A New Thermal-Management Paradigm for Power Devices

Nov 1, 2008 12:00 PM
By Dr. Paul A. Magill, Vice President of Marketing and Business Development, Nextreme Thermal Solutions, Durham, N.C.

Compatible with semiconductor fabrication and processing technologies, thermoelectric thin-film materials allow the cooling function to be integrated within power-semiconductor devices.

Power-semiconductor devices such as thyristors, MOSFETs and IGBTs operate in a high current-density mode in which they dissipate a high level of power, with heat-dissipation management becoming a crucial challenge. Compounding the heat-dissipation challenge is that the drive for most semiconductor devices is toward ever-smaller form factors, resulting in the need to properly control higher heat-dissipating device levels. The price structure for these smaller devices tends to run between $0.50 and $4 each, which means that the thermal-management solution must be a fraction of the cost.

Integrating Heat Management

Thermal-management solutions for power devices will be required that integrate not just high heat-flux capability, but also must meet extreme cost requirements. To meet these requirements, a solution is needed that can be integrated into the electronics-packaging process.

Thin-film thermoelectric materials are available as one attractive choice for thermal management. Thin-film material layers ranging from fractions of a nanometer to several micrometers in thickness are available for this purpose, and their use is growing in popularity. These materials can be grown using a metal organic chemical vapor deposition (MOCVD) reactor, and devices are then fabricated using conventional semiconductor fabrication processes.

One packaging form suitable for thin-film heat management is the flip-chip package, which is one of several package forms in which power-semiconductor devices are housed. Flip chip is a method for interconnecting semiconductor devices, such as IC chips and micro-electromechanical systems, to external circuitry with solder bumps that have been deposited onto the chip pads.

It is possible to package power devices in a flip-chip format or a flip-chip format that approaches surface-mount processes. The flip-chip bumping process offers an opportunity to integrate thermal functionality close to the heat source using thin-film thermoelectric technology.

Thermal Copper Pillar Bump

The core technology for this new thermal-management paradigm is the thermal copper pillar bump, which is also referred to as the “thermal bump.” The thermal bump is a thermoelectric structure made from a thin-film thermally active material that is embedded into flip-chip interconnects (in particular, copper pillar solder bumps) for use in electronics packaging. The thermal bump is compatible with the existing flip-chip manufacturing infrastructure, extending the use of conventional solder-bumped interconnects to provide active, integrated cooling of a flip-chipped component using the widely accepted copper pillar bumping process.

The thermal bump was developed as a method for integrating active thermal-management functionality at the chip level in the same manner that transistors, resistors and capacitors are integrated in conventional circuit designs today. Unlike conventional solder bumps that provide an electrical path and a mechanical connection to the package, thermal bumps act as solid-state heat pumps and add thermal-management functionality locally on the surface of a semiconductor chip or other electrical component.

Thermal bumps today are already extremely small. They are 238 µm in diameter by 60 µm high, and have the capability to be scaled to different sizes. The size advantage of the thermal bump enables the integration of thermal-management capabilities at the wafer, die or package levels.

The thermal bump makes use of the thermoelectric effect, which is the direct conversion of temperature differences to an electrical voltage and vice versa. Simply put, a thermoelectric device creates a current flow when there is a temperature difference on each side of the device. Or, alternatively when a voltage is applied to it, a temperature difference is created. This effect can be used to generate electricity, to measure temperatures, to cool objects or to heat them.

For each bump, thermoelectric cooling occurs when a dc current is passed through the bump. The thermal bump pulls heat from one side of the device and transfers it to the other as current is passed through the material. This is known as the Peltier effect. The direction of heating and cooling is determined by the direction of current flow and the sign of the majority electrical carrier in the thermoelectric material.

When combined with a feedback mechanism, the temperature of a target surface can be controlled and maintained by systematically toggling the direction of the current flow.

The use of thermal bumps in power electronics offers many advantages in terms of size, efficiency and power-pumping capability. The bump adds as little as 100 µm of thickness to a heat spreader, enabling unobtrusive integration close to the heat source.

Thermal bumps have been shown to achieve a temperature differential of 60°C between the top and bottom headers and have demonstrated power-pumping capabilities exceeding 150 W/cm². This makes thermal bumps ideally suited for applications involving high heat-flux flows.

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