Products > Test Equipment

Cheap SMD LCR Fixture, the Good, Bad and Ugly!

(1/9) > >>

mawyatt:
Thought this might be a useful recollection after reading:

https://www.eevblog.com/forum/testgear/best-cheap-lcr-meter-for-small-capacitors-and-inductors/25/

We have a couple cheap non-OEM SMD Fixtures for use with lab grade LCR meters, OK know it's not wise to strap on a cheap fixture to a quality lab type instrument, here's the story. We did have a quality OEM set of Tweezers (Tonghui TH26009B) that helped with some SMD component selection and verification for a major project. When the precision SMD components we required became unavailable we were forced (actually major redesigns 4 times to accommodate available components!!) to use what was available and do selection. These were thousands of components and after using the OEM Tweezers for awhile (tedious as heck!!) decided to get a proper SMD Fixture, however the OEM SMD fixtures were unavailable with deliveries "expected" in many months, so we took a change on a couple available cheap SMD Fixtures that "looked" OK.

The build quality isn't great in both fixtures, but sufficient. After some time playing around before committing to actual use (precision component selection) they seemed OK, but a little concerned about the case lid grounding. One lid was completely painted, the other partially painted but both relied on mounting screws for grounding, fixed this with a knife & drill bit scraping some paint off. Also noticed the internal BNC grounds were not connected on one that had the star ground washers, so we corrected this by soldering in grounding wires. Then repositioned the internal wires towards the middle of the enclosure to minimize parasitic shunt capacitance, this is between the internal shield between the L and H sides.

After we suffered thru the measurements long ago we began to notice more uncertainty in the readings, especially in one fixture, then it began to fail DC Short calibration. We found the threaded insert that is orthogonal to the plunger contact, where the Force and Sense wires come together between the BNCs and plunger was slightly loose. So was the other plunger insert. The plungers were removed and these inserts well tightened and everything reassembled with good results again!! Then we began to experience some increased uncertainty with the other SMD fixture and found the same result, loose threaded insert. Both Fixtures perform well now and serve our needs.

A couple tidbits on getting repeatable results with these SMD Fixtures (probably holds true for the OEM fixtures as well). When you do a Short and Open cal the fixture moveable plunger moves, this also moves the internal wires slightly. For best results we often use a similar size SMD Zero Ohm for the Short calibration, then use a spacer on the plunger arm to open the contacts to about the same space as the expected DUT. Because the wires move, the internal fields in the fixture change slightly influencing the results. Using a similar size Zero Ohm SMD for Short Cal and same contact spacing for Open Cal you are helping keep internals similar for the DUT measurements.

Of the two SMD Fixtures shown the one in front with the 4 BNC lever arms is the better build and mechanically more stable, however both achieve very repeatable and comparable results on either of our TH2830 or IM3536 Lab LCR meters. Recall both were under $100, not a OEM replacement, but if one doesn't mind tinkering around then not too bad a alternative.

BTW the Ugly one has a grey 3D printed SMD support, we needed to replace the original because it was useless for small devices which is what we normally use.

Anyway, hope this helps some folks considering a cheap SMD Fixture for LCR meters.

Best,

mawyatt:
Hers's what the insides look like. First is the better of the two cheap SMD Fixtures IMO. Note the baffle shield between L and H sides and how the Force and Sense wires go from the BNCs to the plunger insert, they are soldered here and this becomes the effective end of the Force/Sense capability. What this means is that the rest of the plunger from this point on becomes part of the DUT, and can only be compensated for by means of Open & Short calibration.

BTW it's a warm fuzzy when you can get a 0603 4.7pF COG to agree within 10fF between instruments & SMD fixtures (4.6912pF vs 4.6927pF), even when swapped (4.6905pf vs 4.6898pF). Repeatability between same instrument and fixture is even better ;D


Best,

mawyatt:
Upon viewing the second image above it became obvious that the internal wires could use a little trimming to shorten up and reduce the uncertainty regarding the internal E fields and reduce the series Z of the fixture. Honestly, don't know why we didn't do this long ago, but we didn't ???

Here's the internal view with shortened leads.

While playing round with this fixture decided to see how well it would work with a KS34465A DMM reading small capacitance, so gathered up some BNC adapters and cable and kludged (hooked) things up as shown in second image. Used the Math MX+B function and Long Averaging Function, and after allowing things to settle did a careful Null (the setup is floating so hand position can influence Null) to remove the residual capacitance (~68pF) with the fixture Open with a spacer in the Lever arm to position the plunger contact tips about the same spacing as the intended DUT, which in this case was the 0603 SMD 4.7pF reference cap mentioned earlier above. Believe it or not here's the result, 4.804pF with last digit bouncing round some but without any trickery :-+

Then we decided to kludge up the adapters for the DE-5000 as shown in the 3rd image, note we used the Guard to the Fixture case ground. With the KS34465A the fixture "floats" and is more susceptible to surrounding effects, with the DE-5000 Guard utilized this effect is reduced considerably. Also note the DE-5000 is using direct Banana to BNC, this defeats the 4 wire Internal Capability but we don't have the unique Split-Banana plugs, so did the best with what was on hand. This resulted in a stable reading of 4.69pF!!

Edit: Added 1, 10, 100pF COG SMD Caps measured with TH2830 & DE-5000 with SMD fixture as shown. 1.01927/1.00pF, 9.95836/9.95pF and 101.080/101.17pF. This was done without an additional Cal on the DE-5000 after a couple hours off after initial Open/Short Cal ~3 hrs earlier, the TH2830 was left On the entire time of ~3 hrs with only initial Open/Short Cal.

Anyway, just some fun stuff with these fixtures and other instruments. As always, YMMV!!

Best, and Happy Thanksgiving

mawyatt:
The kludge setup with the DE5000 above got us thinking. We acquired the DE-5000 long ago on the reliable recommendations provided (ones that had them and made comparative measurements), and planned on using this for some component selections (mostly small Cs), also got the TL-22 tweezers and then modified a TL-21 for proper Kelvin Clips. We found the Kelvin clips were not much better than the TL-21, also have a bunch of 6 1/2 digit DMMs with 4-Wire Kelvin capable and cables so no need for precise resistive measurements from the DE-5000 which isn't anywhere near the DMMs anyway. So the Modded TL-21 with attached Kelvin clips just never got used as intended.

After realizing we could use the SMD Fixture with the DE-5000 as shown above, and get good results (mainly for SMD capacitors), the thought of making an adapter with the TL-21 to 4 BNCs for use with the SMD Fixtures. Also, the core DE-5000 is a true 4 wire system, so we didn't want to defeat this as we did above with the BNC kludge adapters. So we cut the Kelvin clips off the modded TL-21 and reconfigured it with 4 BNC female connectors (could have used male but then would require the BNC barrels) we had on hand. The cables were kept short, maybe a little too short as you can see, and the workmanship isn't good (need to redo this sometime), but the resul is very good!!

We did a cal and pulled a Venkel 0805 COG 0.3pF reference cap we measured with the TH2830 at 0.31914pF, the DE-5000 shows 0.31pF!!

Best

joeqsmith:
I like the idea of a spring loaded fixture like these for rapid change out.  I built something similar but for characterizing parts in several MHz.    They work alright.  You can see the one in the first post.     

https://www.eevblog.com/forum/rf-microwave/shunt-capacitance-of-1206-smd-resistors-jeroen-belleman-december-2010/

Navigation

[0] Message Index

[#] Next page