Cold bonding of integrated circuits and transistors

Indium bead used to cold-bond the metal can to the silicon-chip base on early discrete transistors. Illustration of why oxide-to-metal hardness ratio matters in practice.

So why is that important? It's not as important now as it used to be, but about fifty, sixty years ago when they were still making single transistors — or even sometimes today in critical applications like some nuclear weapons which we don't know about — when they have to bond something at room temperature and actually get a true metal seal, they will often use indium. They used to have single transistors, when they went from vacuum tubes to transistors: a single silicon chip with several leads coming in the base, and a little metal can on the top. That little metal can was just squeezed on, and you had a little bead of indium. You were making an indium solder bond at room temperature. So it was used many years ago.

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Thermocompression gold-wire bonding in microelectronic packaging. Kulicke and Soffa (NW Pennsylvania) makes ~90% of the world's wire-bonders. ~50 trillion bonds/year worldwide; 7 bonds/second per machine; 1-mil gold wire; bond geometry has a central "dead zone" with shear bond in the wings. Limited to ~4-mil pitch by head stiffness.

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.