6-Layer High Frequency PCB: Stack and Materials

Why Six Layers Is the Common Answer

A high frequency design that also has digital and power circuitry tends to end up at six layers. Two layers cannot give the radio frequency section a ground plane and the digital section a separate return; four layers usually forces the radio frequency and the digital routing onto the same layer; six layers allows two signal layers for the radio frequency section with planes on either side, and two more for the digital and power distribution. It is the smallest stack that lets the two domains be separated properly.

The alternative is a hybrid material stack rather than a higher layer count, and in practice most designs use both: six layers, with a low loss laminate on the radio frequency layers and ordinary laminate on the rest.

The Stack

A typical arrangement, reading from the top, is a signal layer, a ground plane, a radio frequency signal layer, a power plane, a digital signal layer and a ground plane, with the layers arranged symmetrically about the centre. Three properties of that arrangement matter.

The radio frequency signal layers are adjacent to a solid plane. That adjacency is what gives the transmission lines their controlled impedance and their low loss return path. A radio frequency layer next to another signal layer, with the plane two layers away, has an uncontrolled impedance and a poor return.

The digital and radio frequency sections are separated by planes. The power and ground planes between them act as a shield. That separation is worth more than any amount of filtering, because it stops the coupling before it happens.

The stack is symmetrical. A hybrid stack with an expensive material on two layers and ordinary laminate on the others is inherently unbalanced in its mechanical properties, and the symmetry of the dielectric thicknesses is what keeps the board flat. Where the material thicknesses cannot be matched, the fabricator balances the stack by adjusting the prepreg.

Material Selection

The laminate is chosen from the loss budget on the longest and fastest line, not from the frequency of the design in general. A design at ten gigahertz with a two millimetre transmission line has a different requirement from one at ten gigahertz with a ten millimetre line, because the loss is proportional to the length as well as to the loss tangent.

The options, in increasing cost, are a high glass transition FR-4 with a controlled dielectric constant, a low loss modified FR-4, a mid loss laminate, and a PTFE or ceramic filled material. The first three are often adequate for a module with short radio frequency traces and a well designed ground plane; the last is used where the line is long, the bandwidth is wide or the insertion loss is part of the specification.

Two numbers matter as much as the loss. The dielectric constant has to be stable with temperature and with frequency, because the filter and the matching network are designed against it. And the batch to batch tolerance of the dielectric constant has to be tight, because a variation of a few percent moves the impedance and the resonant frequency of a printed filter out of specification. Our notes on PCB manufacturing describe how the material and the stack are controlled.

The Hybrid Stack in Practice

A hybrid stack puts the low loss material on the layers that carry the radio frequency signals and ordinary laminate on the digital and power layers, bonded in one lamination cycle. It costs less than building the whole board from the speciality material, and the electrical benefit is almost the same because the lossy layers carry signals that can tolerate the loss.

Three practical problems come with it. The two materials expand differently, so the stack has to be symmetrical enough to remain flat through lamination and reflow. The bond between them is a place where poor process control shows up as delamination after assembly. And the transition between a radio frequency trace on the speciality layer and the rest of the circuit has to be designed as a controlled transition rather than as an ordinary via. A fabricator that runs hybrids regularly will have the lamination recipe and the compensation already developed; one that does not will be learning on the job. Our notes on PCB design and layout cover the transition design.

Via Transitions

On a six layer board every radio frequency signal that changes layer passes through a via, and the via is a discontinuity: it has a capacitance to the planes it passes, an inductance along its barrel and, if it is a through via, a stub below the layer where the signal leaves it.

Four techniques control the discontinuity. Keep the number of transitions to the minimum by planning the routing. Place ground vias next to the signal via so that the return current has a local path and the loop area is small. Use the shortest possible via, which on a six layer board usually means a through via with a short stub rather than a blind via. And where the frequency is high enough that the stub matters, back drill it or use a blind via from the outer layer.

The ground vias matter as much as the signal via. A transition without an adjacent return via has an inductance that raises the impedance, and the mismatch shows up as a reflection. The usual practice is a ring or a pair of ground vias within a fraction of a millimetre of every radio frequency via. Our notes on telecommunications PCBA describe the equipment these boards serve.

Impedance and Verification

The impedance is controlled with a coupon built on the production panel, and the coupon should be built on the same material and with the same geometry as the traces it verifies. On a hybrid stack that means the coupon is placed in the speciality material, not in the FR-4 section, because the impedance error appears where the speciality material is.

Where the design includes a printed filter or a matching network, the coupon should also include a structure that can be measured for its resonant frequency or its insertion loss. A filter that resonates at the wrong frequency is usually a dielectric constant or a thickness error, and measuring it on a coupon rather than on the assembled product separates the board problem from the assembly problem.

Cost and Schedule

Six layers is the point at which the fabrication cost starts to rise steeply with the layer count, and the hybrid material selection adds to it. A six layer board with two layers of low loss laminate typically costs several times a four layer board of the same area, and the premium grows with the line width requirement and the via complexity.

Lead time is longer than for ordinary FR-4 work because the speciality material has to be ordered and the hybrid lamination scheduled. Prototypes typically take one to two weeks and production several weeks. The schedules should be built with that assumption rather than discovered on the first order. Our notes on quality management describe the verification that should accompany the first build, and our notes on PCB assembly cover the assembly that follows.

FAQ

Do I need a speciality laminate for a six layer high frequency board? Only on the layers carrying the long or wideband radio frequency lines. A hybrid stack keeps the cost proportional to the benefit.

How many layers does an RF design need? Six is common because it allows two radio frequency signal layers with planes and two more for digital and power, which keeps the two domains separated.

Why do radio frequency vias need ground vias beside them? To give the return current a local path and keep the loop area small, which lowers the inductance of the transition and the reflection it causes.

What should the coupon verify? The impedance on the speciality material and, where the design has a printed filter or matching network, its resonant frequency or insertion loss.

Conclusion

A six layer high frequency board is the smallest stack that separates a radio frequency section from a digital one properly, and the design is usually a hybrid: low loss laminate on the radio frequency layers and ordinary laminate elsewhere. Give every radio frequency layer an adjacent solid plane, minimise the layer transitions and put ground vias beside the ones that remain, control the impedance with a coupon on the speciality material, and allow for the longer material lead time in the schedule.