Guy,

Thanks for the input.  We use torque specifications relative to the
screw diameter and these seem to dovetail with the device manufacturer's
specs.  In several applications we use either "snap-in" clips or
"screw-down" clips.  These are on our larger extruded heat sinks.  The
stamped small fin heat sinks is where we mostly use the thermal grease.

Phil

-----Original Message-----
From: Guy Ramsey [mailto:[log in to unmask]]
Sent: Monday, August 20, 2001 10:10 AM
To: [log in to unmask]
Subject: Re: [TN] Thermal Compound considerations


I have been out of this loop for a couple of years but as I recall the
grease / mica combination has (potentially) the best themal transfer
characteristics. But is clearly an assembly problem. The TO 220 package
will
have a torque specification, to achieve best transfer without bending
the
tab. Another solution is spring clips that press down on the package.
These
work well with either the pads or the grease. We found them to provide
the
best mechanical interface to the heatsink. They also eliminated the need
for
a torque wrench in assembly. How much grease? Just enough grease to fill
the
gaps without viods, a very thin layer.

Guy Ramsey
Senior Lab Technician / Instructor


E-Mail: [log in to unmask] <mailto:[log in to unmask]>
Ph: (610) 362-1200 x107
Fax: (610) 362-1290



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Phil Nutting
Sent: Monday, August 20, 2001 9:43 AM
To: [log in to unmask]
Subject: [TN] Thermal Compound considerations


Here is this morning's question(s).

I posted a note last week about using the old style thermal grease.  I
agree with your comments about using some of the new Berquist or Power
Devices insulator pads, some of which we currently use.  Today I am
looking for some of your thoughts on an alternate style material (grease
"replacement").

There are several products on the market which are phase change (at
60°C/140°F) thixotropic, non-silicon products which prevent run out
(oozing out from under the device) at operating temperature.

Now assuming we use this material on TO-220 packages and mount the
device with the standard mounting hole in the tab the device tends to
not sit completely flat on the heat sink  (This is a problem not
associated with the thermal compound but the mechanical design of the
device).  On concern brought to my attention is water trapped under the
device.  And our intent is to solder these devices to the PWB (through
hole) and de-flux (OA flux) the boards after assembly in a batch DI wash
with no saponifiers.  Do any of you see this as an issue?  We presently
use the old style thermal grease in this manner mostly with TO-220
devices and heat sinks which are the same size as the body.  If our DI
wash is running at 120°F and the wash chamber is set for 135°F are there
any significant issues with approaching the phase change point that will
degrade the thermal compound's operational characteristics?

We may end up using one of the pad style items, but I wanted to tap into
your collective wisdom to help with the end decision.  This manual
application tends to be messy, time consuming and different by each
assembler.

Thanks in advance.

Phil Nutting

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