Author Topic: Cleaning high voltage PCB with vaporised tracks  (Read 2055 times)

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Offline fourtytwo42Topic starter

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Cleaning high voltage PCB with vaporised tracks
« on: May 30, 2017, 11:36:42 am »
Just had a hard start event caused by my stupid diagnostic code in SMPS that has vaporized the tracks around the TO220 half bridge mosfets leaving copper residue all over the immediate area. The HB operates on a 200Vdc input so kinda the limit already for clearance on TO220 pads so I wonder if anybody has any methods for removing the copper residue to restore the insulation performance before repairing the blown tracks with wire ?

I hope you can see the problem in the attached picture, thanks in advance
« Last Edit: May 30, 2017, 11:38:27 am by fourtytwo42 »
 

Offline DenzilPenberthy

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Re: Cleaning high voltage PCB with vaporised tracks
« Reply #1 on: May 30, 2017, 12:03:20 pm »
My approach to this is usually a good scrub with flux cleaner to get off as much of the loose carbon etc as possible then careful use of a scalpel to scrape the burnt surface away from between the traces.  If the PCB is badly burnt this can extend to cutting quite a deep groove in to the PCB material.
 
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Offline DenzilPenberthy

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Re: Cleaning high voltage PCB with vaporised tracks
« Reply #2 on: May 30, 2017, 12:10:57 pm »
... if you wanted to be really keen you could test the insulation resistance between the traces after you've cleaned and scraped between them with a Megger (or similar).

 Remove the affected component from the pads, cut the traces nearby so that you're not going to back-feed into the rest of the circuit or damage anything with the test voltage. Then after you've cleaned up the burnt area, give it a good test with the Megger to make sure it has adequate high-voltage insulation. Then repair your cut tracks by scraping back the solder resist either side of the break and bridge with a bit of wire and solder.  Maybe re-cover the scraped away bits of PCB and traces with conformal coating afterwards as well...
 
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Offline fourtytwo42Topic starter

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Re: Cleaning high voltage PCB with vaporised tracks
« Reply #3 on: May 30, 2017, 12:29:01 pm »
Thank you Denzil, I like the word scrub, fresh toothbrush comes to mind. I use isopropyl alcohol for most jobs, I will try that first and then maybe something more abrasive (toothpaste) followed by more IPA then a very close inspection under a magnifier. Fortunately there doesn't seem to be any burning of the substrate, the bang was so loud foil d*** come to mind! Good idea the scalpal scraping too :)

I already use conformal coating as this will live outside when its done testing :)
 

Offline Richard Head

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Re: Cleaning high voltage PCB with vaporised tracks
« Reply #4 on: May 30, 2017, 12:45:38 pm »
A good way to remove the copper after a blow up is to roll a molten ball of solder over the area where the metallization has occurred. The solder picks up the metal and leaves behind only flux. Just apply a liberal amount of solder to the iron tip and play it over the are where the metallization is. It works wonders. Once you've removed all the burned flux there isn't a trace of metallization left.
 
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Offline fourtytwo42Topic starter

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Re: Cleaning high voltage PCB with vaporised tracks
« Reply #5 on: May 30, 2017, 01:01:42 pm »
Woww Richard that is a really good idea, I will try that first, many thanks :)
 


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