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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE! |
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1st Jun 2022, 8:01 pm | #1 |
Pentode
Join Date: Nov 2005
Location: Banbury, Oxfordshire, UK.
Posts: 204
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Thermal pads - "clean" cautionary tale
Just narrowly averted MOSFET heatsink failure by chance spotting some poor m/f... Maybe due to a dirty assembly line? or lack of inspection/QA??
When at Retrotech I picked up two 300W 24V battery-mains inverters from a jolly friendly table-seller who helped me out and I've finished adjusting design of my lowering the cut-out, so that they still work nicely down to safe-discharge of 19V (from my reclaimed EV battery)... The low-side quad of IRF540s are heatsinked-to-case by isolating thermal pad. I happened to notice two tiny dinks in the pad (pic) but only on one side, though and was just about to reassemble.. but my bike-tube puncture experiences reminded to "check the tyre", so I looked at the IRF540s as well... See pics! Turned out to be 2 tiny solder spray balls still on one of the IRFs, having been squashed between device and the pad on factory assembly in China, just like on a BGA package... They pinged-off the IRF540 again nice'n'clean with a knife... just in time to avoid a potentially fatal flaw (and most likely some big bangs!).
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1st Jun 2022, 10:29 pm | #2 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
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Re: Thermal pads - "clean" cautionary tale
Taking a battery down to 19 volts will kill it pretty quickly. I always set cutoff @ 22volts.
Just my experience with solar. JOe |
2nd Jun 2022, 9:14 am | #3 | |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,677
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Re: Thermal pads - "clean" cautionary tale
Quote:
Returning to the thermal pad/solder ball issue, well spotted! I've experienced quite a lot of these inverters with the rows of MOSFETs screwed to an extruded aluminium case with thermal pads self-destructing even at low loads. I wonder now whether this defect could have something to do with the failures. Chris
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2nd Jun 2022, 12:34 pm | #4 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,271
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Re: Thermal pads - "clean" cautionary tale
A service bulletin was issued a few years ago from a very well known US manufacturer of active loudspeakers after a number of warranty repair reports showed that aluminium swarf had been left on the diecast heatsinks after the milling process and had been caught under the sil pads, shorting the IGBTs to earth after a time. They had been assembled by a third party contractor in China.
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Kevin |