Ribbon cable / IDC connector failure modes? - Page 1
Forum section: electronics / projects

author: Alex
author_rank: Regular Contributor
timestamp: February 21, 2019, 10:54:19 pm
content: I need to build a cable tester and I am wondering what the failure modes of say 50-way ribbon cable terminated with IDC connectors are.There are only so many ways a cable can fail; I am particularly intrigued by a failure mode where adjacent connections are shorted?I see some cable testers checking for shorts, for example by individually powering each conductor. Is this something you have experienced?Alex

author: wraper
author_rank: Unknown
timestamp: February 21, 2019, 11:07:30 pm
content: It's nearly impossible for them to be shorted. Ribbon cable and connector need to be misaligned so much that shorting would be the least significant problem with that cable assembly.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 21, 2019, 11:14:47 pm
content: I've encountered these failures over the years:1. Blade spreadingThe blade into the cable slowly spread out over time, leading to a disconnection. Using a pliers to bend the blades back in fixed the connection.2. Adhesive flowThe IDC connector had an adhesive on it. Over time it deteriorated, which flowed around the conductors. Once the blades spread, it made a nice insulator.3. Crimp misalignmentSome of the IDC blades were misaligned, so when the crimping was done they got bent over instead of being held in alignment to cut through the insulation properly. This made for only one blade making contact.4. Dangling external conductor strandWhen the cable was cut, a strand of conductor was stretched out. If it makes it to a couple millimeters, at some point it could be bent over and short to an adjacent conductor.5. Dangling internal conductor strandWhen the cable was crimped, a strand of conductor was pushed through the insulation by the terminal blade. Hopefully these are caught in the test.The problem is that these would have worked fine when first manufactured. The failures happened over time, but it's entirely possible that some failed within some reasonable warranty period, as opposed to decades later. A lot of IDC assemblies may not be electrically tested, only visually inspected.Checking for basic shorts and opens is a good start.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: February 22, 2019, 01:13:06 am
content: Quote from: Alex on February 21, 2019, 10:54:19 pmYes, I experienced adjacent shorts not so long ago, when fitting 64-way twisted pair ribbon cable with DIN41612 connectors (64-way IDC), for a test jig.The problem was the ribbon cable - the conductor spacing across the cable was a little uneven. So it would match the connector contacts at one end of the connector, but not at the other end.The solution was to carefully inspect the cable alignment all along the connector, before clamping the cable down into the connector, and stretch the cable (in cross-section) slightly where needed. Sometimes I needed to do this a few times, so the cable would sit just right.The cable was a major brand - Amphenol or 3M - from a recognized supplier - Digi-Key or Mouser. Don't have the exact details handy. DIN41612 connectors were Harting.Once I paid attention to conductor alignment, I had no more adjacent shorts. I tested each cable fully, and now regard adjacent short test as a routine part of the assembly process.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 22, 2019, 03:46:28 pm
content: A fault with ribbon cables I have occasionally seen is where the ribbon has been slashed by a sharp chassis edge (or similar), leading to an open in one or more conductors. Sometimes the open is intermittent and only occurs when the cable is flexed.Also sometimes where the outside edge conductors have had the insulation abraded and worn away, allowing the possibility of short to chassis or components. Obviously, no way to test that in isolation, though. But one would hope that such damage is easily visually apparent.Other than that, as others have said, failure of the IDC connector - loose, etc.

author: Alex
author_rank: Regular Contributor
timestamp: February 22, 2019, 04:38:14 pm
content: It is, in a way, impressive how many things need to be right to get long-term reliability out of IDC and ribbons. I will make sure to account for shorts and intermittent connections in my tester. Thanks very much all!Alex

author: powerchisper
author_rank: Contributor
timestamp: February 23, 2019, 08:16:25 am
content: Recently I had a machine with three ribbon cables, which looed ok.Then I saw the bare tips of the cables, here it was cut, and they were green fom copper oxide, which had penetrated to the pins.Instead of 0 ohms , you can get up to 12-15 ohms per strand.Ths machine I'm woking on shuts down the CPU if any of the data/adress lines are cut and the code can not be executed,I had to copy three new cables as the only solution.

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: February 23, 2019, 02:32:45 pm
content: Back when I was just starting out (1983 ish) the lab I was working for was using T&B Ansley ribbon connectors. The female (socket) version of these had the propensity to break. In particular the contact blades that are normally pressed against the inserted .1 inch header pin would loosen. This resulted in numerous intermittent failures in fielded equipment, What a pain in the ass that was to track down. We switched to samtec and the problems went away. That was nearly 40 years ago and perhaps Ansley fixed the design but I will never touch them again.In real long term use the ribbon cable insulation will go the way of all PVC/vinyl and crack as it loses its plasticizers.

author: poorchava
author_rank: Super Contributor
timestamp: February 26, 2019, 07:58:13 am
content: We had an issue with a 1.0mm ribbon pitch 50p connector. It was very fragile and production would very often break the strain relief when inserting the connector, sice it required quite a bit of force due to high number of pins.