On Thu, 22 Jun 2023 07:56:35 -0700, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:>On Thu, 22 Jun 2023 10:52:57 -0400, Joe Gwinn <joegwinn@comcast.net> >wrote: > >>On Tue, 20 Jun 2023 09:33:56 -0400, Joe Gwinn <joegwinn@comcast.net> >>wrote: >> >>>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/> >> >>An afterthought: Solid rivets expand when set, and it's not clear >>that the semiconductor package can handle the resulting radial stress. >> >>Joe Gwinn > >I plan to blind tap four holes into the copper CPU cooler. There's not >much thickness until the tap would hit fins. I don't know how a rivet >would anchor into the cooler, and replacing a fet would be nasty.Yeah. Can't say that I am able to visualize the arrangement. So I was pointing out possibly relevant options, fasteners with 100-degree cone heads. Is it the hope that this screw into the copper heat sink be non-metallic? It might be simpler to use a steel screw into copper, insulated from the fet package. Or press-fit a delrin plug into a roughened hole in the copper, and run the metal screw into an undersize blind hole in the delrin plug. Joe Gwinn
countersinking a TO247 mosfet
Started by ●June 19, 2023
Reply by ●June 23, 20232023-06-23
Reply by ●June 23, 20232023-06-23
lørdag den 24. juni 2023 kl. 00.12.47 UTC+2 skrev Joe Gwinn:> On Thu, 22 Jun 2023 07:56:35 -0700, John Larkin > <jla...@highlandSNIPMEtechnology.com> wrote: > > >On Thu, 22 Jun 2023 10:52:57 -0400, Joe Gwinn <joeg...@comcast.net> > >wrote: > > > >>On Tue, 20 Jun 2023 09:33:56 -0400, Joe Gwinn <joeg...@comcast.net> > >>wrote: > >> > >>>On Tue, 20 Jun 2023 03:46:47 -0700, John Larkin > >>><jla...@highlandSNIPMEtechnology.com> wrote: > >>> > >>>>On Mon, 19 Jun 2023 18:57:31 -0700 (PDT), Chris <chri...@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/> > >> > >>An afterthought: Solid rivets expand when set, and it's not clear > >>that the semiconductor package can handle the resulting radial stress. > >> > >>Joe Gwinn > > > >I plan to blind tap four holes into the copper CPU cooler. There's not > >much thickness until the tap would hit fins. I don't know how a rivet > >would anchor into the cooler, and replacing a fet would be nasty. > Yeah. Can't say that I am able to visualize the arrangement. So I > was pointing out possibly relevant options, fasteners with 100-degree > cone heads. > > Is it the hope that this screw into the copper heat sink be > non-metallic? > > It might be simpler to use a steel screw into copper, insulated from > the fet package. > > Or press-fit a delrin plug into a roughened hole in the copper, and > run the metal screw into an undersize blind hole in the delrin plug.the hole in a TO-247 is already isolated
Reply by ●June 24, 20232023-06-24
threaded insert, perhaps? Avoids using a nut. Put the screw in from under the PCB. https://www.mcmaster.com/products/threaded-inserts/18-8-stainless-steel-flanged-screw-to-expand-inserts-for-plastic/ May need to open up the hole in the TO-247 to 5/32", to use a 4-40 insert.
