On Mon, 19 Jun 2023 22:54:50 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:>On Monday, June 19, 2023 at 8:50:16?AM UTC-7, John Larkin wrote: >> Has anyone done this? I need to get the mounting screw height down. >> >> I guess I'll have to try it. > >The hole is 3.5mm, so an M3 flat head cap screw with a countersinked washer would >hold it down; here are 1.8mm thick offerings for M3 screws. Is 1.8mm too high? > ><https://www.mcmaster.com/products/countersunk-washers/>That might work, depending on how deep I dare to countersink the mosfet. The ideal height of the screw is zero.> >I'm not seeing any measure of the recess dimensions around the clear hole, though. >The screw would be capable of touching the tab, which might not work for you.TO247s have insulated holes, so that's not a problem. No metal shows up on the one I countersunk. I could use TO-220s, but they would need an insulating shoulder washer, and they are not as good thermally, having a smaller metal footprint than a TO247. I'm trying to get maximum power dissipation with a bunch of mosfets on a K199 copper CPU cooler. 200 watts would be nice.
countersinking a TO247 mosfet
Started by ●June 19, 2023
Reply by ●June 20, 20232023-06-20
Reply by ●June 20, 20232023-06-20
On Tue, 20 Jun 2023 03:46:47 -0700, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:>On Mon, 19 Jun 2023 18:57:31 -0700 (PDT), Chris <chris.863@live.com> >wrote: > >>On Tuesday, 20 June 2023 at 01:50:16 UTC+10, John Larkin wrote: >>> Has anyone done this? I need to get the mounting screw height down. >>> >>> I guess I'll have to try it. >> >>Rivets? > >Rework would be difficult, and a rivet adds height too. >The reason to not just epoxy the fets down would be the rework issue. >And the mess, although there are some instant-set thermally conductive >super-glue things.Aluminum solid rivets with a 100-degree cone angle are also available. Set with an arbor press to avoid shock. .<https://www.mcmaster.com/96685A149/> Joe Gwinn
Reply by ●June 20, 20232023-06-20
mandag den 19. juni 2023 kl. 20.09.37 UTC+2 skrev John Larkin:> On Mon, 19 Jun 2023 10:32:14 -0700 (PDT), Lasse Langwadt Christensen > <lang...@fonz.dk> wrote: > > >mandag den 19. juni 2023 kl. 18.13.01 UTC+2 skrev John Larkin: > >> On Mon, 19 Jun 2023 08:57:14 -0700 (PDT), Lasse Langwadt Christensen > >> <lang...@fonz.dk> wrote: > >> > >> >mandag den 19. juni 2023 kl. 17.50.16 UTC+2 skrev John Larkin: > >> >> Has anyone done this? I need to get the mounting screw height down. > >> >> > >> >> I guess I'll have to try it. > >> > > >> >how low do you need? https://www.mcmaster.com/products/low-profile-head-socket-cap-screws/alloy-steel-ultra-low-profile-socket-head-screws/ > >> The heights are good, the prices aren't! > >> > >> A regular screw and a lockwasher cost me over 0.1", and I need less. > >> With locktite, I can eliminate the lockwasher. > >> > >> A really thin custom AlN insulator will help a bit. Isolated TO-247s > >> are expensive and hard to get. > >> > >> I could just epoxy the fets down, but that has downsides. > > > >maybe something like this? https://www.digikey.dk/da/products/detail/aavid-thermal-division-of-boyd-corporation/MAX08NG/1625322 > I've done that > > https://www.dropbox.com/s/nmyb9pz0qpma2xa/Amp.jpg?dl=0 > > but it adds more height. I'll have boards in a card cage on 1.6" > centers, and I'm stacking a CPU cooler/fan thing with giant mosfets. > The screw heads could hit adjacent boards.like this? https://www.dynatron.co/product-page/k199 can't just clamp the whole stack with the four screws already there?
