W123: Instrument cluster Printed Circuit Board repair

[Graham]

Originally Posted by JimF

It's actually quite amazing how much current that a typical PCB trace can handle.

For 2oz copper (typical), the following table shows the current carrying capability for various trace thicknesses:

Trace Width --------- Current
0.25" ---------------- 3.3 amps
0.50" ---------------- 6.0 amps
0.10" ---------------- 9.9 amps
0.20" ---------------- 11.0 amps

It must have had much more current for it to blow as show in the pics.

Just for info . . .

I was a bit puzzled why the widths are smaller for higher currents?

Your post did spur me to do a search I did find this calculator and it does seem those traces can carry quite high currents. Thanks- I learned something new!

Nevertheless, it does seem that these traces often blow. Bill seemed to have a few examples and there are numerous posts on net about similar failures.

Perhaps they are due to something or someone causing a short. (In my case, nothing on car had been touched for over a year). That would seem to indicate that the 8amp fuse is not protecting the circuit board. In Bills pictures, it seems it is the trace leading to pin 15 that has blown - I think this is a ground.

[JimF]

Originally Posted by Graham

I was a bit puzzled why the widths are smaller for higher currents?

Sorry, can't type either . . . I've corrected the first two table entries.

Anyway they can handle quite a bit of current in general.

You correct, an external 'short' where the pcb's trace is in the loop, the first thing to go could well be the trace since typically, the pcb traces are designed for the "normal" current carrying (with some safety), not the short circuit current.

[Graham]

Originally Posted by JimF

Using a soldering "station" iron (not a Radio Shack blunder-bus iron)

Something like this?



Believe it or not, that is what we used to use. I built and repaired radios as a kid and that was all we had. Use a great big iron for plumbing and a small one for electrical work.

I was not familiar with that "station" term! Had a look at a Weller and it does look like a neat unit. The iron I use for this sort of thing is an American Beauty pencil type - about same size, but not controllable. I also have two of those pistol gun types that I never seem to get to work properly.

[whunter]

Originally Posted by Graham

Something like this?



Believe it or not, that is what we used to use. I built and repaired radios as a kid and that was all we had. Use a great big iron for plumbing and a small one for electrical work.

I was not familiar with that "station" term! Had a look at a Weller and it does look like a neat unit. The iron I use for this sort of thing is an American Beauty pencil type - about same size, but not controllable. I also have two of those pistol gun types that I never seem to get to work properly.

weller solder stations | eBay


.

[hughes81]

Hi,

Thanks for this post! It really helps a lot especially to people having problem repairing PCB trace. You can also use a PCB Trace repair kit to do the job and make things neat. You don't have to worry now when you got broken pcb trace.

[Graham]

Originally Posted by hughes81

Hi,

Thanks for this post! It really helps a lot especially to people having problem repairing PCB trace. You can also use a PCB Trace repair kit to do the job and make things neat. You don't have to worry now when you got broken pcb trace.

Just a heads up. If you have a dead short in your instrument and switch wiring, you WILL blow the traces. The 8amp fuse does not protect against this.