Is your circuit board compromising your circuit? Is it time to consider a change from the standby FR4 board material? With rising operating frequencies and higher data rates, signal fidelity is more important than ever before.
FR4 has been a mainstay for printed circuit boards for decades now. I remember one of my first electronic kits. It was a power supply and it did not use FR4. The kit included a plastic box that had holes on 0.1” centers and the kit was built right onto the box wall. The kit worked pretty well, but the pass transistor was in a TO-3 package and there was no heat sink. During use, the pass transistor got hot and melted the plastic on which it was mounted. Obviously the plastic chosen for this kit was not an appropriate material for the application. In this case FR4 would have been a far superior choice because of its high temperature performance and glass fiber reinforcement. In order to assess the suitability of FR4 we should take a quick look at some key characteristics. Even though mechanical characteristics are very important, I am going to focus on electrical characteristics because this is where FR4 sometimes falls short and it is where the key benefits of newer board materials are found.
One of the primary functions of a printed circuit board (PCB) is to contain and route signals. While board geometry is the primary factor in signal containment, the board materials are the dominant factor in signal loss and signal fidelity. For example, from the Rogers RO4003 datasheet the Rogers dielectric has -0.05dB / inch of loss at 4GHz while FR4 has -0.26dB/ inch. This is a significant difference in electrical loss.
Transmission line impedance is determined by geometry, but also by the electrical properties of the transmission line. Line impedance is directly proportional to the capacitance of the conductor, and the capacitance of a PCB trace is dominated by the dielectric constant of the insulator. In FR4 the dielectric constant is both larger and more variable. This means that over process, frequency and temperature, the FR4 trace will have a higher SWR. High SWR means more loss and more undesired signal radiation as well as inaccurate measurements. For our LMH6554 differential amplifier we used high performance dielectric so that our S parameter measurements are more accurate over a wider range of frequencies.
Transmission lines are not the only area on a PCB where electrical consistency is critical. Filters also depend on consistent board properties. A filter requires good performance both across the pass band as well as the rejection band, meaning that even well beyond the signal bandwidth the board properties are important.
I worked as an RF Engineer from Baseband to 31 GHz. The relatively small variations in effective permittivity and PCB thickness make FR4 and FR4 like PCB materials useful up to at least several GHz. In general Lambda guide is still long enough so as to be very forgiving up though L-Band (2 GHz). I have even used FR4 for low cost designs at 10.6 GHz, where the delay and transmission line effects are much more critical. The key here was to provide the design with ample bandwidth, I.E.: antennae, mixers, amplifiers. Before going to Expensive substrates, I won't mention brand names; I've used most of them. Also there are versions of FR4 like materials that are slightly less lossy and better controlled.
Thanks for your perspective. I know a lot can be done with existing materials before it pays to move to higher performance materials. From our pesrspecitve every bit of extra bandwidth or extra output swing to overcome board loss comes at the expense of more power in the active circuits, and more power also complicates board deisng and ultimately costs money.
Again, thanks for your input.
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