Reply by ●June 24, 20232023-06-24
On Fri, 23 Jun 2023 18:12:33 -0400, Joe Gwinn <joegwinn@comcast.net> wrote:>On Thu, 22 Jun 2023 07:56:35 -0700, John Larkin ><jlarkin@highlandSNIPMEtechnology.com> wrote: > >>On Thu, 22 Jun 2023 10:52:57 -0400, Joe Gwinn <joegwinn@comcast.net> >>wrote: >> >>>On Tue, 20 Jun 2023 09:33:56 -0400, Joe Gwinn <joegwinn@comcast.net> >>>wrote: >>> >>>>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/> >>> >>>An afterthought: Solid rivets expand when set, and it's not clear >>>that the semiconductor package can handle the resulting radial stress. >>> >>>Joe Gwinn >> >>I plan to blind tap four holes into the copper CPU cooler. There's not >>much thickness until the tap would hit fins. I don't know how a rivet >>would anchor into the cooler, and replacing a fet would be nasty. > >Yeah. Can't say that I am able to visualize the arrangement. So I >was pointing out possibly relevant options, fasteners with 100-degree >cone heads.Card cage: https://www.dropbox.com/s/gr57bhafemypi63/P940_box_9.jpg?dl=0 Bottom of board, with fets scrunched down to the CPU cooler. https://www.dropbox.com/scl/fi/gsbc9o7cwc0o3rwfbrhr4/K199_Fets_Pusher.jpg?dl=0&rlkey=fzp5oylmb7ounxjhozu6k8j26 That hardware is cute but too tall, hence the idea of using four flathead screws and countersinking the mosfets.> >Is it the hope that this screw into the copper heat sink be >non-metallic?My concern is that the transistor mounting screws could short to the solder side of the adjacent board in the card cage, and I'm tight on spacing. If I don't c'sink the fets and use a plastic screw, it becomes a spacer, not a short! Zero clerance to the next board is no problem. I've ordered some plastic screws to see how they feel. Apparently PEEK would be the best material, but they are like $5 each, so I'd prefer something else.> >It might be simpler to use a steel screw into copper, insulated from >the fet package.Insulation is not a problem with TO247's.> >Or press-fit a delrin plug into a roughened hole in the copper, and >run the metal screw into an undersize blind hole in the delrin plug. > >Joe Gwinn
Reply by ●June 24, 20232023-06-24
On 24/06/2023 9:20 pm, John Larkin wrote:> On Fri, 23 Jun 2023 18:12:33 -0400, Joe Gwinn <joegwinn@comcast.net> > wrote: > >> On Thu, 22 Jun 2023 07:56:35 -0700, John Larkin >> <jlarkin@highlandSNIPMEtechnology.com> wrote: >> >>> On Thu, 22 Jun 2023 10:52:57 -0400, Joe Gwinn <joegwinn@comcast.net> >>> wrote: >>> >>>> On Tue, 20 Jun 2023 09:33:56 -0400, Joe Gwinn <joegwinn@comcast.net> >>>> wrote: >>>> >>>>> 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/> >>>> >>>> An afterthought: Solid rivets expand when set, and it's not clear >>>> that the semiconductor package can handle the resulting radial stress. >>>> >>>> Joe Gwinn >>> >>> I plan to blind tap four holes into the copper CPU cooler. There's not >>> much thickness until the tap would hit fins. I don't know how a rivet >>> would anchor into the cooler, and replacing a fet would be nasty. >> >> Yeah. Can't say that I am able to visualize the arrangement. So I >> was pointing out possibly relevant options, fasteners with 100-degree >> cone heads. > > Card cage: > > https://www.dropbox.com/s/gr57bhafemypi63/P940_box_9.jpg?dl=0 > > Bottom of board, with fets scrunched down to the CPU cooler. > > https://www.dropbox.com/scl/fi/gsbc9o7cwc0o3rwfbrhr4/K199_Fets_Pusher.jpg?dl=0&rlkey=fzp5oylmb7ounxjhozu6k8j26 > > That hardware is cute but too tall, hence the idea of using four > flathead screws and countersinking the mosfets.What if you countersink the washer? If instead of a washer you use a plate with recesses machined into it for the TO-247s, the un-machined parts would stiffen it. Also more than one screw would vastly decrease the required stiffness. You could glue or (otherwise secure) a piece of thin FR4 over the whole thing to provide durable insulation from the next card. If you have sufficient clearance on the other side of the card, then you could put some washers between the heatsink and your board, which would give more clearance on the side with the mosfet pusher.
