Hi Grey,
Okay, that makes sense. It's a local thing and I understand now.
My experience with techs is that they will not make too great an effort to keep their fingers clean if I'm not looking. I am forced to prevent fingers and grease from coming into contact with each other in the first place. Well, as much as possible. Some days you just can't win.
-Chris
Okay, that makes sense. It's a local thing and I understand now.
My experience with techs is that they will not make too great an effort to keep their fingers clean if I'm not looking. I am forced to prevent fingers and grease from coming into contact with each other in the first place. Well, as much as possible. Some days you just can't win.
-Chris
I must admit that I cheat most of the time and use my fingers to smear on the goo. I have a couple of tubes of thermalcote that may last me until I die (or get heavily into Class A). You can also roll the stuff on in a thin layer using a screwdriver shaft, q-tip handle, whatever. For those who are unfamiliar with the stuff, Go-Jo (or some facsimilie thereof ) can be found at auto supply shops. I never really liked it that much myself, as it leaves your hands a wee bit slimy, but it's loads better than having tons of black (or white) grease caked under your fingernails.
On my most recent build I used Kapton film with arctic silver on both sides. Arctic silver is expensive, but I bought one of those tiny tubes for a PC build years ago and it has been used plenty since. You only need to use a tiny amount of it.
Heatsink calculations are a bit of mathematics too far for me, so I just looked at the pictures on the passdiy site and here and followed visual guidelines.... which have so far been successful...
Heatsink calculations are a bit of mathematics too far for me, so I just looked at the pictures on the passdiy site and here and followed visual guidelines.... which have so far been successful...
I've wondered how much improvement to expect if using a proper clamp over the whole front of a device. Is there an optimum or point of diminishing returns for how much pressure to apply?nonoise said:
I modified a Zen to use steel bars to clamp down the devices onto the sil pads (grey soft things, right?), and I can achieve a LOT of pressure. Seems the heat spreads quicker now, but that could be because I changed the whole arrangement...
Hi Tosh,
For a TO-3 case, the torque should be around 6 ~ 8 in lbs. Any pressure that deforms the mounting surface or semiconductor case is going to create reliability issues.
The real goal is surface smoothness (diminishing returns here too). So, the more surface contact you have, the lower the thermal resistance will be to a point. So minor surface roughness is going to affect things more than a low but reasonable mounting pressure. High mounting pressure can not improve poor interface. It can actually make things worse.
-Chris
Yes, I think so.
For a TO-3 case, the torque should be around 6 ~ 8 in lbs. Any pressure that deforms the mounting surface or semiconductor case is going to create reliability issues.
The real goal is surface smoothness (diminishing returns here too). So, the more surface contact you have, the lower the thermal resistance will be to a point. So minor surface roughness is going to affect things more than a low but reasonable mounting pressure. High mounting pressure can not improve poor interface. It can actually make things worse.
-Chris
Hi Tosh,
The optimal pressure will be a function of the device type and the particular thermal interface used.
The technique of using a metal bar across a TO3-P type package as used in the F4 will give good results thats why Nelson uses those big washer's
The idea is to spread the pressure more evenly across the total contact interface the main reason for this is actually to improve reliability by not subjecting the device to unnecessary mechanical stress which could lead to silicon failure.
A good source of info could be found at http://www.lairdtech.com/
There you will find that the thermal resistance is specified at a particular pressure for any given material type.
Applying these design recommendations will result in optimal thermal transfer for a particular material type.
There are of course many other parameters like surface finish and general dimensional tolerances that should not be forgotten.
The optimal pressure will be a function of the device type and the particular thermal interface used.
The technique of using a metal bar across a TO3-P type package as used in the F4 will give good results thats why Nelson uses those big washer's
The idea is to spread the pressure more evenly across the total contact interface the main reason for this is actually to improve reliability by not subjecting the device to unnecessary mechanical stress which could lead to silicon failure.
A good source of info could be found at http://www.lairdtech.com/
There you will find that the thermal resistance is specified at a particular pressure for any given material type.
Applying these design recommendations will result in optimal thermal transfer for a particular material type.
There are of course many other parameters like surface finish and general dimensional tolerances that should not be forgotten.
hi
artic silver behaves as a varistor
it starts to conduct (fully conduct) when you apply more than 20 to 50V
as longer as you spread it, it becomes more and more conductive...
try it at home - take your DMM (measuring I), variable PSU 0 to 300V (variable U) and let the current flow thru it - with 10kOhm resistor in series- directly thru a bundle of arctic silver ... using Ohm law, and having the U,I measured, it is simple to determine the resistivity of a.s.
Defaultly, it is measured about 100 kOhm, but when you cross certain voltage it drops as close to 10 ohm
theory: all the silver grains moves along the current flow and gets too close, and then...
everybody claims something different about it
but this is actually measured (as described, on 2003 by OK1XVV), found when he switched from using the white grease to arctic silver, succesfully damaging his output Ts
hope this helps
a.
artic silver behaves as a varistor
it starts to conduct (fully conduct) when you apply more than 20 to 50V
as longer as you spread it, it becomes more and more conductive...
try it at home - take your DMM (measuring I), variable PSU 0 to 300V (variable U) and let the current flow thru it - with 10kOhm resistor in series- directly thru a bundle of arctic silver ... using Ohm law, and having the U,I measured, it is simple to determine the resistivity of a.s.
Defaultly, it is measured about 100 kOhm, but when you cross certain voltage it drops as close to 10 ohm
theory: all the silver grains moves along the current flow and gets too close, and then...
everybody claims something different about it
but this is actually measured (as described, on 2003 by OK1XVV), found when he switched from using the white grease to arctic silver, succesfully damaging his output Ts
hope this helps
a.
