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What on earth is this?

Posted: Thu Nov 15, 2018 1:44 pm
by mikro
So I recapped the TT PSU, was waiting only for PC111 opto to make the job complete. Arrived today, so hooray to desolder the old one... I swear to all Gods of Electronics, I didn't do anything else than hot-aired the PCB:
IMG_20181115_143749.jpg
IMG_20181115_143749.jpg (1.39 MiB) Viewed 1454 times
It can't be scrubbed off by isoprohyl alcohol, it seems to be possible only to melt (?) it. It really behaves like some kind of tough chocolate but looks like something burned at the same time. I didn't noticed anything falling off (the pad really looks like it did but I don't think so) so I'm left with mouth wide open wondering what the hell just happened there?

Re: What on earth is this?

Posted: Thu Nov 15, 2018 1:56 pm
by exxos
probably just red hot flux which "exploded" during manufacture would be my first guess. Some flux sets like epoxy!

Re: What on earth is this?

Posted: Thu Nov 15, 2018 2:05 pm
by mikro
Perhaps I should have specified that this happened out of the blue, i.e. the PCB had nothing even remotely similar there before. I simply started desoldering and it appeared like a plague. :)

Bad news - the pad is indeed gone.

Good news - fortunately that pin is not connected.

Still, never seen anything like this. What could cause it? Air too hot?

Re: What on earth is this?

Posted: Thu Nov 15, 2018 2:11 pm
by PaulJ
mikro wrote:
Thu Nov 15, 2018 1:44 pm
So I recapped the TT PSU, was waiting only for PC111 opto to make the job complete. Arrived today, so hooray to desolder the old one... I swear to all Gods of Electronics, I didn't do anything else than hot-aired the PCB:
IMG_20181115_143749.jpg
It can't be scrubbed off by isoprohyl alcohol, it seems to be possible only to melt (?) it. It really behaves like some kind of tough chocolate but looks like something burned at the same time. I didn't noticed anything falling off (the pad really looks like it did but I don't think so) so I'm left with mouth wide open wondering what the hell just happened there?
ITS A Spider!! Nope, look up and right a little bit is the missing pad from the OPTO where the spider is setting. Looks like what ever happened is involved with the desolder since the pad is decoupled.
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Re: What on earth is this?

Posted: Thu Nov 15, 2018 2:14 pm
by rubber_jonnie
It looks like hot air scorching. My experience with hot air and these old fibreglass boards is that they de-laminate very easily, and are very prone to scorching. Looks like maybe too hot plus some flux going nuclear and setting like stone. Lucky that pad isn't in use.

Personally I much prefer wick to hot air for these old boards.

Re: What on earth is this?

Posted: Thu Nov 15, 2018 2:28 pm
by exxos
rubber_jonnie wrote:
Thu Nov 15, 2018 2:14 pm
It looks like hot air scorching. My experience with hot air and these old fibreglass boards is that they de-laminate very easily, and are very prone to scorching.
Very true as well!

Re: What on earth is this?

Posted: Thu Nov 15, 2018 5:18 pm
by mikro
Lesson learned. I just wish it wasn't exactly this way but could have been worse.

Re: What on earth is this?

Posted: Thu Nov 15, 2018 6:29 pm
by czietz
Note that burnt FR4 (PCB material) can get conductive. Since the black spot is also between two pads of the opto-coupler that are used, make sure it doesn't impact the regulation of the PSU.