Using an electroless plating process, future chips may be attached to circuit boards with copper pillars without solder. The research group has developed a method to turn ordinary copper traces on printed-circuit boards into ultra-high frequency coaxial transmission lines.

Solder "has many limitations, especially for the very high-frequency signals in modern computer systems," said Georgia Tech professor Paul Kohl. "For instance, solder will only form spherical bumps, so if you want another shape that works better with high-frequency signals, you're out of luck with solder."

Instead, Kohl's research team invented a technique for growing pure copper connections between copper pillars on a board. Since copper melts at 1,984 degrees F, it is impossible to attach chips to boards with molten copper. Instead, Kohl's group perfected a way of growing solid copper connections between pillars on chips and boards at room temperature.

Scanning electron microscope image of two copper pillars bonded together. (Image courtesy of Tyler Osborn)

"Intel already puts copper pillars on the bottom of their microprocessors, and Fujitsu uses copper pillars too," said Kohl. "Pillars themselves are not new. What is new here is our technique for growing copper between the pillars already on chips and the pillars we propose putting on printed circuit boards."

The technique works through an electro-chemical deposition process in which a whole assembly of chips on a board is submerged in a solution of copper-sulfate. A reducing agent is then added to the solution, giving up electrons to the pillars as it oxidizes and performing electroless deposition. Free electrons strip off copper atoms from the copper sulfate molecules in the solution, elongating the pillars on the chip and the board.

Once the pillars have grown together, forming a solid copper connection, a 180-degree C annealing process realigns the atoms in the connected pillars so they form a single, solid unit with a mechanical strength exceeds solder.

Kohl claims that all the major engineering hurdles have been cleared--all that remains is optimizing the process for mass production. He estimates that the process will be perfected within the next couple years. Texas Instruments, Intel and Applied Materials are already working with Georgia Tech to test the technology for eventual transfer to manufacturing.

In a parallel effort, the researchers are also developing a technique to make pc-board traces into the structural and electrical equivalent of coaxial cables. The technique works by surrounding the high-frequency signals running through a copper core with an insulating dielectric--in this case, air.

"We have learned how to make a coaxial-like structure on a printed circuit board," said Kohl. "We surround the copper traces with air--which has the lowest possible dielectric constant."

This technique works by depositing layers of a new polymer above and below the copper traces in a multilayer circuit board during lamination. After the board was assembled, it was heated to a temperature that vaporized the polymer on each side of the copper traces, leaving small cavities around the copper signal line. Grounded copper traces above and below the suspended copper traces completed the coaxial-like transmission lines.

Funding for the Georgia Tech research was provided by SRC's Interconnect Focus Center and the Defense Advanced Research Projects Agency.