Burning board

[drhex]

Hi,
asking here as well as I had a new (to me) effect the other day on repairing battery damage on a pinball board. Putting the repaired board back in the machine I got nice smoke and crispy board under an edge connector sitting right in the trail of electrolyte. I had replaced that  with a new connector, apparently something made the board conductive enough that the 50V attached fried it around the pins. Only explanation I can come up with is that the electrolyte soaked into the board through the open solder mask at the pin and created a bridge to the ground layer. Have a nice ring of crisp around the pins, could reproduce with my bench power supply until burnt out.
Am I making the right assumption here? Is there any way to fix? Scrape away the charred bits and epoxy?
Thanks,
Florian

[BreakingOhmsLaw]

If it is battery acid, you can use baking soda to neutralize it. Wear safety glasses while doing that.
Then, scrape the board and give the board a good clearing with an IPA pcb cleaner.

[Fraser]

In such cases a picture of the damage is very useful so that a repair treatment may be recommended.

You are correct, it is likely that capacitor electrolyte has contaminated the PCB and caused it to be conductive. Repair of such is normally possible but the approach very much depends upon the PCB material and the specific PCB about in the area of damage. In some cases an area of the PCB may be ground away and filled with a good quality non-conductive filler or epoxy. In more extreme cases, an area of PCB is isolated from electrical connections to other areas of it and a daughter board is created to provide the functionality of the contaminated area, thus bypassing the contaminated PCB material. In modern multi later PCB’s it can be very challenging to repair such damage, but in older, 1 or 2 layer PCB’s it is usually a relatively simple task.

As an example, I have a piece of data analysis equipment that suffered leakage of the Ni-Mh Mempac battery and this made the PCB conductive around the high voltage drive to the electroluminescent backlight drive pins. The high voltage (50V ac) was getting to the 5V rail near to it and crashing the processor. No real current was drawn as it was like a high value resistor introducing noise onto the 5V supply rail. The problem only occurred when the LCD backlight was on. I investigated and discovered that the battery electrolyte had contaminated a relatively sparsely populated area of the PCB around the backlight drive output pins. I was able to cut the backlight drive traces to isolate that area of contaminated PCB and cut away the area where the electrolyte had made contact with the 5V rail. The contaminated area is completely isolated on all edges but I used a Dremmel tool to cut away the contaminated PCB material to prevent future electrolyte migration within the PCB material. I recreated the very simple output circuit of the backlight driver on a separate custom PCB and and connected the new Mempac battery off-board on flying leads. Full functionality of the equipment has been restored.

Fraser

[Fraser]

As a side note, PACE used to produce PCB repair and modification equipment for through hole electronics. They had a series of videos on-line (and they may still be available) that showed how to cut away damaged areas of PCB’s, followed by repair to them. PACE provided through hole eyelets, replacement PCB tracks and the ‘Dremmel like’ tool to o the cutting with burrs. The cutting tool was very expensive but a modern Dremmel or clone is perfect fir the job and cost significantly less. I own the repair kits but not the cutting tool. Very useful fir through hole PCB repair but that technology is now becoming rarer in my repair work.

Fraser

[Fraser]

Here you go, PACE PCB repair videos from the early 1980’s !

Enjoy 


https://youtu.be/HKX-GBe_lUI

https://youtu.be/WqDqu4wCW-A

https://youtu.be/vpgMn88Qfjs

https://youtu.be/7o4ZWlG6mmo

https://youtu.be/e8KRPFOD1RE

https://youtu.be/h_RiRL-flgs

[Fraser]

Out of the above videos, the one that most suits the OP’s situation is here…

https://youtu.be/7o4ZWlG6mmo

PCB damage removal and restoration techniques, including cutting away an area of damaged/carbonised PCB with a ball mill.

I was trained to do this kind of repair and it can be a very satisfying task 

Fraser

[Fraser]

The PACE “Microchine” tool is still available …..$500  and the power supply is extra !

https://paceworldwide.com/mc-65-microchine

There are many much cheaper/better options these days 

Fraser

[DaJMasta]

The char itself can also be conductive - the carbonization of the resin binding the fiberglass effectively becomes a resistor.  Fraser has the right approach, though, you physically remove the charred PCB material and then jump over wires for the traces you removed - at least in small areas and without too many board layers this method works well and can be done with a dremel and some patience.

