Zen Mod, in another thread regarding replacing some of the wires in the Forte Model 3 that involved removing the PCBoards, recommended that the transistors be remounted to the heat sinks using Keratherm thermal material (available on the DIYAudio Store) instead of thermal "goop". He also recommended using large washers under the transistor mounting screws. I've purchased the Keratherm, but would appreciate a bit more clarity on the washers / screws.
The large transistors are 20 mm wide, and the small ones a bit over 10 mm. See attached photo of board / transistors that another owner took. How big should the washers be (i.e., diameter)? Also, why are such washers beneficial?
(All I could find re size detail when looking through Zen Mod's posts was a photo posted by another user who used washers that extended well past the sides of the transistors. See attached. Should they really be THAT big?)
Should the screws and washers be stainless steel or, instead, another material such as regular carbon steel or aluminum?
Should a lock washer be used between the screw head and the large washer? If so, does it matter whether it's a split washer or a tooth washer? See attached.
How tight should the screws be tightened? I have a torque wrench but it only goes down to 4 Nm, so without a smaller torque wrench how would I know when I have tightened the screws correctly?
Thank you!
The large transistors are 20 mm wide, and the small ones a bit over 10 mm. See attached photo of board / transistors that another owner took. How big should the washers be (i.e., diameter)? Also, why are such washers beneficial?
(All I could find re size detail when looking through Zen Mod's posts was a photo posted by another user who used washers that extended well past the sides of the transistors. See attached. Should they really be THAT big?)
Should the screws and washers be stainless steel or, instead, another material such as regular carbon steel or aluminum?
Should a lock washer be used between the screw head and the large washer? If so, does it matter whether it's a split washer or a tooth washer? See attached.
How tight should the screws be tightened? I have a torque wrench but it only goes down to 4 Nm, so without a smaller torque wrench how would I know when I have tightened the screws correctly?
Thank you!
Attachments
role of big washer is to spread pressure to bigger area of transistor, resulting in better contact across entire contact area
culprit - what you can often see, when soft thermal pads are used , is that side of transistor where hole is, is more sunk in that soft pad, transistor is not parallel to heatsink, sorta "inclined"
that in case of small washer or none
if you use bigger washer, amount of "incline" is smaller, and we think ( pray and hope) that thermal transfer is better that way, directly depending of pressure
split washer is there to prevent un-tightening of bolt due to thermal cycling; use split washer, sorta easier to find than tooth washer; if you can find these in quality, no prob - same good
and , of course, split washer goes to bolt head, then goes big washer (size - really biggest you can find, why not..... but not lesser than (say) 15mm Dia), then goes transistor, then goes thermal isolation pad, then goes heatsink
screw/bolt difference - beat me ...... I'm capable of remembering info from Wiki/whatever for 5 min straight, no more
just in case :
mica pads goes with goop
Alumina oxide pads goes with goop
Keratherm 86/82 goes without goop
soft Slilicone pads goes without goop ....... but these are good for nothing, when speaking of Papamps; use those just for looks and really mild heat
culprit - what you can often see, when soft thermal pads are used , is that side of transistor where hole is, is more sunk in that soft pad, transistor is not parallel to heatsink, sorta "inclined"
that in case of small washer or none
if you use bigger washer, amount of "incline" is smaller, and we think ( pray and hope) that thermal transfer is better that way, directly depending of pressure
split washer is there to prevent un-tightening of bolt due to thermal cycling; use split washer, sorta easier to find than tooth washer; if you can find these in quality, no prob - same good
and , of course, split washer goes to bolt head, then goes big washer (size - really biggest you can find, why not..... but not lesser than (say) 15mm Dia), then goes transistor, then goes thermal isolation pad, then goes heatsink
screw/bolt difference - beat me ...... I'm capable of remembering info from Wiki/whatever for 5 min straight, no more
just in case :
mica pads goes with goop
Alumina oxide pads goes with goop
Keratherm 86/82 goes without goop
soft Slilicone pads goes without goop ....... but these are good for nothing, when speaking of Papamps; use those just for looks and really mild heat
Thanks for the prompt response, Honorable Zen Mod, very helpful! A couple loose ends. Do you recommend stainless steel or something else? Maybe the tougher one: how to tell when the screw (yes, actually a bolt) is tightened enough but not too much?
steel - if you can get screws in proper quality, use those
if you have a suspicion that screws you can buy are made of old nails, go buy SS ones - they're usually softer than proper 8.8 hardware, but at least they're undoubtedly better than toy screws
torque - prescribed torque for M3 is 0.9Nm
if you don't have blingie tool that small, use common sense - torque just to flatten split washer and then some more
do not push too much with "some"
if you have a suspicion that screws you can buy are made of old nails, go buy SS ones - they're usually softer than proper 8.8 hardware, but at least they're undoubtedly better than toy screws
torque - prescribed torque for M3 is 0.9Nm
if you don't have blingie tool that small, use common sense - torque just to flatten split washer and then some more
do not push too much with "some"