On 20/06/2023 1:49 am, John Larkin wrote:> Has anyone done this? I need to get the mounting screw height down. > > I guess I'll have to try it. >I would get a piece of metal and use that as a clamp over the top of the FET package. The metal can be countersunk in regions either side of the FET but not over the FET package. If you want to get fancy you could start with a thick plate of metal, maybe stainless that is 1mm thicker than the transistor package, and have a recess machined in it for the FET package to fit into, and drill the screw holes either side of the recess andcountersink from the other side. You may say that this adds manufacturing cost, but you can easily outsource an order of metal clamps to be machined by the lowest bidder, whereas machining the FETs will not be a job many companies will want to do for you, and when they mess it up it will introduce delayed failures. Also when a countersunk FET blows, you can't just order a new FET and fit it, and the technician will have to not over-do the torque, whereas with a metal clamp you can order a standard FET and the torque will be non-critical.
countersinking a TO247 mosfet
Started by ●June 19, 2023
Reply by ●June 21, 20232023-06-21
Reply by ●June 21, 20232023-06-21
On Tue, 20 Jun 2023 14:42:50 -0400, Carl <carl.ijamesxx@yyverizon.net> wrote:>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 used > >Thermal 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.The classic solution is a small belleville washer. .<https://www.mcmaster.com/products/belleville-spring-washers/corrosion-resistant-belleville-disc-springs/> Or a wave wave-spring washer. .<https://www.mcmaster.com/products/disc-spring-washers/?s=wave+disc+spring+washers.> Joe Gwinn
Reply by ●June 21, 20232023-06-21
On Wed, 21 Jun 2023 21:43:31 +1000, Chris Jones <lugnut808@spam.yahoo.com> wrote:>On 20/06/2023 1:49 am, John Larkin wrote: >> Has anyone done this? I need to get the mounting screw height down. >> >> I guess I'll have to try it. >> > >I would get a piece of metal and use that as a clamp over the top of the >FET package. The metal can be countersunk in regions either side of the >FET but not over the FET package. If you want to get fancy you could >start with a thick plate of metal, maybe stainless that is 1mm thicker >than the transistor package, and have a recess machined in it for the >FET package to fit into, and drill the screw holes either side of the >recess andcountersink from the other side. You may say that this adds >manufacturing cost, but you can easily outsource an order of metal >clamps to be machined by the lowest bidder, whereas machining the FETs >will not be a job many companies will want to do for you, and when they >mess it up it will introduce delayed failures. Also when a countersunk >FET blows, you can't just order a new FET and fit it, and the technician >will have to not over-do the torque, whereas with a metal clamp you can >order a standard FET and the torque will be non-critical.My preferred idea was https://www.dropbox.com/scl/fi/gsbc9o7cwc0o3rwfbrhr4/K199_Fets_Pusher.jpg?dl=0&rlkey=fzp5oylmb7ounxjhozu6k8j26 where the metal washer has a rubber washer under it, to apply a uniform pressure to each fet. [1] The screw could be a flathead, but the stack is still too tall to work in our card cage and I don't want any risk of metal touching the solder side of an adjacent board. [2] Machining, even tricky stuff, is no problem for us. Nor is specifying procedures for manufacturing to follow, like fastener torques. We could have two stock numbers and corresponding bins, purchased fets and machined fets, with a drawing to define the machining. All that is standard operating procedures. Thanks for the suggestions. [1] Don't you hate it when a cafe table has four legs and it tilts all over the place and spills your coffee? [2] I wonder if there are any strong non-conductive screws. It wouldn't matter if they had zero clearance to the next board in the crate. Nylon would be too wimpy to scrunch down my mosfets.
Reply by ●June 21, 20232023-06-21
On Tue, 20 Jun 2023 14:42:50 -0400, Carl <carl.ijamesxx@yyverizon.net> wrote:>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 used > >Thermal 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.The stuff a TO-247 is made of is rock-hard. It was really tough to countersink with a hand drill. It's sure not a thermoplastic. A flathead would need some repairable locktite or even an exotic countersink-type lockwasher, which would add height.
