Reducing Thermal Resistance at Die Level
BY DAXI XIONG, Advanced Thermal Solutions Inc.
Faster, denser transistors inside BGAs dissipate more power as they grow smaller. Higher die junction temperatures must be kept below critical levels. Because high temperatures can reduce performance and reliability, new cooling solutions should effectively remove heat and keep the chips at safe temperatures.
In conventional semiconductor packages, heat travels from transistors over two paths. One is through the ball grid connecting the device to the printed wiring board (PWB). The second and more thermally complex path is through the package surface. In assemblies such as BGAs, heat generated from the chip is conducted to the heatsink via the die encapsulant, and subsequently dissipated to the outside by convection.
Die-level localized hot spots create a unique challenge. Thermal management can be provided by spot cooling or spreading the heat over a larger surface. But the growing disparity in size between the cooling surface and the die creates added spreading resistance, which is becoming a dominant factor in cooling of high-power devices.
Thermal Pathways and BGAs
The chain of heat dissipation can be divided into two parts: internal thermal resistance, from die to case, inside the package; and external thermal resistance, from the case to ambient. Several methods are now used to reduce external thermal resistance, such as single- and two-phase liquid cooling. Reducing a package’s internal thermal resistance is still a high priority for the cooling of high power devices. In fact, it is becoming a bottleneck in electronics cooling.
Due to the limits of manufacturing standards, heat conduction is the only adopted method for reducing internal thermal resistance. Efforts focus on applying high-conductivity materials, like copper or copper alloys, in the thermal path to reduce this resistance. Theoretical analysis shows that due to small die size, spreading thermal resistance between the die and the attached copper heat spreader is great. A higher conductivity material, like diamond, may reduce the internal thermal resistance, but is too expensive for broad use. Moreover, because the surface temperature of a copper spreader isn’t uniform, it won’t induce the optimum effect from its attached heatsink.
Current BGA packaging structure offers limited opportunities to enhance the heat transfer needed for high-power devices. A packaging technique that provides higher performance at an affordable cost and is available in different forms for varying requirements is needed.
A combination of die-down BGA and enhanced thermal spreader as one integrated package can be used to enhance heat transfer. The thermal advantage comes from attaching the die to the heat spreader’s bottom surface that forms the top side of the package. The spreader exposes the package surface to available air flow, while minimizing contact and spreading resistances. If needed, the spreader (or slug) can be directly coupled to thermal management devices such as heatsinks.
Current and Ideal Cooling Systems
For external cooling, a number of techniques consisting of liquid-based devices applied to the outside of the package have been developed. Methods include jet impingement, cold plates, spray coolers, vapor chambers, heat pipes, and immersion systems.
Among the drawbacks for liquid cooling systems is the need for a pump or other flow-inducing device. Costs for these systems are a concern, along with the potential for leakage, finite life spans, and their added size and weight. Performance limitations also make some systems unsuitable for high-temperature applications.
Ideally, the package for high-power devices would start with a solid material; i.e., with no internal liquids, and with an inherent thermal conductivity at least as high as diamond’s. This material would have a coefficient of thermal expansion (CTE) like that of its silicon heat source, and cost no more than copper or aluminum. The die would bond directly to a heatsink formation of this thermally conductive material, and the airflow through the sink fins would produce a high convection coefficient of more than 8500 W/m2K. Unfortunately, such an ideal system is expensive and far away in commercial development.
An Active Approach
One liquid loop cooling technique that was recently developed is active BGA packaging,* shown in Figure 1. This approach features a forced thermal spreader (FTS) that combines micro- and mini-channels in a resin package bonded directly to the die. The fluid flow rate inside the channels is 0.5 to 1.0 L/min. The FTS is a liquid-embedded system with an integrated pump that is deployed both internal and external to a package, depending on the application. However, when used outside the package, contact resistance degrades thermal performance.
Figure 1. Diagram of an active BGA packaging system.
The difference between active packaging and regular packaging is enhanced convection cooling provided by the forced heat spreader (FTS) in place of a conductive copper base. This active BGA packaging technique can reduce internal thermal spreading resistance as low as 0.03 K/W, and provides more uniform temperature distribution on the case surface, improving the heat dissipation from an attached external heatsink. When equipped with an external heatsink, the package can dissipate 600 W/cm2 of heat flux, with the potential to reach 1200 W/cm2, without using any additional liquid cooling devices.
Figure 2. Cross-surface of a forced thermal spreader.
Figure 2 shows the structure of the FTS. In the center, the micro-channel array has a high heat-convection coefficient. Once the fluid has carried heat away from the chip via the micro-channels, it circulates around and spreads the heat to the upper surface via peripheral mini-channels. It is reported that a micro-channel-based cold plate can remove heat flux over 800 W/cm2. However, the associated large pressure drop is beyond the capability of the pumps. In this design, the microchannel array’s hydraulic length is short, resulting in a small overall pressure drop in the channels. The total pressure drop in the FTS is only around 15~20 Kpa (2~3 psi), which is within a liquid pump’s capability.
Figure 3. Thermal spreading resistance comparison of solid copper, a vapor chamber, and a forced thermal spreader.
The FTS provides three major advantages over traditional spreaders. It has high thermal performance and the lowest thermal spreading resistance among all the spreaders (Figure 3). Its spreading resistance is only decided by the size of the heat source, whereas other spreaders are affected by their orientation, location, and base plate area. It is continuously active, so there are no dry-out issues as in heat pipes or vapor chambers.
Whole System Thermal Analysis
This active BGA packaging technique is designed to lower the thermal resistance of the whole system. Because the FTS brings uniform temperature distribution on the top surface, it reduces the spreading resistance to the external air heatsink (Figure 4). This allows the air cooling system to dissipate heat flux above 1200 W/cm2 (Table 1).
Figure 4. Air-cooling system with the active BGA packaging.
In comparison, a typical liquid cooling system is much bulkier and has more interconnections between parts. These features have reliability implications, with added cost for maintenance and for system alarms in case of cooling system failure.
Table 1. CFD prediction on whole-system thermal performance with active BGA packaging.
A Forced Thermal Future
FTS systems present a potentially effective packaging technology for high-heat flux cooling. Used with an external air heatsink, it can dissipate heat flux up to 1200 W/cm2, without adding liquid-cooling devices. Some issues that need to be considered before the FTS can be widely used include material selection and fabrication, integrating the FTS with the die, integrating a mini-pump with the FTS, and reliability analysis and testing.
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DAXI XIONG, Ph.D., senior engineer, may be contacted at Advanced Thermal Solutions Inc., 89-27 Access Rd., Norwood, MA 02062; 781/769-2800; E-mail: email@example.com.