Using springs and glue instead of solder to make electronic connections between computer chips could end one of the electronics industry’s most wasteful habits, say researchers at the Palo Alto Research Center and Oracle.
“The whole industry is based on nonreworkable technology like solder or tape,” explains Eugene Chow, of PARC. “If one chip in a module of several doesn’t work after you’ve soldered them down, you have to throw out the whole thing.”
Chow and colleagues are fine-tuning an alternative approach. They pattern a surface with microscale springs that compress slightly under a chip’s weight, and these form a lasting, secure electronic connection when the two surfaces are glued together. “You can turn it on, and if it works great, do a final bond with adhesive,” says Chow. “If it doesn’t work, you can just take off the die that failed and replace it.”
For now, the collaborators are developing their springy approach for the high-performance processors used in supercomputers or high-end servers. These chips are combined in closely packed groups known as multichip modules. Such modules need the processors to be packed closely together in order to speed the transfer of signals between them.
“I think it’s just a matter of course that this approach will get to the lower-end applications, too, though,” says Chow. “Eventually this could be in a high-end cell phone–everyone wants to get more chips into everything, and this can help, because the pitch [the horizontal distance between connections] can be so small.” The team has shown that their springs can be made as close together as six microns, compared to the tens of microns necessary with solder connections.
The springs are flat metallic strips that curve up from a substrate that a chip is fixed to. “Fundamentally it’s the simplest spring you can imagine,” says Chow. The spring-building process starts with the addition of a thin titanium layer to the substrate. On top of this, the spring material is deposited in such a way that builds strain into the top layer. Photolithography is used to carve out the outlines of the many springs before the titanium is etched away from underneath.
“The tension makes the springs simply pop up,” says Chow. “It’s an elegant way of making a three-dimensional structure.” The finished spring is coated with a layer of gold for added strength and a better electronic connection. The manufacturers must design the layout of the springs so that they match up to the contacts on the chips. Small sapphire balls or other peg-like structures on the surface of the substrate fit into notches in the chip to ensure the two are positioned correctly.