Can microwave circuit PCB be made in multilayers?

I am designing a receiver covering in several bands, of which the highest frequency is about 4000MHz. I have to use some switches to seperate the bands. The switches,in detail, SPDTs, all have two control bits. I can't make all the controlling signal lines on the toplayer as the strips . I wonder if I use multilayer PCBs can solve this problems. Eg, the ROGERS 4350, 10 mil substrate, which is used for the RF transmissions, glued on the FR4, 30 mil substrate, which is used for the control signal lines and the power lines. Would this make or it is not practical in fact? Thanks.

Yes.. it's practical.
I designed a VCO on a Rogers substrate glued on a FR4 substrate.
But GND for Microwave lines will have Bottom layer of RF substrate so FR4 will not have TOP layer because of that.
Or you can use 3 layers FR4 and leave TOP layer empty of FR4 and use Bottom layer of Rogers..
There are some such compositions..

saulbit said:

I am designing a receiver covering in several bands, of which the highest frequency is about 4000MHz. I have to use some switches to seperate the bands. The switches,in detail, SPDTs, all have two control bits. I can't make all the controlling signal lines on the toplayer as the strips . I wonder if I use multilayer PCBs can solve this problems. Eg, the ROGERS 4350, 10 mil substrate, which is used for the RF transmissions, glued on the FR4, 30 mil substrate, which is used for the control signal lines and the power lines. Would this make or it is not practical in fact? Thanks.

Click to expand...

Yes, practical; customer designs used 4-layer PCB for tower-mounted WiMAX TX/RX 'head' with top layer RF microstrip and ground pore and lower layers for DC, control and of course, ground.

The board material used was Parks Nelco N4000, a suitable material for work to 2.5 GHz, but it is really not an RF substrate per se (it is meant for high-speed digital as that is the intended use).

To glue a Rogers substrate onto another lower-cost substrate would be innovative, and I do not know it is done commercially or has been experimented with even! It does look to have advantages .. is this for one-of-a-kind engineering eval, eventual volume production or an education project?


Jim

If you are talking about multilayers of microwave type boards bonded together--it is kind of a nightmare. I have tried a few recently, both with FR4 at lower frequency and with rogers material at a higher frequency, and they both have some issues! You have to really pay attention to prepreg layers and the way the vias are installed/plated.

Hi
I have seen some circuit that have complicated DC and control distribution.So designers consider some mother board that distribute DC and control signals and some RF/Microwave boards that are implemented on top of the mother board and they have some mechanical case that provide heat sink and ground for RF and DC signals.the DC/switching signals will come up using some pin headers and going through RF/Microwave board

Yes, many RF/microwave substrates can be used glued to FR4 (f.i Rogers 5880, Arlon diclad 880, etc). I designed many boards with such a layup.
Usually the double layer RF boards have a good reference ground plane at the bottom. Using a multilayer solution you need to move the ground plane internally to the PCB then you must assure a low impedance connection with the ground using many vias (or blind vias) especially in proximity of structers like microstrip filters that can be sensitive to the goodness of the ground plane. Also components that have to be connected to ground will requires two or three vias to compensate for the increased lenght with respect to a double-layer board.

The grounding continuity issue is the key to make a successful multilayer RF PCB. Since most device will be mounted at the top layer, this make the 2nd layer which is a middle layer act as reference ground for RF signal. For most RF device don't need a PCB cut-out this is not a big issue as long as you carefully add PTH vias to connect each layer ground. But for those need cut-out, the multilayer design is big nightmare. For example, in the RF power amplifier the RF power device commonly need to be mounted on top of the heatsink directly. That mean for device their reference ground is not the same as the signal flow on the top layer. Bu use plenty of via to connect these two ground reference is not always usable, e.g. you have very wide microstrip in your matching design and using PTH via which has to be away from the strip some distance will cause big problem. I have try the stair jump PCB design which make the PCB area around the cutout as a 2 layer PCB (it mean a larger cutout at all layers except the first layer in the stack up) and design a small copper coin soldered to the small 2 layer PCB area for device grounding and heat flow. This is better but not always available choice if you have two much constraint in layout area. Blind via is also a possible solution but it highly depend on your PCB vendor capability and I can't trust mine. Somebody else select to metal wrap the edge of the RF device cutout to connect all the layer ground at the cut out slot edge but you need to make sure this will not short the top layer signal trace and from the result I see this is not a reliable solution.
I'd like to know the guru here to provide their excellent solution to this big issue.

Dear guys,

Our factory can help you fabricate the PCB with Rogers substrates with good quality, and also, the technology for pressing FR-4 together with Rogers are in mature.
if you would like to get more, don't hesitate to make your comments .

Yes, it is practical, certainly you should treat ground plane seriously

Yes It is practical,

I have designed a mixed signal board (6 layers) at 10GHz using Rogers 4350B as top layer and other layers as FR4,

It is working fine , Care must be taken not to place any component on the bottom of the PCB

I would recommend a very high quality substrate material. I got into real trouble trying to do 6 Ghz in buried stripline on thin FR4 boards. The circuits worked almost as advertised, except for a much higher loss than expected.

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I would recommend a very high quality substrate material. I got into real trouble trying to do 6 Ghz in buried stripline on thin FR4 boards. The circuits worked almost as advertised, except for a much higher loss than expected.

I think RO4003 is reasonable in cost and performance.

I've seen multilayer PCB designs with RO4003 only and RO4003 mixed with FR4 multilayer boards.

In Australia Lintek can produce these two types. I think any good PCB manufacturer is capable of doing this

cheers,
per_lube

Yes. It is possible.
Go ahead.