When you peel back the cover of some RF designs, the design decisions used in PCB layout and routing sometimes defy logic. When you examine the internal layer arrangement in the PCB stackup for an RF design, it does not always have the typical structure you would see in a digital design. This is because thicker laminates might be used in the outer portion of a PCB layer stackup for an RF system.
This is not typically a choice you would see in a digital system, where thinner layers might be used to ensure traces are not too wide but still have the required impedance value. But sometimes, it’s appropriate to use thin layers in an RF design. We’ll detail when you should prefer thin layers or thick layers in your RF PCB.
PTFE Materials in RF PCB Stackups
RF stackups use a typical laminated structure with alternating core and adhesive layers. In the RF world, we call the prepreg a “bondply” layer, and it bonds the copper coated PTFE cores. Either core PTFE layers or bondply layers can be used for signal routing, depending on the thickness needed and the required dielectric constant. So it should be clear that the layer thickness is an important part of designing a microwave PCB.
Why Thick PTFE?
When thicker layers are used in a PCB, any traces that are routed on that layer and that require impedance control will need to be wider. This means the use of thicker PTFE in an RF PCB forces traces to be wider. This is important in the case of RF systems that have an impedance requirement, which is typically 50 Ohms.
When traces on a PCB layer are forced to be wider, the DC losses and copper losses will be much lower than in narrower traces. This means the skin effect, which creates additional AC impedance at high frequencies, will be reduced. The downside is that these circuits will take up a lot of space on the board, and the trace width can be so large that it is difficult to route into most component pins.
Printed power dividers can typically be routed with very wide traces to ensure very low insertion loss in the printed circuit.
Because wider traces allow very large reductions in copper losses compared to narrow traces, it is much easier to work with printed circuits in an RF PCB when the dielectric supporting a signal layer is much thicker. Structures like couplers, power dividers, printed filters, attenuators, and circulators operating in the GHz range can have very low losses.
Why Thin PTFE?
Thin PTFE layers are not normally used on their own isolated from other layers. In other words, they have to sit on top of some other layers, which might be thicker PTFE core layers, a bondply layer, or an FR4 prepreg/core stack in a hybridized build.
Thin PTFE is used in systems where RF interfaces are present on very fine pitch components, such as advanced MMICs, ASICs, and micro-connectors. Thinner PTFE layers require thinner traces to have the required impedance control, even when a coplanar ground pour is used. These types of components and systems can also have digital interfaces to control the ICs and retrieve data, so they will also need to be routed on thin layers over a ground plane.
Thin Traces on Thicker Layers
In some cases, you can get the benefit of having thin traces, but with thicker layers. This would require coplanar routing (typically on a surface layer) with ground closely spaced to the RF line. The use of a coplanar line allows usage of thicker layers for an RF system, even a 2-layer board, but without needing to use very wide lines. By bringing the coplanar ground close to the signal lines, sometimes as close as 5 mil spacing, the linewidth can be greatly reduced.
This is the type of PCB layer stack that would be used in a case where a design only needs two layers. Some instances where this is helpful are:
- Reducing parasitic capacitance in impedance matching circuits
- Routing into active components with small lead sizes
- Routing into connector bodies with large solder pads
- Simple placement of coplanar defective ground structures
In summary, there are certain instances where thick PTFE might be preferred instead of thin PTFE:
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