Products > Test Equipment
Cheap SMD LCR Fixture, the Good, Bad and Ugly
mawyatt:
--- Quote from: TERRA Operative on September 01, 2023, 11:38:46 am ---I have one of these test fixtures too, the cheapest I could find on Aliexpress, and with some tender love, it works quite well on my HP 4276A LCZ meter.
I improved the grounding inside, cleaned out the overspray from inside with acetone to allow proper contact to the case for the BNC's and the divider panel, tightened everything up, and stuck it all back together with stainless steel screws just for a bit more pretty.
I am wondering though, is it worth using coax (shield connected at the BNC end) for the internal wiring?
Maybe using some RG316 or similar. Would this help improve things, or not worth the effort?
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Interesting thought, not sure if there would be much benefit for using coaxial cables, since the entire interior space is surrounded by a ground shielded case, thus any slight wire movement moves within a grounded enclosure and invokes a tiny shunt parasitic capacitance change. However, the coaxial cable would also move but the internal capacitance would likely change less than the open wire, so might be worth a try.
Best,
mawyatt:
--- Quote from: Kean on September 01, 2023, 11:16:28 am ---
--- Quote from: mawyatt on August 31, 2023, 03:53:23 pm ---As a side note, in the SMD test fixtures, when one does the "Open" calibration the calibration should be done with the fixture plunger spacing the same as the expected DUT. The reasoning is that the plunger fringe capacitance should be the same as when the DUT is evaluated, and the internal two wires on the plunger movement side (L-Cur and L-Pot) slightly move and the fringe capacitance to the fixture interior case (ground) slightly changes. Same goes for the calibration "Short", use a Zero Ohm Short of the same spacing as the expected DUT, here we usually use Zero Ohm SMD devices.
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I was recently measuring some single digit pF 0402 caps on my CKT5000 LCR meter with Tonghui SMD fixture, and I also realised I needed to space the fixture tips in a consistent manner for the open calibration.
I used a small offcut of 0.030" (0.75mm) thick styrene sheet and got the high precision measurements that I expected for those tiny caps that I subsequently lost... :-DD
I didn't use a 0R SMD part for the short calibration as I thought the imperfections in that part and the extra contact resistances would be more significant than getting the spacing right - but I didn't waste time on testing that theory. :-DMM
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What we use for the Open Calibration to get the spacing right is just an insert in the lever arm behind the plunger where the spring is between the plunger case and the lever arm. Just a Bourns plastic blue trimmer tool works well, slide back and forth to get the approximate desired DUT spacing. This keeps everything away from the plungers and they then produce the parasitic capacitance that gets normalized during the Open Cal. We also have achieved some good results with tiny SMD small valued capacitors, and get consistent repeatable results with quality C0G/NP0 capacitors using this technique.
Regarding the Short, Cal. Those Zero Ohm DUT imperfections, and the plunger contact variations is what we wanted to normalize during the Short Cal. Our feeling was (and certainly questionable) that these variations and SMD Zero Ohm imperfections were likely to mimic the actual small resistance DUT parasitic resistance under measurement than just doing a Short Cal with the plunger tips touching, of course as always YMMV.
Best,
mawyatt:
Just did a quick test using our IM3536 LCR meter, this meter can deliver higher DUT test currents and why we prefer it for low impedance measurements.
Using the cheap SMD Fixture measuring a 1206 1 ohm 1% resistor:
DMM6500 using Kelvin Clip leads 990.7738 mohms with 32 uohm SD
IM3536 with fixture direct plunger Short Cal 971.50 mohms
IM3536 with 1206 Zero Ohm Short Cal 981.85 mohms
Also a 1206 2.49 ohm 1% resistor:
DMM6500 2.489757 ohms
IM3536 direct Short Cal 2.46736 ohms
Im3536 using Zero Ohm 1206 Short Cal 2.48366 ohms
Think this shows that we shouldn't rely on the cheap SMD fixtures for low impedance measurements regardless of how the Short Cal is performed, altho in our case the use of a Zero Ohm Shunt seems to help a little.
If you study the fixture design and construction this should be no surprise as the "Kelvin" type connections don't actually make it to the plunger tips where ideally they should be, but are terminated at a long brass headed thin bolt that is screwed into the bottom of the plunger. So from the threaded bolt end to the plunger tip where the DUT makes contact is a single connection and has considerable impedance.
This is one reason we developed the Lever Arm based LCR meter fixture shown above which allows the "Kelvin" connections closer physically and electrically to the DUT, and the PCB test pad has a much larger surface area for the SMD DUT to get pressed against. Also you can create a higher contact pressure with the lever arm adjustment. We anticipate this will provide a better "fixture" for low impedance DUTs and physically larger SMD DUTs with improved contact impedance and area with better repeatability for low Z components. The PCBs went off for fabrication today, so we should have some answers soon.
Edit: As a note, we haven't been able to get good repeatable low impedance measurements with these cheap SMD fixtures and would not recommend considering them for low Z measurements. We've experienced considerable uncertainty with low Z measurements due to the above mentioned shortcomings, however they have provided excellent performance and repeatability with higher Z measurements.
Best,
ExaLab:
These test fixtures are not created with the aim of measuring very low impedance smd components.
In the Keysight one for example (whose cost exceeds a thousand dollars), the connection for the "sense" return path is made directly on the BNC connectors of the case, thus leaving uncompensated the part of the cable between the BNC and the mobile measuring cylinder. Evidently, their intention is to favor other aspects of the measurement tool...
If you wish to cancel the contact resistance, i.e. make it almost irrelevant by the pressure applied on the component, the only solution is to avoid the direct connection between the Force and Sense signals. If you haven't already done so, I would try to divide the PCB contact area with the component into two isolated sections, one dedicated to the force and the second to the sense. During measurement, the metallization of the DUT will connect them together...
mawyatt:
Have seen the mentioned Keysight fixture with the plate across the Force and Sense BNCs, never understood the thinking behind this.
Understand the issue of separating the Force and Sense contact points and trying to figure out if splitting the DUT contacts in half would help solve this and having the DUT make the Force and Sense connections, but decided against that for the PCB because of the issue how to do the Open Cal. Even considered using resistors between the Force and Sense that would allow an Open Cal and the DUT would short them out.
For now the cheap Fixtures have served well, but not applicable for low Z measurements, while hopefully the Lever Arm type fixture will perform better for low Z components.
Might be worthwhile to rethink this Force-Sense splitting for the next version, if we ever do another version.
Best,
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