15
\$\begingroup\$

When you get into the single digit pF capacitors, the capacitance approaches what could be achieved with only a trace (which wouldn't be changeable, but I don't care about that).

Quick calculations tell me that a 5mil trace a 0.6" long would give me about a pF for a clock signal. (or you could do just a pad)

Have you ever seen this done or tried this on FR4?

\$\endgroup\$
11
  • 5
    \$\begingroup\$ Sure you can do it. It's not necessarily a good idea- the trace will have a couple hundred nH inductance and the tolerance and tempco will be crummy on FR4. \$\endgroup\$ Commented Nov 19, 2020 at 21:21
  • 8
    \$\begingroup\$ It's done routinely when designing microwave, and even high RF circuits. We need the context. \$\endgroup\$
    – Neil_UK
    Commented Nov 19, 2020 at 21:42
  • 1
    \$\begingroup\$ @BruceAbbott Avoid a component and just for fun, I'm getting bored \$\endgroup\$
    – Voltage Spike
    Commented Nov 19, 2020 at 21:44
  • 3
    \$\begingroup\$ @VoltageSpike That's why we need to know the context. Many 2.4 GHz boards are done on FR4 for low cost, some are on RO4350 for precision and ability to process like FR4. What's the tolerance on the capacitor? What sub-circuit is it in - which means what other components and TX lines must it be integrated with - which means how are its parasitics absorbed? Yes, a short bit of track will obviously have a small capacitance to ground. How you want to use that is the missing bit of information. \$\endgroup\$
    – Neil_UK
    Commented Nov 19, 2020 at 21:51
  • 2
    \$\begingroup\$ Have you heard of gimmick capacitors? \$\endgroup\$
    – DKNguyen
    Commented Nov 19, 2020 at 22:20

2 Answers 2

18
\$\begingroup\$

Yes, people (including myself) do sometimes use PWB traces to create capacitance or resistance. Often the high tolerances create enough problems that its not worth the space/cost saved by not using a discrete component.

The particular capacitor you propose would likely have over 50% tolerance.

  1. The tolerance on a trace width might be +/- 2 mils. So (40%) for a 5 mil trace.
  2. The thickness tolerance of the PCB might 10%.
  3. There is tolerance in the dielectric constant in FR4.

A better geometry would be something a 50 mil x 50 mil square. The copper feature tolerance would have much less of an impact and so the tolerance would likely be much closer to 20% in that case (rather than 50%). Additionally a square would have much lower inductance.

There are PCB materials specifically sold for the purpose of creating "embedded capacitance" or "embedded resistance".

https://www.3m.com/3M/en_US/company-us/all-3m-products/~/All-3M-Products/Electronics/Data-Center/Electronics-Materials/Interconnect-Solutions/Embedded-Capacitance-Material/?N=5002385+8709318+8709343+8710652+8711017+8734573+8743710+3294857497&rt=r3

Embedded capacitance material typically has a higher dielectric constant than regular FR4, so you get more capacitance per square inch. One typical application is to put the embedded capacitance material between the power and ground planes, so that the plane capacitance increases. This can reduce the plane impedance enough that you can just put decoupling capacitors regionally rather than next to every IC. This can potentially save a lot of capacitors.

\$\endgroup\$
2
  • 8
    \$\begingroup\$ Why not a circle instead of the square? Best ratio of circumference (where the errors happen) to area and no sharp corners where errors are even more likely. \$\endgroup\$
    – Michael
    Commented Nov 20, 2020 at 7:09
  • \$\begingroup\$ I agree, a circle would also work well. \$\endgroup\$
    – user4574
    Commented Nov 20, 2020 at 21:43
1
\$\begingroup\$

Actually it is routinely being done, but only if no such component is available or they are not practical. The design and characterization requires time, which could be saved, or reduced if an off-the-shelf component would have been used. SMD capacitors with values smaller than 1pF are available.

You can always create a small capacitance value with series connection of multiple capacitors if the series resistance and inductance does not matter. That could be also a way to go.

In RF design, matching networks are used to avoid reflections and get out the maximum power from a given device. The simplest case, where I've used a trace capacitance was when I needed to match a bondwire chip connection to a transmission line of the PCB.

In a PCB --- or in any lithographic process --- the trace width is not well controlled. The thinner the copper and the higher the resolution of the technology is, the better. The substrate thickness is fairly well controlled, especially in substrates intended for RF applications. I have no information about FR4, but you can ask such data from your manufacturer. Therefore I would use the capacitance between metal layers, rather than the capacitance between traces of the same metal layer. Especially if you have a ground plate there, its easy to do. If the capacitance value is critical you might run some EM simulation, or parasitic extraction tool, but if you have no licence for such tools, a test PCB run and measurement might do the trick. Since the trace width inaccuracy will alter the plate area of your capacitor a circular cap would fit the best.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.