Different type LCR SMD Fixture

[mawyatt]

We have and use the typical SMD fixtures for our lab grade LCR meters, and these work well for most SMD devices except the larger types. Typical large value SMD inductors, Tantalum SMD capacitors and large Polymer & Electrolytic SMD capacitors have been difficult to measure with these fixtures, and we've had to resort to handheld Kelvin Clips which isn't their intended use as they are for leaded devices not SMD devices.

Awhile back we developed a DIY SMD fixture for our Tek 577, shown here.

https://www.eevblog.com/forum/projects/smd-test-fixture-for-the-tektronix-576-curve-tracer/

This was based upon using the Lever Arm Clamp devices intended for clamping and holding down items. It worked so well we decided to try these Lever Arms for a custom DIY LCR Meter fixture and created a few custom PCBs to give this a try.

It works treat!! We tested a few SMD components that we couldn't test at all with the conventional LCR SMD Fixtures, and we did a couple devices we could test with the conventional LCR SMD Fixture for comparison. The comparison results were very good and we now have a fixture we can rely on for larger SMD devices!!

The first PCB uses both sides with different SMD DUT connection patterns, one for larger contact areas and one for smaller. We added the two Lever Arm assemblies, one on each side. You simply flip over the fixture to use the other side, and maybe do another Short/Open Calibration, altho we've found this isn't necessary unless one is interested in very low or very high impedances.

Also have another type PCB which employs a split SMD contact area which brings the Kelvin connections right up to the SMD DUT and even uses the SMD DUT to make the Kelvin Force and Sense connections, which we'll give a try later when time permits.

Anyway, hope some folks find this interesting and useful, certainly works for us!!

Best,

[moffy]

It is always pleasing to come up with a novel and simple solution.

[mawyatt]

More images.

[mawyatt]

Here's the Tek 577 SMD fixture that provided the inspiration for these LCR SMD fixtures. Note the PCB to the right, this has the Kelvin Type connection moved to the SMD actual pad contacts. Will be interesting to see how this works!!

Best,

[mawyatt]

Another view.

Best,

[Swake]

Really interesting topic and interesting application of the clamps.

What is the diode sign for? It seems reversed with the cathode on the +.

[mawyatt]

Really interesting topic and interesting application of the clamps.

What is the diode sign for? It seems reversed with the cathode on the +.

Just to remind of the polarity of applied DC bias when using such. So if one is measuring a diode for example, then with a + bias and the diode in the indicated direction it will be reversed biased, this would be the "usual" measurement for reverse bias diode capacitance.

We first used these clamps for out Tek 577 custom fixture, and they work so well decided to try with the LCR meter which also works really well!! Before we couldn't measure larger SMD devices with the standard SMD fixtures, now it's simple

Another possibility is to split the fixture contact surface area into a Kelvin like measurement where the DUT SMD makes the actual Force and Sense connection by means of the SMD contact surface. Here's a close up of the fixture contact area.

Making a lever arm tip from teflon or another plastic with smaller pressure area will reduce the tip capacitance, and something we've considered (maybe just glueing on a plastic tip to the arm tip). Of course gold aa a contact area would be better than the standard PCB plating, lots of options.

Best,

[mawyatt]

Mounted onto a cheap LCR fixture, works well.

Best,

[mawyatt]

Another view.

Best

[mawyatt]

Update, seems some interest in this PCB from over here.

https://www.eevblog.com/forum/testgear/cheap-smd-lcr-fixture-the-good-bad-and-ugly!/

Best,

[djerickson]

Nice work, thanks for sharing it!

My favorite impedance fixture is the HP 16092A Spring clip fixture, usable up to 3GHz! Very clever design, it handles down to 0402 parts, even leaded parts! It is not 4 terminal though. 

I used it up to 500MHz with an HP3495A analyzer. It is always impressive to see a little $0.02 part operating nicely at 500MHz. It does use 7mm and N connectors to the instrument, but a similar type fixture could be used at lower frequencies.

https://www.keysight.com/th/en/product/16092A/spring-clip-fixture.html

Dave

[cosmicray]

Do you mind if I riff a little off your concept ?

I want to make a whetstone bridge, with four DUT positions, so that I can qualify matched pairs of common SMD devices. The board under the clamps was a random 100x100 mm board that I was using to make sure it would all fit.

thanks !

[mawyatt]

Created some 3D printed standoffs and a plunger with narrow tip.

Considering ordering a set of PCBS with gold plating as well as the DC Bias Adapter and Tek Curve Tracer Adapters if folks are interested.

Best,

[mawyatt]

Had a few moments today and thought about a way to improve the contact area for the SMD on the fixture. Our plan was later sometime when we ordered some PCBs we would also include this fixture PCB but with gold contact area.

One thought that came to mind was to increase the roughness of the contact surface by means of adding some small conductive "particles" to the surface.

So we mixed up a small amount of solder paste with extra flux and added some copper wire filings by filing some #12 copper wire. This slurry was smeared across the PCB contact area and slightly heated so the flux would flow, then evenly distributed across the contact area as best we could. Using an air gun we slowly heated the PCB surface around the contact area until the solder melted and flowed leaving a nice rough texture!!

After a through cleaning and reassembly, seems this worked and get better SMD contact with less finagling than with the smoother surface. Getting good, repeatable low Z results under 100 milli-ohms with less effort now!!

