Materials for Thermal Management
03/01/2003
BY Nancy Dean
As devices shrink and functionality or clock speed increase, the power density, in addition to power dissipation increases dramatically. This places stringent demands on the materials used in electronics packaging, as well as the physical interfaces between components. This article reviews the materials used for heat transmission, heat spreading and thermal interface applications.
Background
Fueled by exponential increases in device clock speeds, power dissipation in microprocessor electronic devices has grown dramatically in recent years, and the International Technology Roadmap for Semiconductors (ITRS)1 projects this trend to continue to increase. As clock speeds and power dissipation have increased, the function of the microprocessor package has transitioned from that of a mechanical interconnect providing protection to the die, to a conduit for transferring heat from the semiconductor junctions to the environment. As thermal issues threaten to limit device performance, a new industry segment — thermal management — has gained prominence.
 Figure 1. Typical heat paths in a flip chip package with a heat sink. A majority of the heat flows from the die to the heat sink. |
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Thermal management is generally concerned with removing heat from an active semiconductor device and transferring it to the environment. Typical heat conduction paths are from the device through a package to the printed circuit board, and from the device through a package to a heat sink, shown schematically in Figure 1.
Where a heat sink is used, a majority of the heat is conducted through the package to the heat sink and dissipated to the surrounding air. For this path, there is a conductive thermal resistance from the active side to the backside of the die. This is followed by a thermal resistance across the interface between the die and heat spreader (typically termed TIM1). The heat spreader adds conductive and spreading thermal resistances, followed by another interface (typically termed TIM2) between the heat spreader and heat sink. Finally, there is a thermal resistance associated with the heat sink itself.
In many cases now, the majority of the device's thermal budget is not taken up by a heat sink, but rather by the interface materials and heat spreaders.2 Proper selection of heat spreaders and thermal interface materials, and increasingly, ensuring that these materials interact optimally, are critical to thermal management.
Thermal Interface Materials
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There are several classes of interface materials, as described in Table 1. For most high-power microprocessors, very thin bondlines are desired and grease or grease-like materials are used.
Phase change materials and thermal gels are recent additions to the classes of interface materials offered. Both are often described as thermal grease replacements. Phase change materials have the advantage of being preapplied to a component. Thermal greases, however, remain the standard for thermal resistance performance.
Materials that will form a rigid bond, such as adhesives and solders, also may be used to form a thin bondline. However, these materials are usually used in conjunction with small die, or heat spreaders with a low coefficient of thermal expansion (CTE), to minimize die and bond stress that could lead to die cracking or delamination.
Tapes typically are used for lower power applications, as are gap-filling pads. The latter also may be used when compliance to variations in component height are required, as the thermal resistance sensitivity to changes in thickness is not high.
 Figure 2. Percentage of total thermal resistance due to interface materials (estimated from OEM thermal design guides and/or published articles).3-9 |
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It is instructive to examine the role that thermal interface materials have played in the overall thermal budget of high-end electronics. A survey of published articles and thermal design guides for the major manufacturers was used to construct Figure 2, which shows the percentage of the total thermal budget, or thermal resistance, accounted for by interface materials.
While thermal interface materials have improved in recent years, they have not kept pace with the increases in power dissipation and performance of other packaging components. It is apparent from Figure 2 that thermal interface materials now dominate the total thermal resistance of high power microelectronics.
Heatspreading Materials
The TIM1 material attaches the die to a heat spreader. The heat spreader has the primary function of spreading the thermal energy from the small footprint of the die to a larger area in order to make more efficient use of convection from the heat sink or package surface.
The surfaces of heat spreaders are often finished to prohibit corrosion or oxidation, improve cosmetics, or allow for marking of some sort. Typical surface finishes are anodizing (aluminum), sputtering, plating and oxide growth (such as black oxide on copper). The effect of the surface finish on thermal properties should be considered.
Another key aspect of a heat spreader is the surface roughness and flatness. Both these requirements will have an impact on the average bondline of the thermal interface material and, hence, can significantly impact package thermal performance.
Materials Integration
As requirements on the materials increase, it will no longer be sufficient to choose a good performing material for each component of the heat dissipation path — how these materials interact with each other must be considered. A material that spreads heat very well, but is not wet by an interface material may not perform as well as a lower conductivity material that does wet. Phonon scattering at the interfaces and at plating/surface finish interfaces may need to be optimized in a thermal design.10
Four approaches can be taken to reduce total thermal resistance: (1) increase thermal conductivity (k) of the interface materials and spreaders, (2) increase wetting or bonding to decrease contact resistance (qcontact) at the surface, (3) increase flatness of the spreader to decrease the thickness (t) of the interface to reduce heat transfer path, and (4) eliminate one of the interfaces in the heat sinked package. Those challenges are addressed below.