Reply by ●June 21, 20232023-06-21
On Wed, 21 Jun 2023 07:44:00 -0700, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:>On Wed, 21 Jun 2023 21:43:31 +1000, Chris Jones ><lugnut808@spam.yahoo.com> wrote: > >>On 20/06/2023 1:49 am, John Larkin wrote: >>> Has anyone done this? I need to get the mounting screw height down. >>> >>> I guess I'll have to try it. >>> >> >>I would get a piece of metal and use that as a clamp over the top of the >>FET package. The metal can be countersunk in regions either side of the >>FET but not over the FET package. If you want to get fancy you could >>start with a thick plate of metal, maybe stainless that is 1mm thicker >>than the transistor package, and have a recess machined in it for the >>FET package to fit into, and drill the screw holes either side of the >>recess andcountersink from the other side. You may say that this adds >>manufacturing cost, but you can easily outsource an order of metal >>clamps to be machined by the lowest bidder, whereas machining the FETs >>will not be a job many companies will want to do for you, and when they >>mess it up it will introduce delayed failures. Also when a countersunk >>FET blows, you can't just order a new FET and fit it, and the technician >>will have to not over-do the torque, whereas with a metal clamp you can >>order a standard FET and the torque will be non-critical. > >My preferred idea was > >https://www.dropbox.com/scl/fi/gsbc9o7cwc0o3rwfbrhr4/K199_Fets_Pusher.jpg?dl=0&rlkey=fzp5oylmb7ounxjhozu6k8j26 > >where the metal washer has a rubber washer under it, to apply a >uniform pressure to each fet. [1] The screw could be a flathead, but >the stack is still too tall to work in our card cage and I don't want >any risk of metal touching the solder side of an adjacent board. [2] > >Machining, even tricky stuff, is no problem for us. Nor is specifying >procedures for manufacturing to follow, like fastener torques. > >We could have two stock numbers and corresponding bins, purchased fets >and machined fets, with a drawing to define the machining. All that is >standard operating procedures. > >Thanks for the suggestions. > >[1] Don't you hate it when a cafe table has four legs and it tilts all >over the place and spills your coffee?It's worse with beer.>[2] I wonder if there are any strong non-conductive screws. It >wouldn't matter if they had zero clearance to the next board in the >crate. Nylon would be too wimpy to scrunch down my mosfets.Well, there is PEEK, but it's like $5 per screw. .<https://www.mcmaster.com/products/screws/high-strength-high-temperature-peek-slotted-flat-head-screws/?s=plastic+screws> With plastic, beware creep. Joe Gwinn
Reply by ●June 21, 20232023-06-21
John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:><snip>> > [2] I wonder if there are any strong non-conductive screws. It > wouldn't matter if they had zero clearance to the next board in the > crate. Nylon would be too wimpy to scrunch down my mosfets. > > >Amazon bought out Small Parts Inc some years back, and still sells things like Delrin screws, way cheaper than elsewhere. Dunno if it’s all NOS or if they’re still making them. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
Reply by ●June 21, 20232023-06-21
On Wed, 21 Jun 2023 11:39:41 -0400, Joe Gwinn <joegwinn@comcast.net> wrote:>On Wed, 21 Jun 2023 07:44:00 -0700, John Larkin ><jlarkin@highlandSNIPMEtechnology.com> wrote: > >>On Wed, 21 Jun 2023 21:43:31 +1000, Chris Jones >><lugnut808@spam.yahoo.com> wrote: >> >>>On 20/06/2023 1:49 am, John Larkin wrote: >>>> Has anyone done this? I need to get the mounting screw height down. >>>> >>>> I guess I'll have to try it. >>>> >>> >>>I would get a piece of metal and use that as a clamp over the top of the >>>FET package. The metal can be countersunk in regions either side of the >>>FET but not over the FET package. If you want to get fancy you could >>>start with a thick plate of metal, maybe stainless that is 1mm thicker >>>than the transistor package, and have a recess machined in it for the >>>FET package to fit into, and drill the screw holes either side of the >>>recess andcountersink from the other side. You may say that this adds >>>manufacturing cost, but you can easily outsource an order of metal >>>clamps to be machined by the lowest bidder, whereas machining the FETs >>>will not be a job many companies will want to do for you, and when they >>>mess it up it will introduce delayed failures. Also when a countersunk >>>FET blows, you can't just order a new FET and fit it, and the technician >>>will have to not over-do the torque, whereas with a metal clamp you can >>>order a standard FET and the torque will be