Reply by ●June 20, 20232023-06-20
mandag den 19. juni 2023 kl. 22.37.26 UTC+2 skrev John Larkin:> On Mon, 19 Jun 2023 08:49:52 -0700, John Larkin > <jla...@highlandSNIPMEtechnology.com> wrote: > > >Has anyone done this? I need to get the mounting screw height down. > > > >I guess I'll have to try it. > I'm working from home today and I have a hand drill and a very rusty > old countersink, a family heirloom. > > https://www.dropbox.com/scl/fi/y5s1bsc2omn341rnxkicq/TO247_countersunk_4-40.jpg?dl=0&rlkey=cqenr4ev8iic5zgujqor8cns1 > > This looks OK. The 4-40 screw should apply plenty of force and it > centers the fet nicely. It would need locktite so thermal cycling > doesn't loosen it up.I'd be worried about the wedge effect of the countersink maybe xray or decap a transistor to see how much room there is for a counter bore, I think I've seem some manufacturers say that a washer must be used
Reply by ●June 20, 20232023-06-20
On Tue, 20 Jun 2023 08:48:06 -0700 (PDT), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>mandag den 19. juni 2023 kl. 20.09.37 UTC+2 skrev John Larkin: >> On Mon, 19 Jun 2023 10:32:14 -0700 (PDT), Lasse Langwadt Christensen >> <lang...@fonz.dk> wrote: >> >> >mandag den 19. juni 2023 kl. 18.13.01 UTC+2 skrev John Larkin: >> >> On Mon, 19 Jun 2023 08:57:14 -0700 (PDT), Lasse Langwadt Christensen >> >> <lang...@fonz.dk> wrote: >> >> >> >> >mandag den 19. juni 2023 kl. 17.50.16 UTC+2 skrev John Larkin: >> >> >> Has anyone done this? I need to get the mounting screw height down. >> >> >> >> >> >> I guess I'll have to try it. >> >> > >> >> >how low do you need? https://www.mcmaster.com/products/low-profile-head-socket-cap-screws/alloy-steel-ultra-low-profile-socket-head-screws/ >> >> The heights are good, the prices aren't! >> >> >> >> A regular screw and a lockwasher cost me over 0.1", and I need less. >> >> With locktite, I can eliminate the lockwasher. >> >> >> >> A really thin custom AlN insulator will help a bit. Isolated TO-247s >> >> are expensive and hard to get. >> >> >> >> I could just epoxy the fets down, but that has downsides. >> > >> >maybe something like this? https://www.digikey.dk/da/products/detail/aavid-thermal-division-of-boyd-corporation/MAX08NG/1625322 >> I've done that >> >> https://www.dropbox.com/s/nmyb9pz0qpma2xa/Amp.jpg?dl=0 >> >> but it adds more height. I'll have boards in a card cage on 1.6" >> centers, and I'm stacking a CPU cooler/fan thing with giant mosfets. >> The screw heads could hit adjacent boards. > >like this? https://www.dynatron.co/product-page/k199Yes, I'll probably use K199. It measures about 0.25 K/W. It's miles better than machining a heat sink and adding a separate fan. It's copper, too, a great heat spreader.> >can't just clamp the whole stack with the four screws already there? > >I plan to remove the spring thingies and bolt the cooler hard to the PCB, with the fets on the bottom side. We tried squashing the fets metal-side-up, between the cooler and the board, and I didn't like that. Fet contact force would be iffy and nothing would be inspectable. The thermal grease situation would be ikky. With fets bolted on the bottom (with a big hole in the board) each fet is accessable, including for temperature measurement and possible rework.
Reply by ●June 20, 20232023-06-20
On Tue, 20 Jun 2023 08:50:11 -0700 (PDT), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>mandag den 19. juni 2023 kl. 22.37.26 UTC+2 skrev John Larkin: >> On Mon, 19 Jun 2023 08:49:52 -0700, John Larkin >> <jla...@highlandSNIPMEtechnology.com> wrote: >> >> >Has anyone done this? I need to get the mounting screw height down. >> > >> >I guess I'll have to try it. >> I'm working from home today and I have a hand drill and a very rusty >> old countersink, a family heirloom. >> >> https://www.dropbox.com/scl/fi/y5s1bsc2omn341rnxkicq/TO247_countersunk_4-40.jpg?dl=0&rlkey=cqenr4ev8iic5zgujqor8cns1 >> >> This looks OK. The 4-40 screw should apply plenty of force and it >> centers the fet nicely. It would need locktite so thermal cycling >> doesn't loosen it up. > >I'd be worried about the wedge effect of the countersink > >maybe xray or decap a transistor to see how much room there is for a counter bore, I think I've seem some manufacturers say that a washer must be usedI'll break one open, but I don't think the hole is near the silicon. That epoxy is HARD. It was tough to countersink.
