Can this flexible circuit be soldered to? - Page 1
Forum section: electronics / repair

author: HoangLong
author_rank: Newbie
timestamp: October 11, 2025, 01:11:57 pm
content: I have this flex circuit with broken tracks from an old Tektronix TDS 340A oscilloscope's front panel. The track doesn't look like it is copper. Can I solder a wire to the track? Or I must use conductive ink/glue?

author: squadchannel
author_rank: Frequent Contributor
timestamp: October 11, 2025, 01:23:47 pm
content: no, cant soldering it.conductive ink is needed.temporary, aluminum foil and tape might do the trick.

author: marhuum
author_rank: Contributor
timestamp: October 11, 2025, 02:05:36 pm
content: Possible but..many extra pre-requirements- adhere polyimide (Kapton) tape on the copper side pad ie. for SMD components on their side, for through-hole PCB on the opposite side-put stiffener upon it ie. right after prev. step- Soldering iron tip temperature must be 250–290 °C ie. lower than common PCB- durability of tip contact time on pad < 3 sec. so should use pre-tinned leads to help shorten contact time- Solder sholud be Sn63/Pb37 with diameter ~0.5 mm

author: Haenk
author_rank: Super Contributor
timestamp: October 12, 2025, 08:58:45 pm
content: Might try some low melt solder, that goes even down to below 100oC - and will maybe not even melt the plastic carrier at all. Add flux and a wire across the broken trace.However that silver repair paint might be good enough as well, as there is no physical stress involved.

author: Stray Electron
author_rank: Super Contributor
timestamp: October 12, 2025, 10:21:50 pm
content: That doesn't look like copper so I don't know if you can solder to it or not, but I have soldered thin copper foil to copper plated flex circuits many, many times using a low temperature solder. Either the 2% or the 3% silver loadedlead/tinsolder made by Kester and using small tip on a temperature controlled Weller soldering station. Digikey sells the solder and they have data sheets on their website and the data sheets show the melting point. One of the nice things about the kester solder is that it has a very sharp melting point so there is no slushy stage when soldering. I used some copper foil that I found in a surplus store that was gold plated on one side but I don't know where to find any today. The stuff that I used was about like heavy gauge household aluminum foil and it could be cut with scissors and very easily formed into shape. It wouldn't work for a circuit that needs to be flexible but for a flex cable that isn't moved around it works fine. One of the requirements that I had was that the front side (the gold or exposed side) of the copper was used for a low current battery contact and had to remain pure and could not have any solder on it. So I cleaned the broken circuit and the foil very well and made a replacement part from the foil. Then I lightly tinned the back side of the foil and the broken circuit and then put the two together and then carefully heated the foil in an out of the way spot and allowed the solder on both parts to melt and fuse together and then pressed the parts together, removed the heat and allowed the parts to cool before moving anything. That's where the non-slush solder really helped.OTOH if that's not some kind of contact point in your circuit then you can just use a piece of good clean copper wire to jumper the broken section and you won't need to deal with using foil like I did. I was replacing battery contact points (and other nearby conductors) that were completely eaten away by corrosion.My advice would be to get some of the Kester solder and a good temp controlled soldering iron and test the circuit board in some out of the way spot and see if the solder sticks. If it does then you should be good to go.

author: aeg
author_rank: Regular Contributor
timestamp: October 13, 2025, 07:53:28 pm
content: The gray traces are carbon ink. Don't the green traces look like ordinary copper with soldermask?

author: HoangLong
author_rank: Newbie
timestamp: October 15, 2025, 03:00:03 am
content: Thank you everyone for your very helpful replies. I have decided to use conductive ink to connect the traces because of how easy it is and because the trace are not copper but carbon ink so solder won't stick to it.

author: HoangLong
author_rank: Newbie
timestamp: October 15, 2025, 03:11:08 am
content: Thank you for sharing. As for the aluminum foil, solder won't stick to it with regular flux so we can plate it with copper in CuSO4 solution by electrolysis. Or buy some special flux that can solder to aluminum.

author: HoangLong
author_rank: Newbie
timestamp: October 15, 2025, 03:12:54 am
content: Interesting. What green traces are you referring to?

author: aeg
author_rank: Regular Contributor
timestamp: October 15, 2025, 10:58:20 pm
content: Quote from: HoangLong on October 15, 2025, 03:12:54 amI think it was my mistake. For example, the long horizontal trace in the bottom left of the picture is the same color as a copper PCB trace with soldermask. However, now I think it's carbon ink with soldermask.