Copper aims for solder's sockets

Researchers from Georgia Institute of Technology believe they have found a way to improve the connections between chips and boards they sit on by replacing the solder-based process that electronics manufacturers have used for more than a hundred years

Although highly conductive copper is used throughout printed circuit boards (PCBs) and in many chips for wiring, the link between the two is today normally made using some kind of solder. Since the prohibition of lead-based solders in Europe and Japan, manufacturers have had to deal with a range of reliability issues on their replacements.

Professor Paul Kohl of Georgia Tech said replacing solder-ball connections with copper pillars, effectively extending the use of the metal from inside the chips all the way through the PCB, creates stronger connections and the ability to create more connections. "Circuitry and computer chips are made with copper lines on them, so we thought we should make the connection between the two with copper also," Kohl noted.

One advantage of using solder is that it can tolerate misalignment between two pieces being connected but, according to Kohl, so can copper, with the metal being more conductive and creates a stronger bond. It is not the first attempt to build a solderless process. The industry has toyed with elastic connectors but they proved expensive and unreliable. US-based Verdant Electronics has developed an electroplating technique that also uses copper to join chips to the boards that carry them. However, this Occam process uses a 'build-up' technique that is very different to the manufacturing methods used for conventional PCBs.

With funding from the Semiconductor Research Corporation (SRC), Kohl and graduate student Tyler Osborn have developed a novel fabrication method to create all-copper connections between computer chips and the PCB.

The researchers first electroplate a bump of copper onto the surface of both pieces. Then, a solid copper connection between the two bumps is formed by a chemical plating process. The pillar, which is the same thickness as a banknote, is fragile at room temperature, but by heating it in an oven for an hour internal defects are annealed out to generate a much stronger solid copper piece. Osborn found that strong bonds were formed at an annealing temperature of 180C.

Osborn also investigated how misalignments between the two copper bumps affect pillar strength. "I've also studied the optimal shape for the connections so that they're flexible and mechanically reliable, yet still have good electrical properties so that we can transmit these high frequency signals without noise," said Osborn.

The researchers have been working with Texas Instruments, Intel and Applied Materials to test their technology.

Image: A copper plug made using the Georgia Tech process

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