Which components can be measured while soldered on a PCB board already?

Question

I am assembling nonlinear oscillators on PCB boards for applied math research [Colpitts (100 kHz, 3 V_pp), FitzHugh-Nagumo (100 kHz, 15 V_pp)]. I am working with some other people that want to do ODE simulations, so I am pushing to provide some qualitative results quickly. This means I sometimes solder up components before I rigorously measure them, because an approximate measure (~100 ohm vs 99.24 ohm) is enough for the data I need to hand off.

Can I measure these components (resistance, inductance, capacitance) once they are soldered in, or do the connections negate this possibility? Sorry if this may be a basic question, but my background is not in electronics. I understand that I can remove them, measure them, and then re-solder them, and this is probably the obvious approach--I would prefer to avoid doing this because small qualitative changes in the behaviors are important to me, and re-soldering would probably cause that. Should I just take the time/repeatability hit and always measure isolated components?

Also, do any of you have an intuition on what magnitude of stray capacitance occurs on a PCB board? I know that software layouts like PSpice avoid these problems, but I don't know how to use that functionality yet--if I'm being pessimistic, would I see 10 pF? 100 pF?

Edit: I include a picture of one of the soldered circuits so you can get an idea of spacing and the complexity of the circuit (not pictured--the leads for the V+/- power sources for the op-amp).

Edit 2: I include a schematic of the pictured circuit, so I can ask where I might consider that suggested capacitor.

Answer 1

Would have to see your circuit to figure out whether you can measure in-circuit at all without the connections disturbing the measurement. However, if you application requires, as-per your example, 4-digit accuracy on resistance, there's no way you can assume your connections aren't going to disturb the measurement. I'm assuming you're doing this power-off.

Trace capacitance is order 10pF. There are calculators online that will work it out for you. The details of course are dependent on trace length, width and your PWB stackup. For example https://technick.net/tools/impedance-calculator/

Answer 2

If have ony an ordinary meter then you cannot make accurate in-circuit measurements because the other connections will skew the result, but such measurements are possible with specialised equipment.

Most technicians will un-solder one end of a leaded component it the need to measure it, as this is usually the hardest way to mess up the measurement.

With specialised equipment Any 2 terminal component that is not parallel with another can be measured in-circuit by nulling out the other nodes and measuring it's impedance.

^{simulate this circuit – Schematic created using CircuitLab}

Here unknown Z1 can is compared against known R1 If both are resistors the comparison will be the ratio of the voltages

Za and Zb do not affect the reading because V1 is a voltage source, and current in those parts does not effect the voltage a proble A. Zc and Zd do not affect the reading probe B is a virtual ground and thus Zc and Zd have no potential difference and so pass no current. the grounds on Za Zb Zc Zd may be applied with aditional probes (usually clip probes).

Answer 3

Unless you have a ground plane or twisted pairs or close proximity, stray capacitance can be < 1pF. It all depends on length/gap ratios of or between conductors.

• Twisted pair is 0.5~1 pF/cm , single traces much less
• single trace Inductance can be 10 nH/cm,
• paired mutual inductance again depends on length/gap ratio.

For 100kHz range this is almost negligible.

For relaxation oscillators, R & C tolerance contributes to errors. For LC oscillators , L & C tolerance contributes to errors.

Just make it neat with correct parts in the correct locations. A final check on power supply resistance helps prevent the obvious with a DMM.

Compare your solder joints with the best and learn to do it fast in < 2 seconds with a clean tip and clean copper board.

I might solder cut resistor wire for probe test points for signal and ground on outputs.