Mica and Goop

[AndrewT]

My dear companions,
sorry, but it seems you complicate a little bit. Why you simply don't take a bigger screw, f.E. M4 (4 mm) and the torque-problem for TO-247 transistors is almost solved?

you can't do as you suggest, because they won't fit.
To3p takes M3. 1/8th almost passes through, but because the hole is tapered it is likely to foul.
To247 and To264 take M3.5
None take M4

 

[nicoch58]

just drill ....some do it

 

[wrenchone]

I still live in the land of inches, etc., so the TO-220 takes a 4-40 screw, and the TO-247 and TO-3 take a 6-32. I have used a few TO3-P packages (I avoid them when I can). The ones with plastic over the tab do want an M3 screw. The original devices with exposed metal tabs (like the old TI and Motorola/On packages) take TO-3 insulating washers. I have opened up the holes in the plastic TO-3P packages for a 6-32 when I was in a hurry, but otherwise I dutifully bought an M3 tap and drill. A lot of the devices I used in the TO-3P package (TIP142/147 are examples) have been migrated to the TO-247 package, so I can just use 6-32 mounting screws.

 

[ma_coule]

you can't do as you suggest, because they won't fit.
To3p takes M3. 1/8th almost passes through, but because the hole is tapered it is likely to foul.
To247 and To264 take M3.5
None take M4

Dear AndrewT,
first of all, my great respect to your knowledge and your work on diyAudio!
Otherwise, as nicoch58 said: "just drill ....some do it". I've done so few days ago with my ACA amp, no problem at all - until now...
Regards,

 

[DPH]

Bike maintenance nerd rant (where people break bolted joints all the time):

Just make sure you guys lube the threads before torquing (and reduce the amount of torque needed substantially), as that minimizes joint-to-joint variation in running torque. Blindly following a torque wrench is as good a way to overtension or undertension a screw joint.

Or just lube the threads and make sure it's snug but stick with the two finger rule (use only two fingers to apply force on the Allen wrench). All we're trying to do is make intimate contact between the transistor and the heatsink, so just a quick cinch.

 

[anatech]

Hi wrenchone,

I nailed down a Tohnichi adjustable torque driver last night on E-prey. We'll see how that works out.

That's what I use (the brand). Excellent tools! Have you seen the list prices on these?

-Chris

 

[wrenchone]

I am now the proud (?) owner of 10 tubes of Thermalloy Thermalcote II, the older version of Aavid's non silicone thermal paste. It's a bit blue, and it's a lot of goo...

 

[anatech]

Ever consider taking up oil painting?

But seriously, congratulations! Good score. One of these days I'm going to have to find some of that stuff and give it a try. Know anyone who regularly sells that stuff?

-Chris

 

[wrenchone]

The same E-prey seller probably has more. I found it searching for "Aavid Thermal Compound". Both Digi-Key and Mouser sell the current Wakefield non-silicone. The price on the Ebay stuff was too low to resist, and I picked up some cheap IGBTs and trimpots along with it.

 

[wrenchone]

The new Tohnichi torque wrench will take some getting used to, as it gives an indication when the preset torque is reached, but does not slip, so you can still-over-torque if you don't heed the signal. I'll need to practice a bit.

 

[anatech]

Hmmm, mine breaks at the set torque so you can't over-tighten anything.

Still, awesome deal and certainly very useful tool. There are some Motorola and On-Semi app notes that list the proper torques for different packages.

-Chris

 

[wrenchone]

Been there for years...

 

[Bare]

Hmmm, mine breaks at the set torque so you can't over-tighten anything.

Still, awesome deal and certainly very useful tool. There are some Motorola and On-Semi app notes that list the proper torques for different packages.

-Chris

That! assumes :
A) the thing is working well enough to click off
B) it has 'some' accuracy

Saying this as I have a few TQ wrenches from my racing days.
Cheap clickers which quickly proved as unreliable utter Junk
My Decent Clicker wrenches cost several hundred $ each and require regular calibrations.
My Dirt cheap Beam type Tq wrenches, despite being almost No tech, are Much likelier to be accurate than even mid priced Clickers.. just saying

Although TBH I Can't believe one would need a Tq wrench for 4mm clamp down screw

 

[wrenchone]

The Tohnichi is not a "cheap clicker", and its specs say that it indiicates rather than slips at the correct torque level.

 

[anatech]

Hi Bare,
Try looking these up, then tell us what you think ... just saying.

You really do need to set the torque on transistors in order to get the heat transfer performance to the heat sink. Over-tightening a screw (M3 normally) will often distort the package or heat sink. It doesn't take much. Most people crank things down way to tightly though and I have seen warped packages. Some failures occur because the die detaches from the case partially due to the distortion in dimensions. Never mind those happy souls who strip the damn holes. No shortage of those folks.

-Chris

 

[farkincza74]

Hi,
Which best heat transfer, mica or silicon or ceramic?
Under a transistor in a hi-fi.

 

[anatech]

Hi farkincza74,
Depends on the application and cost factors. Then, the difference may make the decision unimportant.

I use mica and goop

-Chris

 

[Mark Johnson]

How about polishing the transistor and the heatsink with Simichrome or Flitz before mounting? Cleaned thoroughly with acetone and isopropyl after polishing, of course.

Applicator bit for Dremel tool

 

[plasnu]

How about bang the top of the transistor with a rubber hammer?
Please don't say no, because I just did it.

 

[anatech]

Hi Mark,
Actually, I have sanded the heat sink surface down when there is too much roughness. It works because you end up with much more intimate surface contact. Any surface roughness will place air between the two surfaces.

There are practical limits of course!

-Chris