Odd connectivity/signal integrity issues with FMC VITA FPGA connector - Page 1
Forum section: electronics / general

author: tom66
author_rank: Super Contributor
timestamp: June 19, 2019, 06:23:16 pm
content: Not sure if this is the right forum as it's more generally about signal integrity issues on connectors - but this connector is also rather specific to the world of FPGAs.I'm using a VITA FMC 57.1 "Mezzanine" connector set up manufactured by Samtec. On one end I have a ASP-134603-01 which is interfacing with a Hittite ADC board using the mating connector (ASP-134606-01).I am having a variety of issues with signal integrity between the two. Specifically the connector will work one day but then not another. I am able to clearly inspect the connector and no surface contamination is visible. Using contact cleaner (just basic WD40 brand stuff) seems to resolve the issue for a couple of hours - sometimes a few days - but inevitably the issue comes back and starts introducing various electrical gremlins.To me it seems likely to be some kind of surface plating. The Hittite connector will be several years older than the fresh connector on my board and perhaps an oxide coating has built up on the connector. Despite all my attempts to clean it the problem returns after a short amount of time.I am certain that it is not a soldering issue because I can probe the connector directly and there is no open circuit issue present. I am also able to directly inspect the respective ball and it is appropriately soldered. The issue is only seen when looking at the signals as they reach the ADC (I've scratched off the soldermask on some traces so I can probe them directly.) Also this is not an issue with the high speed interface - only the low speed SPI interface - although that may be down to luck alone. The connector pins responsible are in a similar area and are affecting the SPI data and powerdown signals.Any input appreciated here - it's been bugging me for quite some time.

author: jmelson
author_rank: Super Contributor
timestamp: June 19, 2019, 07:10:49 pm
content: Quote from: tom66 on June 19, 2019, 06:23:16 pmWD40??!!?? I assume this is a surface-mount type connector. It is very easy for flux to creep along the contact and wick up in the tiny gap between the plastic and metal parts, and then get on the contact area. Also, when reflowed, the flux generates fumes that then condense back ALL OVER the board.You should see what the inside of my oven looks like after a few months. This contamination of the contacts is NOT visible, but it is there. Generally, cleaning with pure alcohol and Q-tips may remove it. Sometimes, ultrasonic cleaning may be required, especially if the contact surfaces are inaccessible or very delicate.Assuming again that the contacts are gold, then oxide should NOT be building up on them, but other things could be contaminating them.Jon

author: tom66
author_rank: Super Contributor
timestamp: June 19, 2019, 07:34:51 pm
content: WD40 contact cleaner - not regular WD40!Trust me, it was a last resort, but it seems like the only bloody stuff that has done a consistent enough job!Have tried ultrasonic cleaning with a bath of IPA and no difference made, also have tried directly brushing on IPA and lightly scrubbing. Aware of ESD issues, so keen to avoid too much brushing of the connector (need to get a good set of conductive brushes.) Light scrubbing with IPA or flux cleaner makes an improvement but fault comes back soon after. Seems to be related to number of cycles too - removing board and reconnecting can make fault return again.

author: iMo
author_rank: Super Contributor
timestamp: June 19, 2019, 08:52:29 pm
content: QuoteClock domain crossing??

author: tom66
author_rank: Super Contributor
timestamp: June 19, 2019, 08:57:55 pm
content: Issue is in real hardware not on the FPGA itself. Signal is not getting past connector - sometimes. The ADC interface is controlled by a separate STM32F4 in any case.My current setup is a bunch of dev kits stuck together with an interface board I've built myself.