BTW for those that don't know how these Lab Grade LCR meters work, they have both Force and Sense for the Hcur and Lcur inputs, thus 4 inputs like DMM Kelvin Inputs. The fixture we've created with this PCB doesn't connect the Force and Sense together like most fixtures do, but rely on the DUT SMD contact ends to make the connections which should improve results with very low Z components. During Open Calibration our two Lab Grade LCR meters will "bark" at you since these Force and Sense aren't connected. A couple solutions are available, one is to include a small shunt resistor between the Force and Sense (see PCB layout earlier or image below, it has these footprints as R1 and R2), the other is to use a very high value (impedance) SMD like a high R resistor or low C cap and do the Open Cal with this component (for low Z measurements this shouldn't have much effect). Lastly one can just use 2 SMD devices and place them back to back but not touching so the ends make the Force and Sense connections (this is what we usually do).

Anyway, just thought that others might this fixture and tips useful.

Best,

[RoGeorge]

Nice finding! 

Looking for similar properties (rough contact surface, combined with some plasticity/elasticity) maybe solder-wick, too, might make a good enough contact pad.

[tszaboo]

It's the first time I see this. Quite interesting and clever, good integration of different parts.
I've some contribution, it's about the hold-down force of these clamps. They can go crazy high, bending and breaking everything. I've started using clamps, that have a small screw to set the hold-down force in my JIGs.

[mawyatt]

The Lever Arm has a threaded rod with the Rubber Bumper (Black) on the end, see previous posts for details. This rod is fixed with two hex nuts and thus adjustable, allowing the DUT pressure to be adjusted.

We've been thinking of replacing the hex nuts with Wing Nuts to allow easier pressure adjustments, but haven't done so just yet.

We also placed a 3D printed cone shaped Bumper (Green) on the other end of the threaded rod, so can use either by flipping over.

Best,

[tszaboo]

The Lever Arm has a threaded rod with the Rubber Bumper (Black) on the end, see previous posts for details. This rod is fixed with two hex nuts and thus adjustable, allowing the DUT pressure to be adjusted.

We've been thinking of replacing the hex nuts with Wing Nuts to allow easier pressure adjustments, but haven't done so just yet.

We also placed a 3D printed cone shaped Bumper (Green) on the other end of the threaded rod, so can use either by flipping over.

Best,

That's not the adjustment kind I was talking about.
Take a look at this:
https://nl.rs-online.com/web/p/clamps/1253863

At the bottom of the lever, there is a small bolt, where the clamping force can individually adjusted from the clamping height.

[mawyatt]

Do you mind if I riff a little off your concept ?

I want to make a whetstone bridge, with four DUT positions, so that I can qualify matched pairs of common SMD devices. The board under the clamps was a random 100x100 mm board that I was using to make sure it would all fit.

thanks !

You want to have 4 SMD devices for the bridge, and each SMD selectable?

Interesting idea, are you going for a custom PCB for this?

We've found various SMD devices have more or less difficulty in making good contact with the pads. This even shows with the concept of splitting the Force and Sense connections to the DUT end terminations, where contact effects should be minimized.

This was the reason we created the rough conductive surface mentioned above, so you might want to consider such if you are going for custom PCBs for your Bridge.

Anyway, interesting idea, please show your setup and results when you can.

Best,

Best,

[mawyatt]

The Lever Arm has a threaded rod with the Rubber Bumper (Black) on the end, see previous posts for details. This rod is fixed with two hex nuts and thus adjustable, allowing the DUT pressure to be adjusted.

We've been thinking of replacing the hex nuts with Wing Nuts to allow easier pressure adjustments, but haven't done so just yet.

We also placed a 3D printed cone shaped Bumper (Green) on the other end of the threaded rod, so can use either by flipping over.

Best,

That's not the adjustment kind I was talking about.
Take a look at this:
https://nl.rs-online.com/web/p/clamps/1253863

At the bottom of the lever, there is a small bolt, where the clamping force can individually adjusted from the clamping height.

Yes, that's a much better plunger fixture than what we are using, and should be a better overall setup, but also more expensive!!

Post some images and results from your setup, curious to see what others are doing!!

Best,

[tszaboo]

The Lever Arm has a threaded rod with the Rubber Bumper (Black) on the end, see previous posts for details. This rod is fixed with two hex nuts and thus adjustable, allowing the DUT pressure to be adjusted.

We've been thinking of replacing the hex nuts with Wing Nuts to allow easier pressure adjustments, but haven't done so just yet.

We also placed a 3D printed cone shaped Bumper (Green) on the other end of the threaded rod, so can use either by flipping over.

Best,

That's not the adjustment kind I was talking about.
Take a look at this:
https://nl.rs-online.com/web/p/clamps/1253863

At the bottom of the lever, there is a small bolt, where the clamping force can individually adjusted from the clamping height.

Yes, that's a much better plunger fixture than what we are using, and should be a better overall setup, but also more expensive!!

Post some images and results from your setup, curious to see what others are doing!!

Best,

I'm using it for different purpose. It's a hold-down clamp for the PCBs for ICT testing, or fixture for the assembly process.
This clamp is quite big. And yes it's more expensive. I'm guessing there will be a supplier in the far East making similar parts.

[mawyatt]

While doing some LCR measurements for a member we encountered the typical uncertainty when using Kelvin Clips for small capacitance measurements.

https://www.eevblog.com/forum/testgear/lcr-cal-test-pcb/

https://www.eevblog.com/forum/testgear/sensitive-lcr-meter-measurements/

We then utilized Guard capability (Hioki IM3536) with a conducting plate (PCBite Base) for the effective "Work Bench" this improved the small capacitance measurements considerably.

This effort caused us to think about a means to utilize the Guard Function with other LCR Measurement "Tools" than just the Kelvin Clips.

Here's the result, with the Guard Function enabled on the SMD Fixture shown in previous posts here. The area underneath the upper surface "Contact Area" was void of any conductors in the previous design, now that area is filled with conductor surface which can be driven by the LCR Meter Guard Function via the Guard 2 pin connector. Also added another 6 pin connector for the wires from the enclosed case.

Will probably be sending this off soon for PCB fabrication.

Best,