Nextreme Thermal Bump Asked Questions
General Questions
Q. What is a thermal bump?
A. Nextreme has embedded a proprietary thin-film thermoelectric material into conventional solder bumped interconnects to provide the possibility of active cooling for an area of a flip chipped component using a modification of the widely accepted copper pillar bumping (CPB) process. Nextreme’s process enhancement also enables power generating capabilities within the copper pillar bumps for energy scavenging applications though these attributes cannot be used at the same time. Before this breakthrough, solder bumps could previously provide only mechanical and electrical functionality.
Q. What are the dimensions of a thermal bump?
A. The bump currently range in diameter from ~100 microns to ~240 microns. When it is placed on a semiconductor chip, there are additional structures required for proper operation. Thus, the dimensions of the surface area of a bump and its associated structures maybe as much as 20% larger.
Q. What products are using the thermal bumping process today?
A. Nextreme builds and ships discrete modules today fabricated using the thermal bump structure. All of our product modules that we are sampling use the thermal bump to provide thermal and power management functionality.
Q. How can I use thermal bump technology for my application?
A. Nextreme’s approach uses a proven, fully scalable technology to deliver new, enabling functionality in discrete or integrated applications. The technology has applicability today for flip chip applications. CPB interconnects, developed for high performance/high density electronics, are considered to be the future of flip chip. This is an industry accepted solution, already being used by Intel (Presler, Yonah), Amkor, Casio and others.
Technical Questions
Q. What is the cooling capacity? There was something that states it can cool a hot spot by 5 to 10 degrees. Is that enough? How does that compare with other TEC solutions?
A. The thermal bump can maintain a maximum temperature differential 60 C across its two faces for a no load (no heat) condition; can pump >150 W/cm² of power while maintaining a zero temperature differential; and can generate up to 10 mW of power per bump provided there is a sufficient heat source.
Q. What are the minimum and maximum heat transfer capacities of the devices?
A. Nextreme’s high performance technology has demonstrated a maximum ΔT (at zero heat pumping) of approximately 60°C between the hot and cold sides, and the maximum heat pumping density (at zero ΔT) of more than 150W/cm². This heat density is equivalent to what can be experienced at a distance of a mile from the ground zero when there is a 1 mega-ton nuclear blast. In comparison, a typical 100W light bulb has the surface heat density of less than 1W/cm², and the outer surface the Sun is at about 5,000W/cm².
Q. What would be the maximum cooling differential for various heat loads?
A. The maximum ΔT and the heat-load are inversely related: as the heat load increases the ΔT decreases, and vice-versa. As an example, given a thermoelectric device under a fixed condition the maximum ΔT decreases from 60°C to 0°C as the heat load increases from 0 to 150 W/cm².
Q. Can you tell me how you can produce such a large ΔT over such a small distance?
A. It is all in the ability of the thermal bump to pump incredibly large amount of heat from the cold-side to the hot-side while the material itself has a very low thermal conductivity. Low thermal conductivity allows the device to minimize the conduction “leak” from the hot-side back to the cold-side. Mainly due to such a small distance between the hot and cold sides though, Nextreme’s TEC still leaks about 10%, or so, of what it pumped. But it pumps enough load to compensate for the loss.
Q. What is the percent of thermal reduction as compared to other thermal management methods?
A. Our thermal bumps rarely compete with other existing thermal solution options. Rather, when those solution technologies have no headroom for further improvements due to cost, form-factor, weight, and acoustic constraints, to name a few, implementing the thermal bumps in conjunction with the solutions provides a very cost effective means for gaining a greater overall performance improvement. At other times, thermal bumps can be employed alone in many products for which no other known thermal solution options are viable due to, for example, form-factor and cost issues.
Q. How does the thermal bump management thermal issues concerning DDR-4, 5+ when used in a stacked die configuration, in a memory module or 3D stack within micro- or graphics processors?
A. A combination of the nano-size, unprecedented performance capability, and unique fabrication process allow the thermal bumps to be implemented on the back, lateral and front sides of a die, as well as embedded within a package or in a stacked die, rendering them to be the only known solution option that can truly address localized target cooling of 3D packages.
Q. Are there any environmental issues with the thermal bumping process?
A. Nextreme is currently producing discrete devices in various forms using the thermal bumping technology without incurring any environmental issues. The discrete devices use Aluminum Nitride as header material. As we move forward, the only difference from the structural stand point is to incorporate the bumping process directly on a silicon die and/or the package substrate rather than onto a pair of Aluminum Nitride headers.
Q. What is the density required to reach the 150W/cm2 performance?
A. A packing fraction of approximately 35% is required to reach this performance.
Q. How much power does the thermal bump consume and how much power is dissipated?
A. This depends on the cooling requirements and hence the COP of the device.
Q. Do I put thermal bumps in series with my power and ground lines, or are they independently powered on the die surface?
A. They would be independently powered.
Q. What impact does the thermal bump make in terms of resistance, impedance or EMF?
A. The resistance of the thermal bump is in the range of 10 mOhms though this is dependent on the contact area.
Q. When I use the thermal bump in power generation mode can I feed the electrical power generated by a thermal bump back to the device, thus reducing the power supply requirement?
A. Yes, although thermoelectrics are not very efficient so only a small fraction (<4%) will be recovered. We are working to improve this.
Contact us for evaluation, analysis, prototype, test, verification or production assistance, or call us at +1 919-597-7300.
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