Higher conductivity materials. While increases have been made recently, the bulk conductivity of most thermal interface materials is relatively low. Many thermal interface materials now have a bulk conductivity in the range of 2 to 3 W/mK. Improving material conductivity by a factor of three to five (to ~0 W/mK) would result in a reduction in bulk and thus total thermal resistance of approximately 0.10°Ccm2/W. This would drive the bulk resistance down to 0.03 to 0.04°Ccm2/W. Further improvements in thermal conductivity would have diminishing returns unless the contact resistance also was improved. Improving the conductivity of the spreader materials through the use of engineered composite materials or heat pipe/vapor chambers also will be required to drive total device thermal resistance down. Because the spreading resistance typically is higher than the through-thickness thermal resistance, the use of anisotropic materials is increasing.
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Increase wetting or bonding forces. Increasing the wetting or bonding to a surface will decrease the contact resistance (Figure 3). For most grease or grease-like interface materials, contact resistance has historically been a fairly low fraction of the overall thermal resistance. However, as the material thermal conductivity has increased and bondline thickness decreased, the contact resistance has begun to become significant. Contact resistance can be reduced through improving the bonding or wetting of the interface material to each surface at the interface. Materials should no longer be chosen individually, but rather the effect of the materials working together will be evaluated. It will be important for suppliers and users of interface materials to work with suppliers and users of heat spreaders to ensure optimal performance. Synergy between metal finishes and interface materials should allow contact resistance to be decreased by 50 percent.
Decrease interface thickness. With everything else held constant, a thinner bondline between components will produce a lower interface thermal resistance. This will require flatter components. In recent years, the tightening of flatness tolerances and shift to die referenced cooling has dropped interface thicknesses by a factor of two or more (from ~50 to 25 µm or less). Further improvements in spreader or lid flatness will enable thinner interfaces and better package thermal performance.
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Elimination of one interface. Most high-performance devices use a thermal lid or heat spreader. This creates at least two thermal interfaces in the heat removal path from the die backside to the ambient — the die to spreader and spreader to heat sink interfaces. With current performance levels, eliminating one of the interfaces will eliminate a significant fraction of the overall junction to ambient thermal resistance. Thermal lids are used to protect the die and spread the heat from the concentrated die footprint to a larger area so that the heat sink is more efficient in dissipating heat to the environment. Heat sinks with thicker bases or high performance bases, will help address the latter concern, minimizing the need for a package heat spreader. When a bare die package is used, the interface material between the die and heat sink is softer, or more compliant to prevent damage during assembly or operation. Improvements in interface materials combined with designs to protect bare die will enable the wider spread use of bare die packaging and its inherent elimination of one of the two interfaces in the heat transfer path.
Conclusion
As package and heat sink design have improved significantly, the materials used now largely determine the thermal performance of electronic packages. Thermal resistance across physical interfaces has progressed from an almost negligible portion of the total junction to ambient thermal resistance to the dominant factor in total thermal resistance. Heat spreading and reduction of hot spots within a die are creating a need for higher conductivity materials. Further increases in device power dissipation will require significant improvements from materials. Increased thermal conductivity (k ≥ 10 W/mK), improved ability to work synergistically with other packaging components and package designs to allow thinner bondlines, and increased use of bare die packaging will be needed to meet the challenges of the coming years.
REFERENCES
1. International Technology Roadmap for Semiconductors, 2000 update, Assembly and Packaging, Semiconductor Industry Association (SIA), 2000.
2. N. Dean and I. Rasiah, "Trends in Thermal Interface Materials," ICMAT 2001, Singapore.
3. Pentium Processor Thermal Design Guidelines, Report 8019, Intel Corp, Santa Clara, CA, 1994.
4. Pentium III Processor Thermal Design Guide, Intel Corp, Santa Clara, CA, 2000.
5. Chiu, CP, Solbrekken, GL, et al., "Thermal Design of Pentium II Xeon Processor Cartridge," IMAPS Symposium, 1999, pp758-763.
6. Dolbear, TP and Tarter, T, "Thermal Management of Pin Grid Array Packaging Flip Chip Connected X86 Microprocessors," ISPS 97 Proceedings, IEEE, P. 279.
7. AMD Publication 23794, "Athlon Thermal, Mechanical and Chassis Cooling Design Guide," Advanced Micro Devices, 2000.
8. AMD publication 21085, "AMD-K6 Processor Thermal Design Guide," Advanced Micro Devices, 1999.
9. AMD publication 20092, "AMD-K5 Processor Thermal Considerations," Advanced Micro Devices, 1996.
10. M. Touzelbaev and K. Goodson, "Photon Transport in Superlattices," ICMAT, Symposium J — Packaging Materials & processes for Microelectronics, Optoelectronics, MEMS and Displays, Singapore, 2001.
Nancy Dean, technical manager, may be contacted at Honeywell Electronic Materials, 15128 E. Euclid Ave., Spokane, WA 99216; (509) 252-8690; Fax: (509) 252-8743; E-mail: [email protected].