countersinking a TO247 mosfet...

[Joe Gwinn]

On Sat, 24 Jun 2023 09:04:57 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

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 Gwinn

The 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.

Yes. How much thread depth is available? If there is enough copper,
then helicoils may not be needed.

With the four smaller holes, it would be one per countersunk FET
package, so no metallic protrusion? Might be a bit to close for
comfort.

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.

One big, thin AlN sheet may be a bit too easy to break in production.
Four FET-size little sheets may be easier.

Joe Gwinn

 

[Lasse Langwadt Christensen]

lørdag den 24. juni 2023 kl. 17.10.41 UTC+2 skrev John Larkin:

On Sat, 24 Jun 2023 21:38:20 +1000, Chris Jones
lugn...@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 <joeg...@comcast.net
wrote:

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.

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.

use a bar with a screw in the middle across each pair of fets,
that way the clamp can \"seesaw\" a bit for even clamping