non-critical. >> >>My preferred idea was >> >>https://www.dropbox.com/scl/fi/gsbc9o7cwc0o3rwfbrhr4/K199_Fets_Pusher.jpg?dl=0&rlkey=fzp5oylmb7ounxjhozu6k8j26 >> >>where the metal washer has a rubber washer under it, to apply a >>uniform pressure to each fet. [1] The screw could be a flathead, but >>the stack is still too tall to work in our card cage and I don't want >>any risk of metal touching the solder side of an adjacent board. [2] >> >>Machining, even tricky stuff, is no problem for us. Nor is specifying >>procedures for manufacturing to follow, like fastener torques. >> >>We could have two stock numbers and corresponding bins, purchased fets >>and machined fets, with a drawing to define the machining. All that is >>standard operating procedures. >> >>Thanks for the suggestions. >> >>[1] Don't you hate it when a cafe table has four legs and it tilts all >>over the place and spills your coffee? > >It's worse with beer.Keep both elbows on the bar.> > >>[2] I wonder if there are any strong non-conductive screws. It >>wouldn't matter if they had zero clearance to the next board in the >>crate. Nylon would be too wimpy to scrunch down my mosfets. > >Well, there is PEEK, but it's like $5 per screw. > >.<https://www.mcmaster.com/products/screws/high-strength-high-temperature-peek-slotted-flat-head-screws/?s=plastic+screws> > >With plastic, beware creep. > >Joe Gwinn
Reply by ●June 21, 20232023-06-21
On Wed, 21 Jun 2023 16:06:13 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote: >> ><snip> >> >> [2] I wonder if there are any strong non-conductive screws. It >> wouldn't matter if they had zero clearance to the next board in the >> crate. Nylon would be too wimpy to scrunch down my mosfets. >> >> >> >Amazon bought out Small Parts Inc some years back, and still sells things >like Delrin screws, way cheaper than elsewhere. > >Dunno if it�s all NOS or if they�re still making them. > >Cheers > >Phil HobbsIf we bolt the fets with plastic screws, they change from being liabilities (shorting to the adjacent board) to benefits (insulating standoffs!)
Reply by ●June 21, 20232023-06-21
onsdag den 21. juni 2023 kl. 16.44.24 UTC+2 skrev John Larkin:> On Wed, 21 Jun 2023 21:43:31 +1000, Chris Jones > <lugn...@spam.yahoo.com> wrote: > > >On 20/06/2023 1:49 am, John Larkin wrote: > >> Has anyone done this? I need to get the mounting screw height down. > >> > >> I guess I'll have to try it. > >> > > > >I would get a piece of metal and use that as a clamp over the top of the > >FET package. The metal can be countersunk in regions either side of the > >FET but not over the FET package. If you want to get fancy you could > >start with a thick plate of metal, maybe stainless that is 1mm thicker > >than the transistor package, and have a recess machined in it for the > >FET package to fit into, and drill the screw holes either side of the > >recess andcountersink from the other side. You may say that this adds > >manufacturing cost, but you can easily outsource an order of metal > >clamps to be machined by the lowest bidder, whereas machining the FETs > >will not be a job many companies will want to do for you, and when they > >mess it up it will introduce delayed failures. Also when a countersunk > >FET blows, you can't just order a new FET and fit it, and the technician > >will have to not over-do the torque, whereas with a metal clamp you can > >order a standard FET and the torque will be non-critical. > My preferred idea was > > https://www.dropbox.com/scl/fi/gsbc9o7cwc0o3rwfbrhr4/K199_Fets_Pusher.jpg?dl=0&rlkey=fzp5oylmb7ounxjhozu6k8j26 > > where the metal washer has a rubber washer under it, to apply a > uniform pressure to each fet. [1] The screw could be a flathead, but > the stack is still too tall to work in our card cage and I don't want > any risk of metal touching the solder side of an adjacent board. [2]if you have space on top you could bend the legs on the fet and push them further through the board
Reply by ●June 21, 20232023-06-21
On Wed, 21 Jun 2023 16:06:13 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote: >> ><snip> >> >> [2] I wonder if there are any strong non-conductive screws. It >> wouldn't matter if they had zero clearance to the next board in the >> crate. Nylon would be too wimpy to scrunch down my mosfets. >> >> >> >Amazon bought out Small Parts Inc some years back, and still sells things >like Delrin screws, way cheaper than elsewhere. > >Dunno if it�s all NOS or if they�re still making them..<http://dmp-ortho.com/products-catalog/acetal-screws/> Acetal is very slippery, so it may be difficult to prevent such a screw from working loose. But acetal does not creep under steady load. Joe Gwinn
