Shields are your friend, except when... (Part 3)
In the past two columns we have looked at the possible side effects of adding a shield to our PCBs. First we showed how to calculate the lowest frequency resonance, backed up with actual data that confirmed that even for a fairly small shield that the cavity resonances were surprisingly low in frequency, compared to the frequencies that our circuits now must work with (see Shields are your friend, except when...).
Then we looked at a few possible solutions to these resonances by the addition of absorbing materials to the shield itself. The absorbers come in many types and thicknesses but they do one thing well and that is they can mitigate many of the resonance issues that the shields themselves introduce (see Shields are your friend, except when....(Part 2)).
So what else can go wrong, you might ask? Well there is another insidious issue lurking that is exacerbated with the growing trend of thin electronics, and that leads us into this part of the series...
If you have looked at any consumer electronics lately you will see an unmistakable trend. That trend is not only a smaller size area wise, but a thinner profile as well.
Just 10 years ago the typical RF module was probably at least 0.25 inch thick; now it is less than 0.1 inch thick. What sets this thickness? The shield probably. As shown in Figure 1 this low-profile shield sure is nice for the user of the RF module, but what about the circuit designer? What effect does this "thinning" trend have on our circuits?
Think about the impedance of our carefully designed traces on our PCBs. Does bringing the shield top cover closer to the surface of the PCB have any effect? Yes it does in an increasing way as it effectively adds capacitance to our PCB traces and circuits, lowering their impedance. Figure 2 shows what happens to a microstrip trace fabricated on a 62 mil substrate, as the top cover of a shield is brought closer and closer to the top of the PCB.
Figure 2: The effect of bringing a shield top cover close to a PCB Microstrip trace is to lower its impedance because it effectively adds capacitance to the trace. At 100 mils spacing from the top of the PCB the impedance of the trace is lowered to around 40 Ohms in this simulation.
As predicted the microstrip trace impedance gets lower and lower. In effect the microstrip structure is really becoming a stripline sort of structure because of the addition of the conductive shield close to the top of the microstrip trace, adding a second reference plane.
You could design your traces to work in the shielded environment by designing them to be stripline equivalent with the shields attached; then this would require the shield covers to be in place when testing the PCB and that would be annoying. But there is a better way.