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Question on TO-220 isolation kits....

guitar_199

guitar_199

Sr. Member
Mar 8, 2011
909
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Deer Park, TX
I can NOT find it !!!!!!! I saw one of Mike's (mikesradiorepair) videos where he did a long talk on the isolation kits for replacing TO-220 pkg finals.
I know that there are two types (for the most part) those that have mica insulators and those that have these rubberized silicone pads. If I recall correctly... Mike seemed to really like the rubberized silicone but I just found some materials doing a Google search that seem to indicate that the rubberized pads alone have about TWICE the thermal conduction of mica with thermal paste. And THAT sent me running to one of my favorite places.... RIGHT HERE!!!!!!

What say you here about this subject? Do you tend to go for the mica/paste kits? Or is the rubberized silicone good enough?

AS always... thank you in advance for any discussion at all!

[edited to add]
re-reading that ... it just didn't look right.
When I said that the rubberized silicone had twice the thermal conduction.. maybe I should have said HALF. What I saw essentially said that mica/paste had ONE capacity to dissipate heat...and the rubberized silicone was WORSE by a factor of two. The graph seemed to imply that the rubberized silicone could only conduct/sink temperature at HALF of the rate of mica/paste. Hope I am getting this across clearly.....

Bob
 
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Sil-Pads work best when the flat tab surface of the transistor is dead flat against the heat sink. A look inside switchmode power supplies reveals clamps that press against the plastic 'belly' of a TO-220 transistor rather than against the mounting hole.

RCI adopted a habit years ago of soldering power components like finals, regulator and modulator transistors to the circuit board first, and then bolting them all to the heat-sink surface. If the transistor leads aren't lapped onto the foil pads at the edge of the circuit board properly, the body of the transistor is pulled away from the heat sink surface. It also puts leverage against the brittle ceramic insulator at the top where the mount hole is. Insulating washers that crack around the mount hole are common for this reason in their radios. I have wondered if the heat-sink compound helps take up that angular gap a little.

Any effort to "upgrade" a radio to Sil-pads will involve unsoldering the transistor's leads from the circuit-board edge before bolting it down, and then soldering it back down.

I'm skeptical of using the mount hole of a tab-mount transistor with Sil-pads unless you're sure the transistor is held parallel to the heat-sink surface.

73
 
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Sil-Pads can work in a pinch, but that's what it will take to make it work in some Radios like the 29' or 25 series using that "accordion" cage...too much Pressure on a Pinch point - they will fail.

Not all of the mounting locations are "pressed/Punched" Flat and Square...

Many MOSFET's will "Deform" and fail because of the Transistor package stiffness and the contact surface of the Sil-Pads. The Sil-pads can PEFORATE when pressed hard against a curved surface by that TO-220.
  • Like the cage - especially the Final side, the bend regions is where this "cut" thru occurs - and short.
  • My repairs - more have been from the attempts to upgrade using Sil-pads
  • They shoudl have used MICA's
  • and NOT that PVC/EPDM and the Conductive Heat Sink Spreader Goo that's "GREY" in color - like Zinc-composition.
The TO-220 may be "deburred" but that doesn't mean it's edges aren't sharp.

I can't recommend them, MICA's work best from the standpoint of firmess and lack of deformation.
 
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nomadradio said:
Sil-Pads work best when the flat tab surface of the transistor is dead flat against the heat sink. I'm skeptical of using the mount hole of a tab-mount transistor with Sil-pads unless you're sure the transistor is held parallel to the heat-sink surface.73
Click to expand...

And I agree 100%. Anytime you have to change an insulator, whether it's mica, ceramic, or silicon, the transistor needs to be unsoldered before reinstalling it on the heatsink. Mica and ceramic insulators are ok, but they require heat sink compound which is toxic and a PITA. While the SIL-PADS are good, I personally use the Berquist type 800 insulating material. It has double-sided adhesive and doesn't require any mounting hardware. I've always recommended the Berquist material because it provides the BEST protection for the transistors or ICs. My 858 page is written for "experienced technicians" (like Nomad and H-ANDY) , and I assumed that people swapping the insulators would know how to do it the right way.
On newer transceivers where replacement parts are readily available, maybe the SIL-PADS are overkill. But on older rigs, like the 858s, they're needed to protect the OEM parts that are NLA.

- 399
 
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unit_399 said:
use the Berquist type 800 insulating material. It has double-sided adhesive
Click to expand...
unit_399
I've found myself in a position to have to remove 2 -T03 type transistors that I had previously mounted with the type 800 insulating material. Any tips on how to get the adhesive to release? I haven't tried any type of liquid (acetone, alcohol etc) to soften it up yet but am interested in any experiences you may have had.

73
David
 
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399 I went to Digi-Key and looked up the Bergquist 800 material and it looks like it is just a bulk sheet. Is that correct? Do you cut the pads yourself or are they pre-cut to a T0-220 size and you just peel them off?

Or another distinct possibility is...... I didn't find the right stuff!

If this is is.... do you bother with the holes for the mounting screw or is the adhesive enough to stay bonded?

Thanks!
Bob

https://www.digikey.com/en/products/detail/bergquist/BP800-0-005-00-1212/4866399

unit_399 said:
I personally use the Berquist type 800 insulating material. It has double-sided adhesive and doesn't require any mounting hardware. I've always recommended the Berquist material because it provides the BEST protection for the transistors or ICs.
Click to expand...
 
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Yes the 800 material comes in a 12" x 12" sheet. I use an Exacto knife to cut and trim it. The adhesive they use is extremely strong and I never use any mounting hardware. Good luck.

- 399

EDIT: When I first started using the type 800 material, I was concerned that the adhesive bond might loosen when the device got hot. So I bonded an old TO-220 transistor to a piece of aluminum and heated it with a heat gun until it was almost too hot to touch. The adhesive bond did not loosen up at all.
 
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Thank you sir.

B

unit_399 said:
Yes the 800 material comes in a 12" x 12" sheet. I use an Exacto knife to cut and trim it. The adhesive they use is extremely strong and I never use any mounting hardware. Good luck.

- 399

EDIT: When I first started using the type 800 material, I was concerned that the adhesive bond might loosen when the device got hot. So I bonded an old TO-220 transistor to a piece of aluminum and heated it with a heat gun until it was almost too hot to touch. The adhesive bond did not loosen up at all.
Click to expand...
 
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Personally, I like mica and compound. While the sil pads do a better job of dealing with imperfections in the two surfaces, when the job is critical, I'd rather lap the surfaces to a flat mirror like finish and use thin mica. Not all mica insulators are created equal. Cheap ones are often way thicker than needed and do not have a flat surface. Look at the reflection off the mica under bright light to spot waves or imperfections, before installing them.

In many cases you can do more to improve cooling by properly preparing the mating surfaces, than you can by using different style insulators.
 
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