Worth mentioning that if you've got a leaky battery, very often the battery chemistry means the leaked fluid is actually alkaline in nature, so while neutralizing it is a good idea, you would normally do a cleaning pass, a mild acid to neutralize the residue and stop the erosion of traces, and then another rinse or two to get rid of as much as possible.  It should be easy to determine the kind of leak it is, you can take a drop of vinegar or a drop of baking soda water (or similar base solution) on the leaked fluid and see if it bubbles.  Just be sure to remove the drop completely unless you are intending to do all the cleanup of it then, as leaving the reacting chemical in place can do more damage if left there.

[Fraser]

I have to be honest and say that I really hate any form of electrolyte leakage damage repair. It can be a challenging job to repair and I am often left with the nagging doubt about the longevity of the repair if I have missed any electrolyte contamination on the PCB. I bought an ultrasonic cleaner to assist in cleaning away contamination on PCB’s and I use the special PCB cleaning chemicals. I have managed to kill a processor PCB with that professional swept frequency ultrasonic cleaner though  The danger with electrolyte leakage is that it can creep into the PCB materials structure and under any IC’s, where it is hidden from view. Thorough cleaning is needed, as stated by DaJMasta 

[james_s]

If it is battery acid, you can use baking soda to neutralize it. Wear safety glasses while doing that.
Then, scrape the board and give the board a good clearing with an IPA pcb cleaner.

No, it's not acid, both alkaline and nickel based rechargeable batteries use a strong base, so baking soda is exactly not what you want. Wash it instead with diluted white vinegar to neutralize followed by rinsing with clean water.

[tooki]

The PACE “Microchine” tool is still available …..$500  and the power supply is extra !

https://paceworldwide.com/mc-65-microchine

There are many much cheaper/better options these days 

Fraser

Such as…?

For those who don’t know, what makes the Microchine special is that the tip is a continuity tester. You connect a clip to the trace you need to excavate, and if the tip makes contact with it, it stops rotation so you don’t mill right through it. So an ordinary dremel-like tool is not a substitute.

[Fraser]

Tooki,

In all my career I have never used a burr handpiece that had, or needed, a continuity function. We were taught care when excavating a PCB rather than to rely upon some form of continuity system that requires the operator to be connected to the vulnerable sub surface layers. Slow and steady was our mantra and I have never abraded a PCB layer that I was not fully intending to. I tended to use high speed motor units that supplied drive to a thin handpiece via a Bowden cable. Very easy to manipulate in the hand. Some Dremmel type multi tools offer similar Bowden drive options but the whole ‘multi tool’ has got smaller and lighter so such is not always necessary.

I own a significant amount of PACE soldering and rework equipment and I am sure that the Microchine is very well made and of clever design, but it remains a very expensive option for many users. As a side note, I own several of the PACE VF100 pad/land cleaning units that are used to remove solder from BGA and other pads during rework. It looks like a conventional solder sucking gun, but it provides axial hot air preheat to the PCB plus a specialist nozzle for removing the solder residue efficiently from small pads. Very clever, very nice to have, but also specialist and very expensive ! Many users would just use solder wick and flux to achieve the same result and be happy with that results. It is horses for courses I suppose and PACE design for Industry where efficiency is important and high equipment costs can be tolerated. I do like my PACE equipment though 

http://competence.com.mx/prods/pace/html/_ShowSystemDetails-1464.html

Fraser

[drhex]

Thanks for the information and suggestions - I washed the board with vinegar, water, alcohol, then removed all affected components, washed again as previous and soldered new components. That usually works, battery damage is pretty common on pinball boards sadly. This is the first time I was left with a conductive board though, when it started smoking I first though one of the power transistors was burning up. I let it burn out now as this seems to be the easiest way to get rid of the contamination, will dremel out and put epoxy in - this is a simple two layer board, only problem is that the edge connector is mechanically taxed. Will post before/after pics once I get around to it...