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

[KungFuJosh]

Interesting! You don't usually see the 4 retainer clip version for that price.

[mawyatt]

That look good, just make sure the cover makes good contact with the main case. Also make sure the main case is making good contact with the copper surfaces.

Actually you don't need to go to this extreme, just making sure everything has a good electrical ground connection is sufficient, this effectively creates a somewhat like Faraday Shield around everything inside the case. You want all the parasitic capacitances to remain fixed, and a good mechanical structure keeps things from moving around.

You folks with these SMD fixtures should consider the DC Bias Adapter, kinda interesting to "see" how the DC Bias capacitance changes with ceramic caps, diodes and such!!

Good work and keep us up to date on your progress

Best,

[mawyatt]

Also should mention, when doing the "Open Cal", use something to space the fixture plungers the same distant apart as they will experience when engaged with the DUT. We just use something that keeps the Lever Arm at the proper spacing, a pencil works!!

I was just about to write this myself.  But a pencil??? 

Yeah, works well. It just sits behind the Lever arm between the arm and the low side brass plated (silver) plunger housing. Anything will work as a spacer, since it's not electrically involved.

Best,

[Kean]

Yeah, works well. It just sits behind the Lever arm between the arm and the low side brass plated (silver) plunger housing. Anything will work as a spacer, since it's not electrically involved.

OK, I was imagining the graphite being placed between the contacts... 
I don't know if your original description made it clear otherwise.

On the topic of your DC bias adapter, I've got some of your PCBs on the bench (5pcs added onto a JLC order) but I've not yet reviewed the BOM or ordered the parts to make one up.

[mawyatt]

Yeah, works well. It just sits behind the Lever arm between the arm and the low side brass plated (silver) plunger housing. Anything will work as a spacer, since it's not electrically involved.

OK, I was imagining the graphite being placed between the contacts... 
I don't know if your original description made it clear otherwise.

On the topic of your DC bias adapter, I've got some of your PCBs on the bench (5pcs added onto a JLC order) but I've not yet reviewed the BOM or ordered the parts to make one up.

The DC Bias PCBs were designed to fit inside this type 65x110x50 enclosure (sawed in half). Use the latest files for the PCB, they have the BNC terminals correctly spaced and added the fast charge capability (and a few other minor changes) mentioned earlier. We haven't used the latest PCB yet, waiting on a PCB order to piggyback on, which should be soon!!

Anyway, hopefully this DC Bias Adapter works as well for you as it has for us!!

Best,

https://www.aliexpress.us/item/3256801465819165.html?spm=a2g0o.order_list.order_list_main.638.27e31802vJOM1n&gatewayAdapt=glo2usa4itemAdapt

[hpw]

Good work and keep us up to date on your progress

So on second tweak as  while the connections wires are too thick, also shows the common 2mm diameter connection connector as one was loosely too.

Finally, last picture, RG178 inner silver wire as more flexible and in addition with torx screws for better alignment. Enlarged also inner holes to 4mm diameter as better to adjust.

May at last, shorten or use different connection cables 

BTW: The Hantek uses for cap measurements only 2 wires and each time switched on, the open calibration is a requirement.

Hp

[KungFuJosh]

It seems like those screws are always loose. I tightened them by unscrewing the parts so they could be turned without damaging the wires.

[mawyatt]

Yes, one of ours was also loose and causing erratic readings. These look different than what we have, ours are just long thin brass bolts without the insulator. These look better with the insulator!!

BTW this image illustrates why getting repeatable low Z measurements is difficult with these type fixtures. The effective Force and Sense point is the top of the brass screw insert where the two wires are soldered, this leaves the entire electrical path from the solder connections to the plunger tip not sensed.

We developed another inexpensive DIY type SMD LCR fixture for this reason where the Force and Sense electrical connection is made by the DUT SMD terminal, this has shown promise with repeatable low milli-ohm SMD DUTs readings.

Best,

[hpw]

It seems like those screws are always loose. I tightened them by unscrewing the parts so they could be turned without damaging the wires.

The loose part(s) are the cause of the not flexible and more harder cable (even after longer time as SMD size movements). In addition caused by the "Heat shrink tubing" as cable would not brake at the solder points.

The other question about the used Chinese Gold as may thin as it gets??

That's why I used the more flexible inner RG178 coax cable.

Hp

[gitm]

@mawyatt

I would be interested in purchasing a PCB from you, especially with the gold contacts, if you move forward with the design.  I'm in the US.  Please let me know if you make them available.

Thanks,
gitm

Anyway, just an update, hope some folks find this interesting as we've found getting good repeatable low Z measurements with SMD is not trivial.

Edit: These are DCR measurements. Also no disrespect for the standard SMD fixture, these aren't intended for low Z measurements. If one studies the design the Kelvin type connections are terminated on a long thin brass bolt and screws into the bottom of the brass plated plungers, so everything after the thin brass bolt head is not part of the Kelvin measurement. This includes the plungers, and the plunger to DUT contact surfaces, which can only be compensated for by the Short Cal. Whereas the Lever Arm structure with the split contact area, the DUT contact "ends" make the Kelvin termination, and thus should improve results for low Z components, which apparently works is some cases as we've demonstrated.

BTW, if some folks want to participate we might roll the next PCB with gold contacts, which should improve contact area performance.

Best,

                     

[mawyatt]

@mawyatt

I would be interested in purchasing a PCB from you, especially with the gold contacts, if you move forward with the design.  I'm in the US.  Please let me know if you make them available.

Thanks,
gitm

Anyway, just an update, hope some folks find this interesting as we've found getting good repeatable low Z measurements with SMD is not trivial.

Edit: These are DCR measurements. Also no disrespect for the standard SMD fixture, these aren't intended for low Z measurements. If one studies the design the Kelvin type connections are terminated on a long thin brass bolt and screws into the bottom of the brass plated plungers, so everything after the thin brass bolt head is not part of the Kelvin measurement. This includes the plungers, and the plunger to DUT contact surfaces, which can only be compensated for by the Short Cal. Whereas the Lever Arm structure with the split contact area, the DUT contact "ends" make the Kelvin termination, and thus should improve results for low Z components, which apparently works is some cases as we've demonstrated.

BTW, if some folks want to participate we might roll the next PCB with gold contacts, which should improve contact area performance.

Best,

                     

More details over here for those interested.

https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/msg5061028/#msg5061028

If we can get a few folks in US interested (shipping overseas is too expensive and better for those folks to just direct order PCBs), we'll consider ordering a small batch of the PCBs.

Best,

[TERRA Operative]

I wouldn't mind a PCB or two but I'm in Japan. Have you released design files or Gerbers etc?