RF PCB Vias and thermal relief

pcb layers & thermal relief

I need to know the constraints on GHz PCB, Vias and thermal relief

pcb via thermal relief

Your question is too general to answer in any detail.

Thermal reliefs are only needed for thru-hole components to aid soldering. In general, thru-hole components have several orders more inductance than a thermal relief will add to Ghz circuits - so the thermal relief is the least of your problems.

The effect of vias on the circuit depend on how big in diameter they are, how thick the board is, and where exactly they are in the circuit. You should be concerned with series inductance (impedance), parallel inductance (coupling), and possible self resonance. Depending on the circuit, you probably should model it in a 3D simulator for the best estimate of possible problems.

If you want a detailed answer, you'll have to be more specific in your question.

rf thermal relief

Ok, let us be more specific if I have a 3 layer PCB design, and I have to route a 2 GHz RF signal from top layer to the bottom layer "the middle layer is the ground layer".
What kind of Vias may I use, what are the precautions?

thermal relief via rf

First of all, you would generally not want to make a 3 layer board. To prevent cupping or warping, circuit boards are made in symetric pairs of layers. Your board would either be 4 layers, top- ground plane- ground plane- bottom - or you would make a two layer board with signal return traces or fills opposite the controlled traces.

Second, you asked about thermal vias. I already responded that thermal vias are ONLY for making soldering of thru-hole components easier. The ONLY kind of via you are going to be using is a plain plated hole with a copper annulus on the top and bottom of the board. The fab you choose will tell you how small a hole they can plate, and how much annulus they need to plate.

Assuming the 4 layer board is used - passing a 2Ghz signal thru a via is no big trick. You want to make sure that the "anti-via" on the ground planes is sufficiently big so as not to present appreciable capacitance to ground as the signal passes thru the planes. A typical value is plated hole size + 25mils. The exact size will depend on the diameter of your via plated hole, the risetime of the signal passing thru the via, the allowable losses, trace size, and the thickness of the board. The diameter of the plated hole will also determine the inductance of the via.

For a normal board of thickness less than 100mils, the via is going to have negligible effect on the signal compared with the capacitance and inductance of connected components and the signal traces themselves.

I suggest you get some books and do a little studying. A couple of good starters are:

High-Speed Digital Design A Handbook of Black Magic. Howard W. Johnson
Martin Graham. © 1993 by PTR Prentice-Hall, Inc. ISBN 0-13-395724-1

High Speed Signal Propagation: Advanced Black Magic. Autor: Johnson, Howard W. ISBN: 0-1308-4408-X