Reply by ●June 24, 20232023-06-24
On Sat, 24 Jun 2023 21:38:20 +1000, Chris Jones <lugnut808@spam.yahoo.com> wrote:>On 24/06/2023 9:20 pm, John Larkin wrote: >> On Fri, 23 Jun 2023 18:12:33 -0400, Joe Gwinn <joegwinn@comcast.net> >> wrote: >> >>> On Thu, 22 Jun 2023 07:56:35 -0700, John Larkin >>> <jlarkin@highlandSNIPMEtechnology.com> wrote: >>> >>>> On Thu, 22 Jun 2023 10:52:57 -0400, Joe Gwinn <joegwinn@comcast.net> >>>> wrote: >>>> >>>>> On Tue, 20 Jun 2023 09:33:56 -0400, Joe Gwinn <joegwinn@comcast.net> >>>>> wrote: >>>>> >>>>>> 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/> >>>>> >>>>> An afterthought: Solid rivets expand when set, and it's not clear >>>>> that the semiconductor package can handle the resulting radial stress. >>>>> >>>>> Joe Gwinn >>>> >>>> I plan to blind tap four holes into the copper CPU cooler. There's not >>>> much thickness until the tap would hit fins. I don't know how a rivet >>>> would anchor into the cooler, and replacing a fet would be nasty. >>> >>> Yeah. Can't say that I am able to visualize the arrangement. So I >>> was pointing out possibly relevant options, fasteners with 100-degree >>> cone heads. >> >> Card cage: >> >> https://www.dropbox.com/s/gr57bhafemypi63/P940_box_9.jpg?dl=0 >> >> Bottom of board, with fets scrunched down to the CPU cooler. >> >> https://www.dropbox.com/scl/fi/gsbc9o7cwc0o3rwfbrhr4/K199_Fets_Pusher.jpg?dl=0&rlkey=fzp5oylmb7ounxjhozu6k8j26 >> >> That hardware is cute but too tall, hence the idea of using four >> flathead screws and countersinking the mosfets. > >What if you countersink the washer?There's a neoprene washer under the metal one, so it's still kinda high. And I'd still have metal about to touch the next board. But that idea suggests something: Use a giant plastic washer for the pusher, something that flexes a bit and avoids the 4-leg-table issue, c'sink that, and use a plastic screw. Interesting idea.> >If instead of a washer you use a plate with recesses machined into it >for the TO-247s, the un-machined parts would stiffen it. Also more than >one screw would vastly decrease the required stiffness. > >You could glue or (otherwise secure) a piece of thin FR4 over the whole >thing to provide durable insulation from the next card. > >If you have sufficient clearance on the other side of the card, then you >could put some washers between the heatsink and your board, which would >give more clearance on the side with the mosfet pusher. >
Reply by ●June 24, 20232023-06-24
On Saturday, June 24, 2023 at 4:21:09 AM UTC-7, John Larkin wrote:> On Fri, 23 Jun 2023 18:12:33 -0400, Joe Gwinn <joeg...@comcast.net> > wrote: >> >Is it the hope that this screw into the copper heat sink be > >non-metallic? > My concern is that the transistor mounting screws could short to the > solder side of the adjacent board in the card cage, and I'm tight on > spacing. If I don't c'sink the fets and use a plastic screw, it > becomes a spacer, not a short! Zero clerance to the next board is no > problem.Shorting is only a problem where the 'next board' is too close? Can you put a few strategically placed holes in that 'next board' so that a sticking-up screwhead avoids contact by going into a void?
