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| Cheap SMD LCR Fixture, the Good, Bad and Ugly |
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| 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:
--- Quote from: KungFuJosh on December 22, 2023, 05:29:57 pm ---It seems like those screws are always loose. I tightened them by unscrewing the parts so they could be turned without damaging the wires. --- End quote --- 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 --- Quote from: mawyatt on September 27, 2023, 06:55:14 pm ---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, --- End quote --- |
| mawyatt:
--- Quote from: gitm on February 27, 2024, 10:47:51 pm ---@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 --- Quote from: mawyatt on September 27, 2023, 06:55:14 pm ---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, --- End quote --- --- End quote --- 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, |
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