Residue between FET legs is blowing controllers - solution?

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Skedgy Sky   100 W

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Residue between FET legs is blowing controllers - solution?

Post by Skedgy Sky » Apr 14 2014 12:38pm

I've noticed that in building or repairing controllers, residue is left behind from the solder.
I've done (almost) my best to clean it but this sometimes comes up:
10156961_10201867558036612_708293492_n.jpg
10156961_10201867558036612_708293492_n.jpg (70.69 KiB) Viewed 994 times
Watching the board closely after powering up, I first see smoke. I cut power, examine.. and find a charred path between the FET legs.

If I don't clean the problem up, it results in blown FETs.
It's as if any residue between the pins becomes a conductive path, and results in a short that blows FETs on that phase.


Has anyone experienced this?
Any cleaning techniques that I can try to eliminate shorts occuring? ... maybe high pressure spray using some sort of solution that cuts through flux residue/any other dirt?

This probably is only apperant in controllers where high voltage is run. I don't believe low voltage wouldn't cunduct enough current between the FET legs.

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Re: Residue between FET legs is blowing controllers - soluti

Post by zombiess » Apr 14 2014 1:20pm

Flux is not normally conductive, but some types are hydroscopic after use. I've never had an issue with it and I have some personal controllers I've used at 125V just covered in flux residue from building up traces. Never had a problem.

BTW that EB324 board needs to have the top traces built up with solder to help carry current.

If you short a drain to gate and run > 20V or 30V depending on the MOSFET you will destroy it because the gate will only handle 20 or 30v max.

First power up of a new build should always be done on a current limited power supply which should help limit damage, then you slowly bring power up once it appears things are OK. I limit my supply to 100mA for first power up since a controller only draws around 30mA-40mA with a Cycle Analyst plugged in.
Last edited by zombiess on Apr 14 2014 5:10pm, edited 1 time in total.
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wesnewell   100 GW

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Re: Residue between FET legs is blowing controllers - soluti

Post by wesnewell » Apr 14 2014 1:26pm

You can buy resin flux residue remover. but I found that Mennen after shave lotion and a good stiff brush worked good and made the boards smell good too. Actually, I just washed them with soapy water and rinsed with hot water after cleaning them. It was a lot cheaper that way and I've done thousands of boards. Just be sure and dry them good.
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Re: Residue between FET legs is blowing controllers - soluti

Post by dnmun » Apr 14 2014 2:53pm

your pcb is burned. that is not flux. the board is useless. the carbon left behind is shorting out the mosfet gate.

Skedgy Sky   100 W

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Re: Residue between FET legs is blowing controllers - soluti

Post by Skedgy Sky » Apr 14 2014 3:29pm

dnmun wrote:your pcb is burned. that is not flux. the board is useless. the carbon left behind is shorting out the mosfet gate.
Yes, it is burned.. but why did arcing occur between the legs in the first place? It was not burned before power was applied. I've started to keep a very close eye on the area between FET legs the past few weeks after realizing the existance of the problem, but I'd like a solution so I don't have to worry about this happening each time I power on a new controller (either after building it up or repairing it).

I have found that scraping away this burned section (and any other dirt between the legs) prevented damaged from occuring to the FETs (so controller is still GOOD), but I'd like a way to make sure the FETs don't end up like this to begin with. This is most likely why some reworked controllers have failed after repair for me in the past.
wesnewell wrote:Actually, I just washed them with soapy water and rinsed with hot water after cleaning them. Just be sure and dry them good.
I will try something like this - and maybe compressed air.
zombiess wrote:First power up of a new build should always be done on a current limited power supply which should help limit damage.
I've started to use a power supply (limited to like 10A) instead of plugging my LiPo in. ;) Maybe a small fuse would be good idea.

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TMaster   1 kW

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Re: Residue between FET legs is blowing controllers - soluti

Post by TMaster » Apr 14 2014 6:47pm

I use a toothbrush and acetone. Then squirt bottle rinse off with acetone. Sometimes 97% isopropyl alcohol. But the Acetone cleans better. :D
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