High reliability ribbon cables

[toybuilder]

Were your cable suppliers involved in the selection of the mating socket connectors? 
If both socket and plug were from the same manufacturer, you might want to contact the manufacturer's rep -- at 1726 (x2) units, there's enough volume for them to at least take your call.

[jmelson]

Ohh, I thought of another possible cause.  How were the headers soldered to the boards?  Were they wave soldered?  Or, were the boards cleaned after soldering?  Some wave solder processes get flux sprayed all over the place.  If flux got on the header pins, and/or the boards were not completely cleaned of all flux residue, then flux on the pins could cause delayed failure and intermittent contact.  Generally, wiggling the connector might "fix" it for a few days, but then it might fail again.
Jon

[Retirednerd2020]

Responding to other comments:

The power pins are maybe 30mA-50mA at worst.

The boards were cleaned as part of the specified processing.  The gold pins looked clean on visual inspection but may have a closer look.

I don't know how the manufacturer chose the cable to go with the connector.  I do know that they are reputable in general.

[Smokey]

It's hard to fix a problem you don't know the root cause of.  X-ray a fault cable maybe?

Make a revision for the cable assemblies with different everything (cables, connectors, mates, and assembly). There are about a million options for 0.1in ribon cable parts and assembly.  Instead of IDC, use individual wires and crimp pins?

[tooki]

Y’all…

Perhaps used poor qual connectors, cables or tooling

could it be good 3M connectors, but some dodgy supplier of the actual ribbon? is "3M" printed on the ribbon?

Were your cable suppliers involved in the selection of the mating socket connectors? 
If both socket and plug were from the same manufacturer, you might want to contact the manufacturer's rep -- at 1726 (x2) units, there's enough volume for them to at least take your call.

Am I the only person who bothered googling the part number?!?

It resolves to two variants of Digikey custom cable assembly, one with gray cable, one with rainbow cable:
Gray: https://www.digikey.ch/en/products/detail/3m/M3DDA-2606J/230229
Rainbow: https://www.digikey.ch/en/products/detail/3m/M3DDA-2606R/230228

On the product page, it has a BOM of the components used to make the assembly: two connectors, two strain reliefs, and a length of cable.

So we know exactly what connectors and cable they used, down to the part number. It’s not whatever random cable a procurement bean counter managed to scrounge up that week. 

[thermistor-guy]

...  Out of 1726 in the field for <2 years, 78 have had intermittent connections and were replaced.  The application is not challenging, clean, dry, low vibration, low stress and no movement...

I use 3M twisted pair ribbon cable with Harting IDC DIN41612 connectors (64-way), in  test jigs. The combination has proven reliable, but there is a fault that
can occur during initial cable assembly.

The cable can sometimes have a distorted conductor-to-conductor pitch, so that it doesn't line up exactly with the IDC connector tines.
If the misalignment is bad enough, it results in adjacent conductor short-circuits.

Conceivably, if the misalignment is not that bad, but still present, it could lead to unreliable conductor-to-tine contacts; contacts that are not
gas-tight and that will fail over time.

To guard against this, I check the cable alignment against the IDC before assembly, then check for continuity and no adjacent shorts after.
Sometimes I do have to massage (stretch or compress) the ribbon cable into good alignment, beforehand.

[mkiijam]

I think your "trusted supplier" of predone IDC cable use cheap alternative to real (expensive) crimping tool.

[tooki]

I think your "trusted supplier" of predone IDC cable use cheap alternative to real (expensive) crimping tool.

Very unlikely. DigiKey is not some el-cheapo fly-by-night operation. I would expect their agreements with 3M require them to use the official tooling, not to mention that the volumes they do pretty much preclude using cheap manual tools.

[thm_w]

Do you have the ones in hand that were replaced? Inspect them, there may be some strain that occurs that you didn't think about.

Could consider getting someone other than Digikey to build them. A company that will do resistance and hi-pot tests, etc. I don't know if digikey does this, I don't see anything specifically advertised on their site.

Very unlikely. DigiKey is not some el-cheapo fly-by-night operation. I would expect their agreements with 3M require them to use the official tooling, not to mention that the volumes they do pretty much preclude using cheap manual tools.

Most likely you are right, but I'd like to see some video evidence out of curiosity, I can't find anything on their site other than just spooled wires that they cut by hand.
https://www.youtube.com/@digikey/search?query=cable