Reply by ●June 24, 20232023-06-24
On Sat, 24 Jun 2023 08:10:16 -0700, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:>On Sat, 24 Jun 2023 21:38:20 +1000, Chris Jones ><lugnut808@spam.yahoo.com> wrote: > >>On 24/06/2023 9:20 pm, John Larkin wrote: >>> On Fri, 23 Jun 2023 18:12:33 -0400, Joe Gwinn <joegwinn@comcast.net> >>> wrote: >>> >>>> On Thu, 22 Jun 2023 07:56:35 -0700, John Larkin >>>> <jlarkin@highlandSNIPMEtechnology.com> wrote: >>>> >>>>> On Thu, 22 Jun 2023 10:52:57 -0400, Joe Gwinn <joegwinn@comcast.net> >>>>> wrote: >>>>> >>>>>> On Tue, 20 Jun 2023 09:33:56 -0400, Joe Gwinn <joegwinn@comcast.net> >>>>>> wrote: >>>>>> >>>>>>> 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/> >>>>>> >>>>>> An afterthought: Solid rivets expand when set, and it's not clear >>>>>> that the semiconductor package can handle the resulting radial stress. >>>>>> >>>>>> Joe Gwinn >>>>> >>>>> I plan to blind tap four holes into the copper CPU cooler. There's not >>>>> much thickness until the tap would hit fins. I don't know how a rivet >>>>> would anchor into the cooler, and replacing a fet would be nasty. >>>> >>>> Yeah. Can't say that I am able to visualize the arrangement. So I >>>> was pointing out possibly relevant options, fasteners with 100-degree >>>> cone heads. >>> >>> Card cage: >>> >>> https://www.dropbox.com/s/gr57bhafemypi63/P940_box_9.jpg?dl=0 >>> >>> Bottom of board, with fets scrunched down to the CPU cooler. >>> >>> https://www.dropbox.com/scl/fi/gsbc9o7cwc0o3rwfbrhr4/K199_Fets_Pusher.jpg?dl=0&rlkey=fzp5oylmb7ounxjhozu6k8j26 >>> >>> That hardware is cute but too tall, hence the idea of using four >>> flathead screws and countersinking the mosfets. >> >>What if you countersink the washer? > >There's a neoprene washer under the metal one, so it's still kinda >high. And I'd still have metal about to touch the next board. > >But that idea suggests something: Use a giant plastic washer for the >pusher, something that flexes a bit and avoids the 4-leg-table issue, >c'sink that, and use a plastic screw. > >Interesting idea. > > > >> >>If instead of a washer you use a plate with recesses machined into it >>for the TO-247s, the un-machined parts would stiffen it. Also more than >>one screw would vastly decrease the required stiffness. >> >>You could glue or (otherwise secure) a piece of thin FR4 over the whole >>thing to provide durable insulation from the next card. >> >>If you have sufficient clearance on the other side of the card, then you >>could put some washers between the heatsink and your board, which would >>give more clearance on the side with the mosfet pusher.Delrin might work for that big washer. In the space between the FET packages there is added depth available, enough that an ordinary flat-head steel screw would fit, and could be screwed directly into the copper heat sink, perhaps with a heli-coil (pure copper is pretty soft, so an insert may be necessary). Joe Gwinn
Reply by ●June 24, 20232023-06-24
On Sat, 24 Jun 2023 11:44:44 -0400, Joe Gwinn <joegwinn@comcast.net> wrote:>On Sat, 24 Jun 2023 08:10:16 -0700, John Larkin ><jlarkin@highlandSNIPMEtechnology.com> wrote: > >>On Sat, 24 Jun 2023 21:38:20 +1000, Chris Jones >><lugnut808@spam.yahoo.com> wrote: >> >>>On 24/06/2023 9:20 pm, John Larkin wrote: >>>> On Fri, 23 Jun 2023 18:12:33 -0400, Joe Gwinn <joegwinn@comcast.net> >>>> wrote: >>>> >>>>> On Thu, 22 Jun 2023 07:56:35 -0700, John Larkin >>>>> <jlarkin@highlandSNIPMEtechnology.com> wrote: >>>>> >>>>>> On Thu, 22 Jun 2023 10:52:57 -0400, Joe Gwinn <joegwinn@comcast.net> >>>>>> wrote: >>>>>> >>>>>>> On Tue, 20 Jun 2023 09:33:56 -0400, Joe Gwinn <joegwinn@comcast.net> >>>>>>> wrote: >>>>>>> >>>>>>>> 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/> >>>>>>> >>>>>>> An