A Silicone Job

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A lot of people want or need to add heatsinks to
devices that have no provisions for mounting one:
video card memory chips, the clock
generator on Asus P3V4X motherboards, some
UltraATA controllers.

Popular methods involve thermal
tape (commonly nicknamed “frag tape”) or dots of super
glue.

These approaches are good in some ways, but bad
in others.

Super glue is rather unforgiving and tends
to grip the plastic chip surfaces too tightly,
breaking off the surface off the chip in some extreme
cases. Super glue is also rather rigid, and
tends to pop loose if the heatsink expands too much.

Thermal tape doesn’t have stellar holding
power (especially upside-down) and transfers heat
rather poorly.

A better possibility is a few small dabs of clear
silicone (NOT caulk – the clear silicone of the sort
used on aquarium glass, etc. as it doesn’t emit quite
as much nasty stuff while curing) or RTV rubber
(potting compound is great) and heatsink paste to
literally “glue” the heatsink to the chip by the
heatsink’s corners.

It works well; it’s easy to remove without
damaging anything, it can be reapplied once the previous
application is carefully scraped off, and it doesn’t
conduct electricity. Best of all, if applied
correctly, four dots 1/4″ in diameter can hold a
quarter-pound heatsink in place upside-down for just
this side of ever.

To do it:

1. Disconnect everything (cords, cables, POWER CORD –
hint hint!) and if at all possible remove the board
holding the item(s) to be worked on and place it on an
electrically conductive, grounded surface. (In a
pinch, use heavy-duty aluminum foil and set the PC
chassis on part of it to act as a ground source. Best
bet is a grounded antistatic electronics mat.)

You
will need to be able to get to all sides and corners
of the target device(s) unless obstructed by other
objects on the board, so do what you must to make this
possible.

2. Use a cleaner like rubbing alcohol (NO GASOLINE OR
SIMILARLY VOLATILE CHEMICALS! NO CHEMICALS THAT WILL
ATTACK PLASTICS!) to clean the top surfaces of
everything you intend to apply heatsinks to, as well
as the edges and neighboring quarter-inch or so of
board in all directions, again unless obstructed.

Make sure to clean thoroughly and make sure you don’t leave
residue anywhere.

Let the item dry thoroughly before
proceeding. Cleanliness is critical for proper
adhesion! (Besides, leaving liquid alcohol on any
surfaces is not a good thing when one fires up the
computer…).

3. Dry-fit your heatsink on each chip to be “sunk.”
Note where the heatsink overlaps (if it’s larger than
the chip) or how well it will center (if it’s
smaller.) Decide on how it will be positioned, making
notes if necessary.

4. Take the heatsink back off and apply a THIN layer
of heatsink compound to its contact surface, BE
CAREFUL NOT TO GET ANY ON OR IMMEDIATELY NEAR THE
EDGES/CORNERS OF THE HEATSINK
.

The reason for this is
that the heatsink compound will make the silicone not
“grab” properly if the two overlap. Start from the
center and apply outward to within say a tenth-inch
from all edges.

5. Again, dry-fit the heatsink and remove it, to make
sure you have JUST enough thermal compound to do the
job. Remember, you want JUST enough to uniformly fill
any gaps, NO MORE. You also don’t want so much that
any oozes out from under the heatsink.

The surface
tension of the heatsink compound should be strong
enough to feel when pulled straight away from the
device it’s on – if not, you might need to add a VERY
small amount of extra “goop.” (This helps make sure
the heatsink remains in contact even if upside-down.)

6. Squirt a dab of silicone (say 1/4″ – you can always
squirt out another dab if you use it all) onto a piece
of cardboard you can dispose of later. Get a few
toothpicks (preferably rounded ones, not those flat
cheap crappy ones!) to use as applicators – don’t use
anything metal that could scratch or cut the board
near the chip, the chip itself, or any of its pins.
Make sure you can see what you’re about to do (light,
magnification, etc.)

7. Reapply the heatsinks, one at a time. While holding
the heatsink firmly in the desired position, use a
toothpick to apply tiny dabs of silicone onto the
heatsink’s edges and roll the silicone onto the chip.

Make sure you don’t try to run any silicone over any
heatsink compound, as this will guarantee that it
won’t stick. Avoid getting the silicone onto pins or
any neighboring components – if the heatsink is larger
than the chip, glue the heatsink’s corners only with
silicone by bridging the chip’s corners to the
heatsink with silicone.

If the heatsink’s smaller than
the chip, either tack it down by its corners or by its
edges, but don’t totally enclose all edges. It usually
only takes four small spots, one at each corner, for
most heatsinks up to several ounces in weight – more
is overkill and may complicate removal later.

NOTE: If the heatsink is large/heavy, bridge the
silicone from the heatsink to the chip to the circuit
board, so that the chip’s leads aren’t bearing the
weight by themselves.

8. Let the “glued-down” heatsink(s) sit undisturbed
(read: NO TOUCH!) overnight to cure. (If you’re using
quick-cure stuff, give it at least twice the label’s
cure time as a safety precaution.) Then check your
work before reassembling the computer.

If you get one
that didn’t stick COMPLETELY, CAREFULLY trim away any
silicone that did stick with a small, sharp knife (get
it ALL but BE CAREFUL! One well-placed nick can KILL a
motherboard, video card, etc.!) and clean it
thoroughly as before, and repeat. Again, make sure you
don’t try to run any silicone over any heatsink
compound, as this will guarantee that it won’t stick.

9. Reassemble and fire up your machine. After a couple
minutes or less, you should be able to detect a slight
to serious increase in the heatsinks’ temperature,
depending on how much heat is radiated by the device
underneath. If you don’t detect ANY temp change, it’s
possible that the heatsink isn’t stuck down well
enough and has pulled away from the device, in which
case you’ll probably have to remove it and try again.

I’ve stuck heatsinks to an amazing number of
electronic parts (including a notched heatsink to a
50-watt power resistor!) with this technique and not
one has come loose without being “helped.” Sure beats
the crap out of frag tape in terms of holding power,
longevity, and outright thermal conductivity…

–O d d O n e, who happens to be an old-school
electronics tech with a few tricks up his sleeve, as
well as a overclocker psycho…

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