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Electronics => Beginners => Topic started by: Dabbo56 on October 17, 2022, 06:30:29 pm

Title: How to fix TO-126 (a.k.a. SOT-32 a.k.a. TO-225AA) to heatsink without shorting
Post by: Dabbo56 on October 17, 2022, 06:30:29 pm
This is a small, through-hole transistor package. There is a hole for fixing to a heat sink. I'm using a 3mm bolt, having tapped a hole in the heat sink with my tap-and-die set, and using a mica insulator. BUT, I'm not happy because the metal collector pad is incredibly close to the fixing hole. Checking for connectivity between the heat sink and the collector I'm getting intermittent short, possibly due to barely visible swarf from the tapping process coming between the bolt and the collector pad. Is there an alternative e.g. plastic bolts? Am I using the right technique?
Title: Re: How to fix TO-126 (a.k.a. SOT-32 a.k.a. TO-225AA) to heatsink without shorting
Post by: Kleinstein on October 17, 2022, 06:38:18 pm
Normally there is sufficient insulation around the hole, but it may depend on the parts / manufactuer. A more flexible silicone pad instead of the thin mica plus thermal grease could also be an advantage, as under pressure it moves closer to the hole.  A clean surface of the heat sink may be needed to avoid damage to the insulator. It helps to add a bevel to the hole before tapping.
Title: Re: How to fix TO-126 (a.k.a. SOT-32 a.k.a. TO-225AA) to heatsink without shorting
Post by: Benta on October 17, 2022, 06:49:18 pm
Quote from: Kleinstein on October 17, 2022, 06:38:18 pm
A clean surface of the heat sink may be needed to avoid damage to the insulator. It helps to add a bevel to the hole before tapping.
+1
Swarf should not be there, sloppy machining/tapping.
Title: Re: How to fix TO-126 (a.k.a. SOT-32 a.k.a. TO-225AA) to heatsink without shorting
Post by: strawberry on October 17, 2022, 07:10:34 pm
dull drill bit presure lift edges around hole
Title: Re: How to fix TO-126 (a.k.a. SOT-32 a.k.a. TO-225AA) to heatsink without shorting
Post by: Benta on October 17, 2022, 08:47:00 pm
Quote from: strawberry on October 17, 2022, 07:10:34 pm
dull drill bit presure lift edges around hole
Doesn't matter. A hole should always be bevelled/chamfered. Anything else is sloppy work.
Title: Re: How to fix TO-126 (a.k.a. SOT-32 a.k.a. TO-225AA) to heatsink without shorting
Post by: Dabbo56 on October 17, 2022, 08:59:06 pm
That’s the answer :-+
Title: Re: How to fix TO-126 (a.k.a. SOT-32 a.k.a. TO-225AA) to heatsink without shorting
Post by: David Hess on October 18, 2022, 06:48:19 pm
Quote from: Dabbo56 on October 17, 2022, 06:30:29 pm
I'm using a 3mm bolt, having tapped a hole in the heat sink with my tap-and-die set, and using a mica insulator.

Threading and attaching directly to the heat sink is not recommended because it distorts the surface making it difficult to get good contact and risking damage to the insulator.

Title: Re: How to fix TO-126 (a.k.a. SOT-32 a.k.a. TO-225AA) to heatsink without shorting
Post by: Benta on October 18, 2022, 07:33:50 pm
Quote from: David Hess on October 18, 2022, 06:48:19 pm
Quote from: Dabbo56 on October 17, 2022, 06:30:29 pm
I'm using a 3mm bolt, having tapped a hole in the heat sink with my tap-and-die set, and using a mica insulator.

Threading and attaching directly to the heat sink is not recommended because it distorts the surface making it difficult to get good contact and risking damage to the insulator.
Which chamfering the hole fixes. Works for engine blocks, gearboxes, all kinds of precision mechanics. And yes, also for power transistors on a heat sink.
Title: Re: How to fix TO-126 (a.k.a. SOT-32 a.k.a. TO-225AA) to heatsink without shorting
Post by: David Hess on October 19, 2022, 12:09:58 am
The old classic Motorola application note 1040 (https://www.nxp.com/files-static/rf_if/doc/app_note/AN1040.pdf) has quite a bit to say about TO-225 mounting.