Bonded mosfet - Page 1
Forum section: electronics / repair

author: jheatac
author_rank: Regular Contributor
timestamp: February 08, 2019, 01:47:25 am
content: Hello!I need to replace a mosfet bonded to an aluminum bar. The aluminum bar is then screwed to a heatsink.How to I remove the mosfet? What product do I use to reattach?This is in a fosgate car audio amplifier.Thanks!

author: james_s
author_rank: Super Contributor
timestamp: February 08, 2019, 04:59:44 am
content: That's a strange setup. I think I would use either thermally conductive epoxy, or more likely I would drill and tap a hole to use a screw through the mounting hole in the mosfet or a clamping bar across the group of them like a sensible design would have.

author: DaJMasta
author_rank: Super Contributor
timestamp: February 08, 2019, 06:15:20 am
content: Could it be epoxy under there that just isn't oozing out anywhere? I don't think there's a great way to either bond the aluminum to a metal pad on the back or the plastic casing without some kind of adhesive. You can be pretty sure heating it up isn't going to release it, though

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 08, 2019, 06:51:51 am
content: Common sight in plasma TVs. Get a replacement board, or junk it.Tim

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 08, 2019, 07:41:16 am
content: Try prying the old mosfet off without bending the bar, failing that, if that aluminium bar is only for mounting and nothing else, I'd chop the bar to remove the old one and screw the new MOSFET directly to the heatsink then solder it to the PCB (order of operation is important!).

author: bundy
author_rank: Newbie
timestamp: February 08, 2019, 08:39:28 am
content: Seeing the gold plated X (sort of fiducial/alignment for production) above a mounting hole this seems to be metal substrate PCB with a copper pattern.So my best guess is that these packages are soldered onto this board.

author: Ian.M
author_rank: Super Contributor
timestamp: February 08, 2019, 09:52:33 am
content: If that's the case, I would suggest removing the assembly from the PCB, filing, grinding or milling away the faulty MOSFET till the top of its thermal pad is exposed, then preheating the assembly to about 100 deg C and attempting to desolder the thermal pad remains from the assembly with a high power iron or soldering gun. Clean the pad it was on thoroughly to remove as much of the old soler as possible. Reassembly would be by reflow with low melting point solder (preferably between 125 & 150 deg C) with a jig to hold the MOSFET leads in the correct place for easy refitting to the PCB. If you don't want to get low temperature paste you'll probably never finish before it goes bad, you could use low temperature solder wire - tin both surfaces, flux and sweat together

author: PKTKS
author_rank: Super Contributor
timestamp: February 08, 2019, 12:17:31 pm
content: There is a thread around (not so long) which the folk managed to"UNBOND" the component with care by HOT AIR only...Can't remember the title associated ...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 08, 2019, 12:22:13 pm
content: Chop the leads, then crack it off with hammer and chisel-like implement

author: Ian.M
author_rank: Super Contributor
timestamp: February 08, 2019, 12:47:33 pm
content: If its epoxied onto a solid metal heat spreader, you stand a good chance of removing it with hot air and a chisel, provided you clamp the heat spreader to a hard flat heat resistant low thermal conductivity surface while chiselling so its supported to avoid twisting or bowing the heat spreader.However, I'm with Bundy on this, its probably an aluminum substrate PCB, and if you attempt to chisel the MOSFET off you'll most likely rip the pad off and may damage the insulating layer as well. At that point, there's no good way to bond on a new one with comparable insulation and thermal conductivity. You may get away with it you may not, depending on how close to the edge of its SOA it runs at. Its certainly a big risk if you turn it up to 11 on a hot day.If its soldered, you *may* be able to get it off with hot air while cooling the others enough to keep their solder solid, but they will be quite well 'cooked' by the time you've replace it, probably well in excess of their permissible reflow profile, hence my recommendation above to mechanically remove enough of the MOSFET to allow direct heat to be applied to the top side of its thermal pad, where the die was attached. Another consideration is popcorning, depending on how much ambient moisture the good remaining MOSFET packages have adsorbed, heating the whole assembly up to near reflow temperature without a long slow bakeout may destroy them.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 08, 2019, 10:52:52 pm
content: You have some small look a like capacitors on the metal plate substrate, i would try an paint heat gun on the rear of the defective mosfet, the heat combined with expansion would surely crack the bonding, or make it more brittle ??I would try to slide an exacto blade under it, not a chiselI dont know if thoses fc2 marked parts are important

author: amyk
author_rank: Super Contributor
timestamp: February 09, 2019, 01:06:10 am
content: Mill it off.

author: ogden
author_rank: Super Contributor
timestamp: February 09, 2019, 01:16:40 am
content: Quote from: coromonadalix on February 08, 2019, 10:52:52 pmDon't. You will either crack (brittle) substrate or strip copper off it. Preheating whole plate and hot air gun (on the back of mosfet) shall be tried first.