Intel high-power chip thermal management (Pentium/Itanium)

Tab-bonded Intel Pentium 1 package with 400 I/O lines (100 per side) used as physical specimen.

The last type of cold welding I want to talk about is actually probably the most common welding in the world, at least in terms of number of units. Anybody have any idea? I estimated once, 20 years or so ago, they make 50 trillion of these welds a year.

Student: [guess]

That's not a cold weld. This is wire bonding in microelectronics. If I have my little tab-bonded tape — this is the Intel thing I passed around before — there are 400 input and output lines on it, 100 on each of the four sides. This one's soldered. But after they've soldered the chip to this little composite polyimide-copper tape, to join it to the rest of the package, they come in with a one-thousandth-of-an-inch diameter gold wire, and they squeeze that wire onto the surface and push it to the side. They often add a little bit of heat, like 200 or 300 degrees, and it's called thermocompression welding.

We've talked about flip chip and the problem of thermal expansion. Coefficient of 2.6 for the integrated circuit; thermal expansion for glass aluminum epoxy, glass polyester resin — which is basically what that sheet of plastic is — of 6 to 27. You've got a real problem. The plastics have large coefficients of thermal expansion. Silicon is a ceramic, fairly low. Sometimes people are bonding from silicon to a copper heat sink. When you get to a powerful chip like a Pentium or one of the Itaniums, or whatever the Intel chips are now, these little chips will put out 30 watts when they're humming along. 30 watts per square centimeter is roughly equivalent to taking a plumber's propane torch and putting it right on the surface of something, just holding it there. That's the heat transfer rate of that flame against the surface. When these chips are operating, you've got to cool them down.