-section editor. You add zones using the standard Add Shape or Add Rectangle pop-up options of the Setup – Zones menu. You can either map a stackup to a zone when creating the Zone or you can use Zone Manager to assign a stackup to a zone after it is created. You can also assign Constraint Regions and Rooms to zones at creation or through the Zone Manager.

We used the first two layers for RF routing in the RF ZONE, which does not allow any digital interface due to routing in this zone. Rest of the routing layers are used for digital routes.

Conclusion

This solution should excite you if your products are mixed-signal devices, which is most probably the case if you are toying with a new 5G-enabled design. Do let us know if you have any queries. We are eager to hear from you and know if we can be of help.