As part of my studies, I am designing a measurement setup for measuring leakage current of SMD capacitors in the ranges up to pico Amps.
At the moment I am in research periond and later I will have access to some high end measurement devices such as a picoammeter, a SMU and a low leakage relay switch matrix.
What I have in mind at the moment is soldering 10 SMD capacitors on a cartridge and connect each capacitor to the SMU and picoammeter through the switch matrix
One of the concerns, as I approached here, Is thermocouple effect. If I know that my measurement performance should be in nano and pico Amps range, would this make a huge issue? I am asking this because when I go to PCB design phase, I would have to consider using connectors and PCB routes of same material (Copper).
For example, considering the connection between the test fixture and SMU, picoammeter and relay matrix should be done using coaxial or triaxial (the later can be the best option, but I do not have at the moment any idea of how economic will it be, considering the co to tri converters and theire own price).
So my question, in short, is that should I be worried about this effect having in mind reaching picoamp measurement?
Please let me know if I am not clear enough with the question.
Best Answer
The thermocouple effect should be no issue to you. It is very small as Jason already pointed out, but more importantly it causes a voltage offset. Your system is measuring current. A small voltage offset in series with the current source should be irrelevant.
As Jason also said, you do need to think about leakage current carefully. A guard trace around the high impedance node will be necessary. You may also have to use reed relays so that you can place the magnetic part yourself to insure extra low leakage from the magnetic drive circuit to the contacts. Check the datasheet of the relay you plan to use. It probably either doesn't say or doesn't promise leakage to the coil connections that low.