Reply by ●June 20, 20232023-06-20
On Tue, 20 Jun 2023 09:17:23 -0700, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:>On Tue, 20 Jun 2023 08:50:11 -0700 (PDT), Lasse Langwadt Christensen ><langwadt@fonz.dk> wrote: > >>mandag den 19. juni 2023 kl. 22.37.26 UTC+2 skrev John Larkin: >>> On Mon, 19 Jun 2023 08:49:52 -0700, John Larkin >>> <jla...@highlandSNIPMEtechnology.com> wrote: >>> >>> >Has anyone done this? I need to get the mounting screw height down. >>> > >>> >I guess I'll have to try it. >>> I'm working from home today and I have a hand drill and a very rusty >>> old countersink, a family heirloom. >>> >>> https://www.dropbox.com/scl/fi/y5s1bsc2omn341rnxkicq/TO247_countersunk_4-40.jpg?dl=0&rlkey=cqenr4ev8iic5zgujqor8cns1 >>> >>> This looks OK. The 4-40 screw should apply plenty of force and it >>> centers the fet nicely. It would need locktite so thermal cycling >>> doesn't loosen it up. >> >>I'd be worried about the wedge effect of the countersink >> >>maybe xray or decap a transistor to see how much room there is for a counter bore, I think I've seem some manufacturers say that a washer must be used > >I'll break one open, but I don't think the hole is near the silicon. > >That epoxy is HARD. It was tough to countersink.https://www.dropbox.com/s/3u6p3cfmndqbgvw/IRFP4321_decap.jpg?dl=0 https://www.dropbox.com/scl/fi/h7solf2qefsuronrn8quj/IRFP4321_Decap2.jpg?dl=0&rlkey=fkeuy62rgjfafyabfodxss6gh Looks safe to countersink.
Reply by ●June 20, 20232023-06-20
On 6/20/23 11:50 AM, Lasse Langwadt Christensen wrote:> mandag den 19. juni 2023 kl. 22.37.26 UTC+2 skrev John Larkin: >> On Mon, 19 Jun 2023 08:49:52 -0700, John Larkin >> <jla...@highlandSNIPMEtechnology.com> wrote: >> >>> Has anyone done this? I need to get the mounting screw height down. >>> >>> I guess I'll have to try it. >> I'm working from home today and I have a hand drill and a very rusty >> old countersink, a family heirloom. >> >> https://www.dropbox.com/scl/fi/y5s1bsc2omn341rnxkicq/TO247_countersunk_4-40.jpg?dl=0&rlkey=cqenr4ev8iic5zgujqor8cns1 >> >> This looks OK. The 4-40 screw should apply plenty of force and it >> centers the fet nicely. It would need locktite so thermal cycling >> doesn't loosen it up. > > I'd be worried about the wedge effect of the countersink > > maybe xray or decap a transistor to see how much room there is for a counter bore, I think I've seem some manufacturers say that a washer must be usedThermal expansion on the plastic is going to be what, 5-10x that of the metal screw (SS is best here) so each heat/cool cycle is going to mash the countersunk screw into the plastic a little more, then loosen it on cooling. It all depends on the flow characteristics of the plastic well below the yield point. Bolt a complete assembly together and cycle the fet power from zero to max with the fan on, holding at hot and cold for as long as you think appropriate, and rinse and repeat as many cycles as you can. Check the torque (either loosening or tightening) on one of the screws every 100 cycles, every 1000 cycles on the next screw, every 10,000 cycles on the next screw ... On second thought, log the fet temp at max power each cycle, you would probably see the temp rise before you find a screw loosening up. -- Regards, Carl
Reply by ●June 20, 20232023-06-20
