SMD Test Fixture modifications - Page 1
Forum section: electronics / testgear

author: mawyatt
author_rank: Super Contributor
timestamp: February 10, 2022, 06:51:44 pm
content: We all aresuckersattracted to cheap test equipment and fixtures. Recently we needed to measure a large quantity of SMD and acquired a TH2830 LCR meter and TH26009B Tweezers to do so. This setup combo worked well but took some time to complete.Later when we needed to again measure and characterize a large quantity of SMD, decided to get a proper SMD fixture. Rather than get the quality Tonghui SMD fixture we opted for a cheaper clone since if this didn't work well, we could always fall back and utilized the TH26009B tweezers and pay the time price. Since these needed to be very accurate we wanted to compare to the KS34465A for the resistors, and did so but this required using a set of Kelvin tweezers with the DMM.Didn't find any SMD fixtures like those for the LCR meters that would directly work with the DMM, so we thought about modifying the SMD fixture to work with a DMM. This would require adding SMD fixture connector, cable and a few Banana jacks. To verify that this would work we opened the SMD fixture to check the wiring a spacing for the connector.We found the SMD fixture had the cases painted after the machining and the fixture holders, rear cover and other parts were not grounded!! Luckily the main U frame was grounded thru the 4 BNC connectors back to the instrument, but nothing else wasWe ended up clearing all the mounting holes with a drill to remove the paint around the holes and also added 2 ground leads internally to attach to the rear lid mount.Honestly I'm surprised we were able to get repeatable capacitance readings with everything floating aroundAnyway, we've ordered another different type SMD fixture and will decide which to modify when it arrives.Best,Edit: Added image per request. Also the readings seem more stable, which would make sense with the fixture properly grounded.

author: andyB2022
author_rank: Regular Contributor
timestamp: February 10, 2022, 07:22:10 pm
content: This looks really nice! I like it a lot. Can you make more photos in the real "probing" area? I mean where the two needles touch smd's.

author: mawyatt
author_rank: Super Contributor
timestamp: February 12, 2022, 03:54:48 am
content: Here's what the inside looks like. Note the added two black ground wires on each end to the rear lid mounting brackets. Just made some measurements of a 0.5pF COG 0805 Venkel cap at 100KHz with a Tonghui TH2830 LCR meter. Initial reading was 0.51444pF and the cap was removed and later remeasured at 0.515pF (don't recall the finer digits tho).Also noted the fixture is now not sensitive to perimeter effects like my hand moving around and touching the fixture.Best,

author: bwcrypt
author_rank: Newbie
timestamp: January 02, 2023, 07:22:32 pm
content: I've been looking at this unit for a while. Thanks for the tear down photos.Do you have anything idea whether those are 50 or 75 BNC? I want to wire this up to my DE-5000

author: mawyatt
author_rank: Super Contributor
timestamp: January 02, 2023, 07:30:29 pm
content: Don't think it matters. For the DE-5000 we did a modded version of the TL-21 with 4 BNCs Female connectors to use with this SMD fixture and other special LCR Bench fixtures. A search should find this if you are interested. Works very well indeed with the DE-5000, highly accurate and repeatable resultsBest,

author: scopeman
author_rank: Frequent Contributor
timestamp: January 02, 2023, 10:32:00 pm
content: I have one of those and they are 50 Ohm BNC connectors. 75 Ohm have smaller pins and the insulator does not extend out past wherethe pin exits the insulator.SamW3OHM