4 wire measurement of a 0402 chip resistor

[PartialDischarge]

Unfortunately, this is still a 2 wire connection to the DUT. We do have such a tweezer, but readings are quite unstable and very dependent on applied pressure

This is why you should make a test board with the resistor soldered. I would use first the lowest temp possible and then apply heat with a gun and see how reversible or not the effect of heat is.

[coppice]

I don't think the OP has defined what they mean by a low resistance. What works well for hundreds of milliohms can work very poorly for the one milliohm or less resistors people widely use as things like shunt sensors. At a milliohm or less EVERYTHING is significant part of the impedance. I've never successfully measured an SMD resistor that small without is being soldered to a PCB that is tracked to make a 4 wire connection, following the guidelines in https://www.analog.com/en/analog-dialogue/articles/optimize-high-current-sensing-accuracy.html . We did our own research, and came to the same conclusions as those guys at Analog Devices, but they published their note just before we published ours, making our pointless. We had covered exactly the same ground, probably for similar reasons, and got exactly the same results.

[mawyatt]

I don't think the OP has defined what they mean by a low resistance. What works well for hundreds of milliohms can work very poorly for the one milliohm or less resistors people widely use as things like shunt sensors. At a milliohm or less EVERYTHING is significant part of the impedance. I've never successfully measured an SMD resistor that small without is being soldered to a PCB that is tracked to make a 4 wire connection, following the guidelines in https://www.analog.com/en/analog-dialogue/articles/optimize-high-current-sensing-accuracy.html . We did our own research, and came to the same conclusions as those guys at Analog Devices, but they published their note just before we published ours, making our pointless. We had covered exactly the same ground, probably for similar reasons, and got exactly the same results.

That's an excellent article, must read for anyone trying to achieve high accuracy low Z SMD measurements/uses.

Thanks for posting!!

BTW the crude SMD fixture mentioned earlier isn't too bad wrt to low value SMD resistor measurements. We have a few somewhat precision 2512 SMD resistors used to check things, of course these are 2512 and not 0402 the OP referenced. This is just used for sanity checks, and the fixture still needs additional effort as it's not totally foolproof!!

Anyway, here's a few measurements just for reference, and related to earlier measurement from over a month ago, mainly to show some repeatability and consistency between various instruments and repeat measurements.

All in milliohms:

Resistance  TH2830(10/31/23)   TH2830 (9/27/23)   IM3536 (9/28/23)    DMM6500 (9/28/23)

5                 4.999
10               10.001                   9.949                     10.04                      10.0262
15               14.919                                                14.97                      14.868
20               20.003                                                20.06                      20.016
40               39.965                   39.848                   39.90                      39.916
50               50.107                   49.993                   50.03                      50.065

Best,

[mendip_discovery]

I don't think the OP has defined what they mean by a low resistance.

The resistor is a 49.9 Ohm 0.1% TC10
In production we are seeing some PCB's where the resistors are outside these tolerances, so we need to figure out weather it is the initial resistance or the reflow process that are giving us problems

Low ohm but not low low ohm.

btw looking at the AD article, I found it interesting that the method used before was considered a good way to do it. I had a chat with an old Marconi chap once about shunts and he talked about thinking about the flow of electrons across the shunt and how you need the point where you take the voltage reading to be in a good flow part of the shunt otherwise you get errors. The way I thought about it was to imagine water flowing through a void in the shape of the shunt.

[coppice]

btw looking at the AD article, I found it interesting that the method used before was considered a good way to do it. I had a chat with an old Marconi chap once about shunts and he talked about thinking about the flow of electrons across the shunt and how you need the point where you take the voltage reading to be in a good flow part of the shunt otherwise you get errors. The way I thought about it was to imagine water flowing through a void in the shape of the shunt.

The big deal is to get every last scrap of high current carrying copper out of the path over which you sense, or the horrible temperature coefficient of copper wrecks accuracy as the temperature changes.

[mawyatt]

In the case of the SMD fixture we mentioned, the "assumption" was the conductive end caps of the SMD DUT were at equipotential on each end wrt the current flowing thru the SMD DUT.

https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/

Weren't sure just how valid this "assumption" was but the idea was to let the SMD DUT conductive end caps make the sense connection in the 4 wire Kelvin measurement process. Thus the DUT is making the Force/Sense connections on each end of the SDM DUT, which should help with the connection uncertainties involved.

So far measurements indicate this wasn't too bad an assumption for making a low Z SMD measurement without soldering the component. Of course one could argue all sorts of "reasons" why this isn't a good approach, and the real "proof" would be to solder the test DUT SMD onto a proper PCB with the optimum layout as shown in the ADI article, and then compare the mounted and soldered results with the fixture results. We may give this a try someday when time permits, but so far reasonably happy with the results we've been able to glean from this crude SMD test fixture.

Should mention also in application trying to keep the 2 sense points at the same temperature helps with the TEMF of Cu and the other conductive metals involved.

BTW find it interesting that the ADI article also chose the 2512 size, seems popular for low value resistors.

Anyway, as always YMMV.

Best,

[coppice]

BTW find it interesting that the ADI article also chose the 2512 size, seems popular for low value resistors.

People using really small shunt values are usually dealing with large currents, so this is not surprising. People generally cop out with lower currents, and use a bigger shunt resistance, even if careful design could have kept it lower.

[mawyatt]

BTW find it interesting that the ADI article also chose the 2512 size, seems popular for low value resistors.

People using really small shunt values are usually dealing with large currents, so this is not surprising. People generally cop out with lower currents, and use a bigger shunt resistance, even if careful design could have kept it lower.

Yes, and the I^2R resistor heating with the TC, along with Thermal EMF issues, not a simple Ohm's Law direct solution!!

Best,

[Aheld]

Hi,
Think about a simple Waferprober setup with chuck, microscope and 4 simple manipulators.
With applied needles for contact your can measure direct at the resistors contact areas in real kelvin mode.

If the chuck is not large, used equipment is quite cheap. Or if you already have some microscope, you simply need some manipulators or needle holders.

This really depends on the sample count you want to measure on regular base.
For first impression how it can looks like, watch homepage from HPPI in Germany. They‘re also offering a small, simple prober.

Br. Axel

[lugaw]

Just calibrate it after soldering.

[mendip_discovery]

Just calibrate it after soldering.

I think the OP is trying to work out an issue they are having.
Are the resistors incorrectly specified?
Is there a process issue such as too much heat during soldering?
Have they a bad batch?

Reducing time spent faffing about soldering and desoldering faulty parts could save a lot of time.

[gamalot]

For the measurement of 49.9 ohm 0.1% resistance, a LCR meter with an SMD fixture is enough.

The picture shows my LCR measuring a 1 ohm 1% resistance in 0402 package.