countersinking a TO247 mosfet...

[John Larkin]

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.

 

[Joe Gwinn]

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

 

[Lasse Langwadt Christensen]

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?

 

[Lasse Langwadt Christensen]

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

 

[John Larkin]

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/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.

 

[John Larkin]

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.

 

[John Larkin]

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.

 

[Carl]

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.

--
Regards,
Carl

 

[Lasse Langwadt Christensen]

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

 

[whit3rd]

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.

 

[Chris Jones]

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.

 

[Joe Gwinn]

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

 

[John Larkin]

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.

 

[John Larkin]

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.

 

[Joe Gwinn]

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

 

[Phil Hobbs]

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

 

[John Larkin]

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

 

[John Larkin]

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 Hobbs

If we bolt the fets with plastic screws, they change from being
liabilities (shorting to the adjacent board) to benefits (insulating
standoffs!)

 

[Lasse Langwadt Christensen]

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

 

[Joe Gwinn]

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