Multilayer PCBs for microwave and RF

Carlos Rodríguez Alemparte | Researcher


Not surprisingly, PCB design (see the photo above) is still the supporting technology for  electronic radiocommunication prototyping. Some PCBs are simple, using traditional materials such as FR4 with just 2 layers. Others are more complex, like some PCBs designed at Gradiant, based on  several metallization layers and different dielectrics,  FR4 alternating with special microwave substrates, more homogeneous and having smaller losses. Multilayer PCB technology closely resembles a sandwich construction in which “cores” alternate with “prepegs” .

While cores are solid and resemble a traditional bilayer PCB with copper on both sides, prepegs are originally gelatinous layers with not cured epoxy resin, being similar to cheese slices within a sandwich. By using heat during the pressing process, prepeg layers are used to glue the different cores in order to form the multilayer PCB. It is important to choose a prepreg with the appropriate dielectric constant and fluence to ensure good bonding and good filling for via holes, which are vertical channels for electrical connection between the different metallization levels.

The designer has to take into account restrictions due to manufacturing techniques, such as aspect ratio between via hole diameter and depth (> 1/8),  in order to avoid problems in the via hole metallization  process. The cores and prepegs pressing process generates 3 types of via holes:

Through: passing “through” all metallization levels, being visible at both PCB sides.

Blind: connecting an external face with several neighboring inner layers, but being not visible from the other side.

Buried: only connecting internal layers, being not visible from the outside.

When  thin dielectrics are used, holes metallization has important implications on the RF transmission lines calculation. Each remetallization adds thickness to copper layer, affecting TX line impedance value. During the  layout design, copper surface density must be kept almost constant in order to avoid very thick RF tracks and too thin copper layers over the ground planes. Maintaining a symmetrical sandwich structure also helps to reduce tensions which can produce undesirable PCB warping.

Correct width for RF tracks prevents RF power losses due to impedance mismatch between stages. Via holes grounding through the external ground planes avoids EMI problems (Electro Magnetic Interference) and unpredictable behavior. Spacing via holes <= lambda/40 is recommended, being lambda the wavelength of the maximum used frequency.

With a good design, we can get from PCB layout all the transmission lines in our circuit, the system antenna or a good heatsink.

Gradiant has been developing during the last months  a hybrid digital-analog prototype based on  multilayer PCBs technology, two-side  components and integrating RF and FPGAs within a single design. This project consolidates the new RF laboratory and the electronic prototyping team.