Thermal Copper Pillar Bump

The thermal copper pillar bump, also known as the "thermal bump", is a thermoelectric device made from thin-film thermoelectric material embedded in flip chip interconnects (in particular copper pillar solder bumps) for use in electronics and optoelectronic packaging, including: flip chip packaging of CPU and GPU integrated circuits (chips), laser diodes, and semiconductor optical amplifiers (SOA). 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 chip or to another electrical component. The diameter of a thermal bump is 238 μm and 60 μm high.

The thermal bump uses the thermoelectric effect, which is the direct conversion of temperature differences to electric voltage and vice versa. Simply put, a thermoelectric device creates a voltage when there is a different temperature on each side, or when a voltage is applied to it, it creates a temperature difference. This effect can be used to generate electricity, to measure temperature, to cool objects, or to heat them.

For each bump, thermoelectric cooling (TEC) occurs when a 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. Thermoelectric power generation (TEG) on the other hand occurs when the thermal bump is subjected to a temperature gradient (i.e., the top is hotter than the bottom). In this instance, the device generates current, converting heat into electrical power. This is termed the Seebeck effect.

The thermal bump was developed by Nextreme Thermal Solutions 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. Nextreme chose the copper pillar bump as an integration strategy due to its widespread acceptance by Intel, Amkor and other industry leaders as the method for connecting microprocessors and other advanced electronics devices to various surfaces during a process referred to as “flip-chip” packaging. The thermal bump can be integrated as a part of the standard flip-chip process (Figure 1) or integrated as discrete devices.

The efficiency of a thermoelectric device is measured by the heat moved (or pumped) divided by the amount of electrical power supplied to move this heat. This ratio is termed the coefficient of performance or COP and is a measured characteristic of a thermoelectric device. The COP is inversely related to the temperature difference that the device produces. As you move a cooling device further away from the heat source, parasitic losses between the cooler and the heat source necessitate additional cooling power: the further the distance between source and cooler, the more cooling is required. For this reason, the cooling of electronic devices is most efficient when it occurs closest to the source of the heat generation.

Use of the thermal bump does not displace system level cooling, which is still needed to move heat out of the system; rather it introduces a fundamentally new methodology for achieving temperature uniformity at the chip and board level. In this manner, overall thermal management of the system becomes more efficient. In addition, while conventional cooling solutions scale with the size of the system (bigger fans for bigger systems, etc.), the thermal bump can scale at the chip level by using more thermal bumps in the overall design.