brand new PCB with variable resistance between bus lines

This is the most bizarre thing I have heard of regarding a PCB. How do you know the short is on the board and not one of the parts? I have ***no*** idea how a chip would "ooze" back and white foam from the power pins. Have you checked any bare PWBs? If the problem is the PWB you should try working with a bare PWB and do tests on that.

Rick

If you're "blowing" shorts with a 5V power supply, That's *not* a good idea. I found out a number of years ago that the shorts can grow back after hours or days of operation. I would guess that the act of blowing the short leaves a bit of carbon between the traces, which is conductive.

I have seen something like this before.

In my case, the PCB laminate was slightly porous, and the tin plating oozed into cracks. I would set me bench supply to 5V, and up the current limit until I burned them out. I would need to repeat this ever other day until I got new PCB's in.

AL

Ah, the joys of RoHS. You probably have tin whiskers. See here:

Rodent urine ?

-jg

We have struck some field reliability issues we suspect is tin whiskers/ fretting corrosion on PLCC sockets. - the joys of RoHS indeed.... :(

-jg

Wouldn't it work better with a higher voltage supply (with a large current capability) to more permanently blow out the shorts? It would also make it easier to see where the problems are (look for the black marks!).

...

In order to remove the short circuits on my homebrew PCBs I used an old

Falk

When I worked in an old (Strowger) telephone exchange, the 50v bus bars were huge, and we did everything on live equipment. When a pice of wirewrap (remember when you wire-wrapped with Lindstrom pliers) wire fell into the frame, we just changed the rack fuses for something unmentionably large.

The day a spanner fell across the bus bars was memorable. A single cell of the exchange battery was a cube about 5 feet in each dimension!

Stephen

I don't know if it is foam but is so dense. We speculate about heated flux. Arround the "resistance" of the tracks, can be possible for a liquid proceeding from the manufacture of the pcb to stay unther the green barnish ? If it where an acid there were possible to change the resistance with the temperature and the copper ? Is possible to do something similar with defluxer drained in a via and interacting with something inside the pcb?

Thank you Narcis Nadal

The theory was that anything over 5V would let the magic smoke out of the IC's. I was doing this to stuffed boards.

AL

The black substance could be melted IC casing plastic, if the ICs are getting hot enough. I once had a lamp power cord short out, due to excessive flexing cracking the synthetic-rubber insulation. Some of the rubber insulation was transformed into a viscous green liquid, which remained liquid even after everything cooled down. Apparently the heat had triggered a chemical reaction between the insulation and the copper wiring, hence the green color.