This has been mentioned a couple times here - just picked some off the bay - 50 pieces of 2.2" x 2.4" x 1mm thick for under $20.
6W/mK conductance too.
http://www.lairdtech.com/pages/products/T-pli200GapFillers.asp
Its the T-Pli 240 seen here:
http://www.lairdtech.com/pages/products/T-pli225-250-POP.asp
Anyone have any experience with it?
6W/mK conductance too.
http://www.lairdtech.com/pages/products/T-pli200GapFillers.asp
Its the T-Pli 240 seen here:
http://www.lairdtech.com/pages/products/T-pli225-250-POP.asp
Anyone have any experience with it?
On my A75 with IRF 630/9630 (case TO220) i used Kapton at first, but after some weeks three Kapton insulators failed, as far as i could see they had very small cracks around the mounting hole.
I wanted to replace all Kapton with ceramic filled silicone pads, 0,2 mm thick, specified at 0,3 K/W.
Compared to the 0,07 K/W of Kapton this seemed a lot, so i did some maths:
Ambient temperature 25°C
IRF630/9630 have 1,67 K/W each
One heatsink has 0,34 K/W
Power dissipation 120 W per channel
Since there are 24 IRF´s per channel, the junction temperature of the IRF´s with Kapton is
((((1,67 K/W +0,07 K/W)/24)+0,34 K/W) * 120W) + 25°C = 74,5 °C
and for the silicone pads
((((1,67 K/W +0,3 K/W)/24)+0,34 K/W) * 120W) + 25°C = 75,65 °C
If you had only six output transistors, the difference would be 100,6 °C to 105,2 °C.
So at least for an A75, you can spend your money on even bigger sinks .....
norb
I wanted to replace all Kapton with ceramic filled silicone pads, 0,2 mm thick, specified at 0,3 K/W.
Compared to the 0,07 K/W of Kapton this seemed a lot, so i did some maths:
Ambient temperature 25°C
IRF630/9630 have 1,67 K/W each
One heatsink has 0,34 K/W
Power dissipation 120 W per channel
Since there are 24 IRF´s per channel, the junction temperature of the IRF´s with Kapton is
((((1,67 K/W +0,07 K/W)/24)+0,34 K/W) * 120W) + 25°C = 74,5 °C
and for the silicone pads
((((1,67 K/W +0,3 K/W)/24)+0,34 K/W) * 120W) + 25°C = 75,65 °C
If you had only six output transistors, the difference would be 100,6 °C to 105,2 °C.
So at least for an A75, you can spend your money on even bigger sinks .....
norb
I have a question realing to the refurbishing of the old Sansui...
It has those TO3 cans... on an anodised (measures infinate resistance with DMM) heatsink.
The amp is at least as old as I am, and maybe a year or 2 older... i.e. early 30s...
Does the thermal paste dry up in amps like it does in PCs...? I find my PC needs a fresh application every summer to keep it from going to hell (literally).
I can't see any isolateing material uder the transistor... just white goop.
Good idea /bad idea to refresh the paste on the amp?
P.S. nelson maybe you can use an adhesive sticker to keep the connecting bits smooth while blasting the surface....?
In my experience, the smoother the two surfaces meeting each other, the more area is exposed on a molecular level to form a thermal bridge. and the better the performance of the heatsink....
Have quite a bit of experience in PC heatsinking, which allows you to digitaly track small modifications and diffirent setups...
Is there any aluminium in common transistor cases?
I would like to use some Liquid Metal (another PC thermal material, literaly liquid metal), has several times beter thermal transfer behaviour than other pastes.
The catch is if you use it on aluminium you can throw the aluminium away, or rinse it away under a tap for that matter....
Of course I still need to test it on anodised aluminium.
Oh a trick for gettign realy thin layers of paste... wear a streched jiffy bag over your finger...
It has those TO3 cans... on an anodised (measures infinate resistance with DMM) heatsink.
The amp is at least as old as I am, and maybe a year or 2 older... i.e. early 30s...
Does the thermal paste dry up in amps like it does in PCs...? I find my PC needs a fresh application every summer to keep it from going to hell (literally).
I can't see any isolateing material uder the transistor... just white goop.
Good idea /bad idea to refresh the paste on the amp?
P.S. nelson maybe you can use an adhesive sticker to keep the connecting bits smooth while blasting the surface....?
In my experience, the smoother the two surfaces meeting each other, the more area is exposed on a molecular level to form a thermal bridge. and the better the performance of the heatsink....
Have quite a bit of experience in PC heatsinking, which allows you to digitaly track small modifications and diffirent setups...
Is there any aluminium in common transistor cases?
I would like to use some Liquid Metal (another PC thermal material, literaly liquid metal), has several times beter thermal transfer behaviour than other pastes.
The catch is if you use it on aluminium you can throw the aluminium away, or rinse it away under a tap for that matter....
Of course I still need to test it on anodised aluminium.
Oh a trick for gettign realy thin layers of paste... wear a streched jiffy bag over your finger...
The clear factory goop on my 25+ year old 400a is still liquid and functional. It has an obvious silicone "feel" to it. Is it just silicone lubricant? It certainly looks neater than the white and messy aluminum oxide compounds in use today.
Anyone using copper slugs or phase change heat tubes instead of extruded aluminum?
...j
Anyone using copper slugs or phase change heat tubes instead of extruded aluminum?
...j
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