Board-level Thermal Management: 3D Heat Removal

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By Paul A. Magill, Ph.D., Nextreme Thermal Solutions Inc.

The paradigm driving PCB-level design has always been cost. However, IC-generated heat on these boards has increased in modern designs. Frequently, the heat generated by these die leads to hot spot formation on the circuit boards. As board designs progress, a combination of active and passive and bottom, top, and lateral thermal management is needed.

As the need for speed improvement at the die level continues to grow, companies are now looking at 3D packaging. This type of die stacking also presents challenges for thermal management. To address these challenges, we need to move beyond current methods for thermal management and consider heat removal in all three dimensions (Figure 1). If this is to happen, the board must handle this heat flux that traditionally has been taken out the top and removed by a fan.

Figure 1 illustrates the thermal management concept. By extending the only currently available option of passive back-side cooling to also include active back-side cooling, front-side heat removal, and lateral heat removal, thermal management of the 3D stack is significantly enhanced.

nextremeone.jpgFigure 1. 3D thermal management.

I introduce these concepts here in terms of a thermally active flip chip that has integrated thermoelectric material. These bumps act as solid state heat pumps to move heat from one location to another. It is possible to use these same concepts with either active or passive systems.

To keep pace with these emerging thermal management issues, the thermal management system must be designed in parallel with the electrical product, so tradeoffs can be made at the appropriate time. This increases the burden on the board manufacturer to reduce the thermal resistance of the circuit board or module.

3D Die/Chip CoolingSeveral options are available for heat removal from a 3D chip stack.

Back-side cooling. Back-side cooling the traditional method for heat removal can be enhanced by the introduction of thermal bumps, either in the heat sink to form an active heat sink or into the heat spreader as shown in Figure 2.

nextremetwo.jpg Figure 2. Integrated 3D thermal management.

Lateral cooling. In the lateral cooling concept, current flows from left to right, but the heat flows from the center of the module outward. For a 3D chip stack, this lateral heat removal concept can be combined with an interposer through which the heat can be removed. Here, the thermoelectric material is underneath the substrate and the heat is pulled from the center segment to the side. Active-side cooling. The last approach is active-side cooling. Figure 3 depicts the active side of a microprocessor. The smaller structures represent conventional copper pillar bumps next to the larger thermal bump.

nextremethree.jpgFigure 3. Thermal and electrical bumps integrated on a single substrate.

Heat FlowActive and passive thermal management options can be applied for the schemes suggested above. The efficiency of the thermal management system plays a fundamental role in the board-level design. How this heat is managed is therefore an extremely important piece of the overall puzzle.

The heat equation is an important partial differential equation describing the distribution of heat (or variation in temperature) in a given region over time. For a function u(x,y,z,t), which is a measurement of the temperature (T), of three spatial variables (x,y,z), and the time variable (t), the heat equation is

nextremeeqone.jpgu = u(t, x, y, z) is temperature as a function of time and space; or equivalently

nextremeeqtwo.jpgwhere k is a constant.

The other important governing equation is Q = U + W. In thermodynamics, the change in heat Q is equivalent to the heat flowing into the system. Combining these two equations and in the absence of any work being performed, Q is proportional to the change in temperature.

Q = kΔT

Hence for passive systems, cooling by the conduction of heat is a linear function of temperature and a constant related to the material properties of the solid. This constant (k) may be a function of many variables including T, P, and V.

Passive Systems for Electronic Thermal ManagementThe primary systems used for passive cooling of electronic and optoelectronic systems are materials configured as thermal interface materials (TIM), heat spreaders, and heatsinks.

Heatsinks are environments (e.g., water or air) or objects that absorb and then dissipate heat. This occurs in different ways, including direct and radiant transfer of heat. Heat sink performance is a function of material, geometry, and the overall surface heat transfer coefficient along with the temperature of the heatsink.

TIMs are used to fill the gaps between thermal transfer surfaces, such as between microprocessors and heat sinks, to increase thermal transfer efficiency. Most often, heat spreaders are simple metal plates that have high thermal conductivity, but carbon-based heat spreaders with anisotropic characteristics are also in use. These heat spreaders act as a heat exchanger: moving heat between a localized heat source and a secondary heat exchanger that is larger in area.

nextremefour.jpgFigure 4. TD is the temperature of the device; TC is the temperature of the cold side of the TEC; TH is the temperature of the hot side of the TEC; and TA is the temperature of the ambient or surrounding environment.

Active Systems Benefit from Thermoelectrics Given that passive heat removal is only a linear function over distance of the temperature difference, the system must be modified and advanced to obtain a greater rate of cooling and a smaller device.

Active thermal management devices such as thermoelectric coolers (TECs) provide this additional heat pumping and temperature stabilization capability. An example of the type of heat pumping offered by a TEC for cooling below ambient is shown in Figure 4.

A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump that transfers heat from one side of the device to the other side against the temperature gradient (from cold to hot), with consumption of electrical energy. Such an instrument is also called a Peltier device, Peltier heat pump, solid state refrigerator, or thermoelectric cooler.

One of the drawbacks to using TECs is that, in doing work to move the heat in an almost discontinuous manner, they add heat to the system. This heat will be added downstream of the device to be cooled. The key to any of these solutions is to scale the thermal management system to the size of the heat problem.

ConclusionTo more appropriately handle the heat generated by future IC packages, some of it likely will have to pass through the board. This will place extreme constraints on the board design that will necessitate metal cores for lateral and vertical heat movement. In addition, the material used for the board must be able to survive the temperature excursions as some of the heat is dropped across the thermal vias. All in all this points to many of the thermal management challenges being passed to the board manufacturers as they keep pace with the ever increasing need for higher chip speeds and more functionality.

Paul A. Magill, Ph.D., VP of marketing and business development, Nextreme Thermal Solutions Inc., may be contacted at (919) 597-7300; Read his recent contribution to SMT, "Cooling Needs to Start at the Chip Level." For more on the 3D thermal management concept, read his article "Cooling 3D Packages with Thin-film Thermoelectrics" in Advanced Packaging.



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