Conductive glues used in RF component assemblies, any experience? - Page 1
Forum section: electronics / rf-microwave

author: joeqsmith
author_rank: Super Contributor
timestamp: April 06, 2025, 05:04:33 pm
content: I ran into a problem the other day and tracked it down to what appears to be some bad attenuators. The manufacture used something like this to connect the element to the case:https://mgchemicals.com/products/adhesives/electrically-conductive-adhesives/conductive-adhesive/The parts are spec'ed for +/- 0.5dB, DC-6GHz. Below 100MHz the parts fall way out of spec. A brand new one, in the factory sealed bag had the same problem. I cut one open, removed the element and cleaned it up. I then soldered the element to a coplanar waveguide and tested it. It performed correctly. I suspect what ever they used for glue is causing the problem.Have you ever ran into a problem like this?

author: Xena E
author_rank: Frequent Contributor
timestamp: April 06, 2025, 06:31:51 pm
content: Yeah I have*, the stuff seems to degrade over time which is a PITA from the point of view of angry customers... which is shit because its hella expensive to buy in the first place. Four years from application and its causing problems.* Circuitworks CW2400 silver epoxy by Chemtronics, used to seal screening cans.X

author: joeqsmith
author_rank: Super Contributor
timestamp: April 06, 2025, 07:12:47 pm
content: These parts have less attenuation at the lower frequencies, almost like the glue is not as conductive as it was when applied.Looking the part up on Digi-key, they are no longer offered. The data sheet they link from 2008 has a hand written note where they had changed from a thin to thick film element. These were made in China.https://www.digikey.com/en/products/detail/cinch-connectivity-solutions-johnson/141-3901-806/1922534

author: tszaboo
author_rank: Super Contributor
timestamp: April 06, 2025, 10:20:50 pm
content: We used epoxy like this in an assembly of an ultrasonic probe.Real PITA to handle. You have to keep it in the freezer until used, and costs like gold despite it only has silver in it. I honestly don't know how it handles long term, the entire project was a failure.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 06, 2025, 11:08:19 pm
content: I never thought about RF attenuators having a shelf life.

author: coppercone2
author_rank: Super Contributor
timestamp: April 07, 2025, 08:24:32 am
content: I fixed an attenuator that had impossible time constants for soldering (if you can get a laser beam through a 0.5 degree opening and slam the connector on the blob, it might be possible to solder) using conductive epoxy. The mg chemicals one is IMO a dodgy structural material, simply put it is weak compared to adhesive epoxy.I recommend curing a blob of the glue you are planning on using on a piece of paper and see how it holds up. IMO its a weak interconnect that is designed to get things out the door with minimum design time (i.e. figuring out how to make it solderable)I think the reliability is up there with conductive ink key pads.It seems to be one of those things that gets the people that save 0.005 cents on the dollar on low scale manufacturing happyI also used it to mount LED modules on a piece of sanded aluminum, in the hope that it could get rid of the negative wire and also act as thermal paste. The resistance even on a big lap joint on the bottom of the module was some where in the ohm's and I thought it was pitiful, so I ended up running a ground wire.I am not very interested in conductive adhesives anymore. The 'dry blob' test makes me think that it will break and that its a compromise on top of a compromise.It's interesting how conductive plastics creeped their way into capacitors. The way they get SMT capacitors to... normal reliability levels for car electronics and such is by using a 'flex joint' of conductive adhesive. I wonder how this will age..https://www.tti.com/content/ttiinc/en/manufacturers/kemet/products/fp_kemet_ftcap.html

author: joeqsmith
author_rank: Super Contributor
timestamp: April 07, 2025, 02:25:44 pm
content: Quote from: coppercone2 on April 07, 2025, 08:24:32 amTo be clear, I have no idea what material they used. I only showed the MGC as an example of what appears to be a similar material.Quote from: coppercone2 on April 07, 2025, 08:24:32 amI have no idea how old those parts are. Consider the drawing was from 2008. I doubt they started going bad in the first month or two. Maybe it took years. To try and test the glues in this application would require some major effort. They spec the flatness of the part. How many years does it take to fall out.I was surprise just how close the part that I had cut apart and the new part in the sealed bag behaved. I wouldn't be surprised if they were not purchased at the same time and came from the same lot.There is no repair for these. They are crimped and had to be cut apart. Outside of removing the element to test it with a known good connection, I had no plans to do anything further with them.I wonder if they were original made in the USA before being outsourced to China and if they ever used a different technique to mount the elements. It could have very well been an attempt at cost reduction. It could also explain why they are no longer sold.

author: ftg
author_rank: Frequent Contributor
timestamp: April 07, 2025, 02:45:43 pm
content: Quote from: joeqsmith on April 06, 2025, 07:12:47 pmDrat, now I get to measure all of these I have at the office.I have those exact affected 6dB attenuators and the 3dB versions.As for conductive glues, maybe they had a process problem in curing them?The conductive epoxies I have seen hams use to assemble mmW RF mixers all need to be baked.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 07, 2025, 03:38:08 pm
content: Quote from: ftg on April 07, 2025, 02:45:43 pmI am very interested in knowing what you find. These were from the office as well. Cost me some time collecting bad data and head scratching. Couldn't believe when I traced the problem back to these parts.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 07, 2025, 03:52:26 pm
content: This was an office problem. I took a LiteVNA64 into work and started testing components. It was good enough to track down the problem.Shown is S21 brown and black, using the LiteVNA to sweep to 100MHz. The back was the brand new in the bag part. In all my years, never seen something so crazy....

author: joeqsmith
author_rank: Super Contributor
timestamp: April 07, 2025, 04:00:33 pm
content: Just for completeness, showing 10MHz with the element mounted to the waveguide. I did run it up to 1GHz with the LiteVNA64. Its easily within the +/- 0.5dB specified. Fixture is not good enough to run it to the full 6GHz.