afterthought: Solid rivets expand when set, and it's not clear >>>>>>> that the semiconductor package can handle the resulting radial stress. >>>>>>> >>>>>>> Joe Gwinn >>>>>> >>>>>> I plan to blind tap four holes into the copper CPU cooler. There's not >>>>>> much thickness until the tap would hit fins. I don't know how a rivet >>>>>> would anchor into the cooler, and replacing a fet would be nasty. >>>>> >>>>> Yeah. Can't say that I am able to visualize the arrangement. So I >>>>> was pointing out possibly relevant options, fasteners with 100-degree >>>>> cone heads. >>>> >>>> Card cage: >>>> >>>> https://www.dropbox.com/s/gr57bhafemypi63/P940_box_9.jpg?dl=0 >>>> >>>> Bottom of board, with fets scrunched down to the CPU cooler. >>>> >>>> https://www.dropbox.com/scl/fi/gsbc9o7cwc0o3rwfbrhr4/K199_Fets_Pusher.jpg?dl=0&rlkey=fzp5oylmb7ounxjhozu6k8j26 >>>> >>>> That hardware is cute but too tall, hence the idea of using four >>>> flathead screws and countersinking the mosfets. >>> >>>What if you countersink the washer? >> >>There's a neoprene washer under the metal one, so it's still kinda >>high. And I'd still have metal about to touch the next board. >> >>But that idea suggests something: Use a giant plastic washer for the >>pusher, something that flexes a bit and avoids the 4-leg-table issue, >>c'sink that, and use a plastic screw. >> >>Interesting idea. >> >> >> >>> >>>If instead of a washer you use a plate with recesses machined into it >>>for the TO-247s, the un-machined parts would stiffen it. Also more than >>>one screw would vastly decrease the required stiffness. >>> >>>You could glue or (otherwise secure) a piece of thin FR4 over the whole >>>thing to provide durable insulation from the next card. >>> >>>If you have sufficient clearance on the other side of the card, then you >>>could put some washers between the heatsink and your board, which would >>>give more clearance on the side with the mosfet pusher. > >Delrin might work for that big washer. In the space between the FET >packages there is added depth available, enough that an ordinary >flat-head steel screw would fit, and could be screwed directly into >the copper heat sink, perhaps with a heli-coil (pure copper is pretty >soft, so an insert may be necessary). > >Joe GwinnThe CPU cooler in the pic is tapped for the single big screw. My machinist guy says we can tap the copper cooler, and he did that one. We could also tap four smaller holes. I do need an AlN insulator under the fets, probably one big thin custom one with four holes. I figure that will add a trivial thermal resistance, like 0.04 K/W per fet.
Reply by ●June 24, 20232023-06-24
On Sat, 24 Jun 2023 08:31:56 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:>On Saturday, June 24, 2023 at 4:21:09?AM UTC-7, John Larkin wrote: >> On Fri, 23 Jun 2023 18:12:33 -0400, Joe Gwinn <joeg...@comcast.net> >> wrote: >> > >> >Is it the hope that this screw into the copper heat sink be >> >non-metallic? >> My concern is that the transistor mounting screws could short to the >> solder side of the adjacent board in the card cage, and I'm tight on >> spacing. If I don't c'sink the fets and use a plastic screw, it >> becomes a spacer, not a short! Zero clerance to the next board is no >> problem. > >Shorting is only a problem where the 'next board' is too close? Can you >put a few strategically placed holes in that 'next board' so that a sticking-up >screwhead avoids contact by going into a void?There will be a lot of possible neighboring boards, at least 10 ultimately, some very dense, and it would be a nuisance to lay all of them out to avoid screw heads on this one. Plastic screws sound cool, if they work mechanically. I probably don't need a lot of hold-down force.