tirsdag den 20. juni 2023 kl. 18.15.01 UTC+2 skrev John Larkin:> On Tue, 20 Jun 2023 08:48:06 -0700 (PDT), Lasse Langwadt Christensen > <lang...@fonz.dk> wrote: > > >mandag den 19. juni 2023 kl. 20.09.37 UTC+2 skrev John Larkin: > >> On Mon, 19 Jun 2023 10:32:14 -0700 (PDT), Lasse Langwadt Christensen > >> <lang...@fonz.dk> wrote: > >> > >> >mandag den 19. juni 2023 kl. 18.13.01 UTC+2 skrev John Larkin: > >> >> On Mon, 19 Jun 2023 08:57:14 -0700 (PDT), Lasse Langwadt Christensen > >> >> <lang...@fonz.dk> wrote: > >> >> > >> >> >mandag den 19. juni 2023 kl. 17.50.16 UTC+2 skrev John Larkin: > >> >> >> Has anyone done this? I need to get the mounting screw height down. > >> >> >> > >> >> >> I guess I'll have to try it. > >> >> > > >> >> >how low do you need? https://www.mcmaster.com/products/low-profile-head-socket-cap-screws/alloy-steel-ultra-low-profile-socket-head-screws/ > >> >> The heights are good, the prices aren't! > >> >> > >> >> A regular screw and a lockwasher cost me over 0.1", and I need less. > >> >> With locktite, I can eliminate the lockwasher. > >> >> > >> >> A really thin custom AlN insulator will help a bit. Isolated TO-247s > >> >> are expensive and hard to get. > >> >> > >> >> I could just epoxy the fets down, but that has downsides. > >> > > >> >maybe something like this? https://www.digikey.dk/da/products/detail/aavid-thermal-division-of-boyd-corporation/MAX08NG/1625322 > >> I've done that > >> > >> https://www.dropbox.com/s/nmyb9pz0qpma2xa/Amp.jpg?dl=0 > >> > >> but it adds more height. I'll have boards in a card cage on 1.6" > >> centers, and I'm stacking a CPU cooler/fan thing with giant mosfets. > >> The screw heads could hit adjacent boards. > > > >like this? https://www.dynatron.co/product-page/k199 > Yes, I'll probably use K199. It measures about 0.25 K/W. It's miles > better than machining a heat sink and adding a separate fan. It's > copper, too, a great heat spreader. > > > >can't just clamp the whole stack with the four screws already there? > > > > > I plan to remove the spring thingies and bolt the cooler hard to the > PCB, with the fets on the bottom side. We tried squashing the fets > metal-side-up, between the cooler and the board, and I didn't like > that. Fet contact force would be iffy and nothing would be > inspectable. The thermal grease situation would be ikky. > > With fets bolted on the bottom (with a big hole in the board) each fet > is accessable, including for temperature measurement and possible > rework.you have room on the top side? if you space out the cooler and mount the TO-247s through a hole with the legs surface mount they only stick up about 2.5mm
Reply by ●June 20, 20232023-06-20
On Tuesday, June 20, 2023 at 3:54:19 AM UTC-7, John Larkin wrote:> On Mon, 19 Jun 2023 22:54:50 -0700 (PDT), whit3rd <whi...@gmail.com> > wrote: > >On Monday, June 19, 2023 at 8:50:16?AM UTC-7, John Larkin wrote: > >> Has anyone done this? I need to get the mounting screw height down. > >> > >> I guess I'll have to try it. > > > >The hole is 3.5mm, so an M3 flat head cap screw with a countersinked washer would > >hold it down; here are 1.8mm thick offerings for M3 screws. Is 1.8mm too high? > > > ><https://www.mcmaster.com/products/countersunk-washers/> > That might work, depending on how deep I dare to countersink the > mosfet. The ideal height of the screw is zero.The countersink for M3 is 90 degrees, that for #4-40 is 82 degrees; with the right countersunk washers, head is flush with the washer top, so total height is the washer thickness. It does NOT need any modification of the MOSFET, it's just a washer with a flush-to-surface screw.
