Thermal Copper Pillar Bumps Yield 'Cooler' Flip-Chips

A novel approach targeted at resolving some of the electronics industry's biggest thermal and power management challenges in high-end flip-chipped devices is, not surprisingly, attracting lots of attention. Nextreme Inc. (Research Triangle Park, N.C.) has developed thermally active copper pillar bumps that, when an electrical current is passed through, are capable of rapidly cooling one side relative to the other. Just reverse the process and pass heat through the thermal bump to generate power. And there's no limit to how small the geometry of the bump can go — it can scale right along with the bumping process and the chip side.

Nextreme was founded in December 2004 with the goal of finding solutions to the difficult thermal problems hindering the electronics and optoelectronics industries from moving forward. It started out working with various electronics suppliers to develop what would look like a staggered tech solution to the hot spot problem. Hot spots typically require 5-25°C of cooling, but also require an enormous amount of power (on the order of 100 W/cm²) to be pumped. So Nextreme developed a device that provides this kind of cooling in a package so it wasn't right against the hot spot, but was pulling heat out the backside. While the company was happy with the performance, it didn't fit into the manufacturing flow used in the packaging world.

Enter the thermal bump. The company was already using solder bumps to build devices and thinking about building a device more like a copper pillar bump, explained Paul A. Magill, Nextreme's vice president of marketing and business development. "Since we're using solder — it's essentially a thermoelectric sandwich with solder — we can fit this into or make it compatible with virtually any flip-chip bumping process," he said.

Thermal copper pillar bumps, such as this one, deliver both cooling and energy harvesting functionality. (Source: Nextreme)

Nextreme's technology incorporates a thin film of proprietary nanomaterial (made of bismuth telluride, at a thickness of a single crystal that is anywhere from 5 to 20 µm) into the solder bumping manufacturing process. And its thin-film-embedded thermoelectric generator (eTEG) directly converts heat into electricity using the Seebeck effect, where electricity is produced from a temperature differential applied across the device. The temperature difference between the hot and cold sources leads to a difference in the Fermi energy across the thermoelectric material to yield a potential difference, which drives a current.

A temperature difference of 60°C has been achieved across the 60-μm-high thermal bump. Nextreme also demonstrated maximum power pumping capabilities exceeding 150 W/cm² and a capability to generate up to 10 mW of power per bump.

Because this technology isn't a bolt-on solution, Magill thinks it might just be the first truly scalable solution for the electronics industry. "We also like to point out that if we can integrate these thermal bumps close to the hot spots, devices will only need to pull the temperature delta the hot spot actually needs," he added.

The next step is to integrate it into a package you could flip-chip a die onto, according to Magill. "People are asking if this will affect signal parasitics, but we're not envisioning it being in the signal current path," he elaborated. "There would be a separate array of bumps in the hot spot area that would require its own power and ground. This isn't a big step to go through, and this phase would involve working with the manufacturers of design tools and the chip designers. We'd eventually like to see a chip design tool for thermal and power management that designers could just place as they would capacitors, resistors and interconnects into their total package stackup."

Nextreme is using its thermal and power management technology in conventional approaches for thermal management in lasers and sensors to cool high-temperature electronics and trickle charge miniature batteries. Potential flip-chip applications for the thermal bump include microprocessors, display drivers, chipsets, RF devices, medical devices, watches, smartcards and analog/mixed-signal devices.

Long-term, Nextreme hopes to work with a partner to implement a methodology to put its bumps down during the bumping process itself. With the exception of one step, the rest of the process uses electroplated deposition. Magill envisions sputtering the material down or placing it down in another operation.

Nextreme is currently in pilot production and ramping up, expecting a capacity of a million devices next year.

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