author: ftg
author_rank: Frequent Contributor
timestamp: April 07, 2025, 04:18:40 pm
content: Mine seem to be within the datasheet spec.Sample consists of 3x 141-3901-806 and 2x 141-3901-803. All less than four years old.I have attached some quick measurements from the Siglent SSA3032X.Narda_4772-3.png is an ancient Narda 4772-3 3dB DC-6GHz attenuator I also measured for comparison.I might check some of them on the LiteVNA on LF-100MHz to make sure.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 07, 2025, 04:59:17 pm
content: What is causing that huge disturbance at your lower end? The problem I am seeing is all sub 100MHz, not several GHz as you show. My bad parts start to settle down once you get above 100MHz or so. Its like the glue's capacitance is enough to improve the coupling to the case.Maybe just repeat that 6dB1.png plot. Calibrate from your lower end to say 20MHz. The tail you show sure looks like what I am seeing but you're too zoomed out to tell.***Attached showing one of the bad attenuators measured on PNA from 300k - 4GHz. It looks very similar to your plot.

author: tszaboo
author_rank: Super Contributor
timestamp: April 07, 2025, 09:29:52 pm
content: Quote from: ftg on April 07, 2025, 02:45:43 pmYes, the baking. I wonder if the problem is something simple, like they fully assembled it before baking, and partially assembling it in earlier batches. Trapping some additive in those epoxies, that were doing something bad.

author: coppercone2
author_rank: Super Contributor
timestamp: April 08, 2025, 03:00:12 am
content: I think basically the epoxy if it needs heat or UV activation its a molecule that is not particularly active while cold which breaks down into something reactive from UV light or heat. Like it gets cleaved by heat or ultraviolet light.I would imagine in most cases this is a detriment to the glue, it just is a shelf life or work life mod, I suspect some of these chemicals would be 'good' for 10 hours after being synthesized in the chemical reactor, unless they are modified for inactivity by additions that are removed by heat or light.I wonder if the remenants (parts that were just used to inhbit the reaction and separate after heat or light) could make it off-gas or something like that during cure, where the ideal mixture might not (and not be commercially viable either)a epoxy used in this would have a offgas spec or something. easy to ignore that one. factory shit.. sometimes it get tiring to think about what creative scam they are trying to run in a factory lolyou actually test epoxies for off-gassing when you make some parts as part of QCmaybe they hired some people with experience doing artsy epoxy floors that week that had their own ideas, or those fill in 'epoxy river between two drift wood parts' tables. i got this bro

author: ftg
author_rank: Frequent Contributor
timestamp: April 08, 2025, 08:18:50 am
content: Did some more measurements of the 6dB attenuators with LiteVNA and the SSA3032X.With averaging off the LiteVNA results looked similar. With averaging on they got weird.Please note that I put the attenuators in their bag after yesterday.So attenuators 1, 2 and 3 don't have the same labels as last time.On the SSA3032X I think part of the low frequency weirdness is the DC blocking caps and what I suspect is the analyzer digitizing the first few MHz directly.Yesterday I used a Johanson made RG316 jumper as the connecting cable, today I used the same ss405 cable that came with the LiteVNA for measurements on both the LiteVNA and the SSA3032X.I have attached screen grabs from NanoVNA saver for the 6dB attenuators and also enclosed the corresponding S-parameter files.In case someone wants to take a look at those.The more narrow band measurements of the 1st 6dB attenuator were done with 0/3 averaging.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 08, 2025, 11:38:01 am
content: Comparing your two bad parts with mine, we see the same behavior but not to the same extent. Because there is no way to know when these parts were made, it's a pure guess but I suggest my parts are older than yours. Maybe rather than tossing them, set those parts aside and measure them again in 5 & 10 years.Thank you for providing that data. Now with four parts ran independently at two completely different locations, it's pretty safe to say they had a major problem. Hopefully you didn't waste as much time as I did because of them.

author: ftg
author_rank: Frequent Contributor
timestamp: April 08, 2025, 12:03:28 pm
content: The units I tested were ordered at some point 3 to 4 years ago, so they are likely newer.But as said, the degradation can be observed to have started.I did not loose any time, but the degrading contact to the case makes me worry about their thermal contact and if they could have failed when presented with extended high power use.Which is something I could actually test.Just by running them overnight with +23dBm or +26dBm on something like 1GHz and see how the measured parameters change.I mostly used them to improve matching or to protect other things from high powers on frequencies above 100MHz.So I was not severly impacted by them.

author: coppercone2
author_rank: Super Contributor
timestamp: April 08, 2025, 12:06:36 pm
content: another thing it can be is corrosive action, that its contaminated and the silver slowly reacted with something, like sulfidethis is like concrete and rebar + swelling from rustand its cheaper to do chemically silver plated copper or aluminum then pure silver too, and for instance to precipitate copper powder for plating from dirty recycled copper solutionepoxy version of bad caps?if you hear about copper nano particles, chances are its precipitated from something

author: joeqsmith
author_rank: Super Contributor
timestamp: April 08, 2025, 12:34:52 pm
content: Quote from: ftg on April 08, 2025, 12:03:28 pmIn my case, the one part was brand new in a factory sealed bag. The only thing it was exposed to was running it on the two VNAs. It was kept in our lab since purchase. Seems to point to an aging problem.

author: ftg
author_rank: Frequent Contributor
timestamp: April 08, 2025, 12:38:49 pm
content: Quote from: joeqsmith on April 08, 2025, 12:34:52 pmOne of the 6dB units was also factory fresh. Sadly I did not think to mark which one it was.Definitiely aging related.