Thermal Materials Solve Power Electronics Challenges

Feb 1, 2006 12:00 PM
By Carl Zweben, Ph.D., Advanced Thermal Materials Consultant, Devon, Pa.

Applications

A number of second- and third-generation advanced thermal materials are now in commercial and aerospace production systems, including power systems, servers, plasma displays, notebook computers, aircraft, spacecraft and defense electronics, and a variety of optoelectronic products. These materials include natural graphite; natural graphite/epoxy; HOPG; C/Ep; C/C; diamond particle-reinforced copper (diamond/Cu), an MMC; and diamond particle-reinforced silicon carbide (diamond/SiC), a CMC.

Al/SiC has been used in insulated gate bipolar transistor (IGBT) module bases for some time. Fig. 2 shows an IGBT module that is widely used in traction applications. Because the CTE mismatch between Al/SiC and ceramic substrates is small compared to that of copper, the Al/SiC modules last many times longer under thermal cycling.

Fig. 3 shows a liquid-cooled Al/SiC power module base used in aircraft applications.

HOPG, a brittle, highly anisotropic material, is typically encapsulated with aluminum, Al/SiC or C/Ep. The CTE of the encapsulant tends to dominate. Fig. 4 shows an aluminum-encapsulated HOPG aircraft VME format power supply housing that is conduction cooled and dissipates 235 W. The effective inplane thermal conductivity reportedly is 1160 W/m-K.

A key limitation of Al/SiC is that its thermal conductivity is only in the range of aluminum. However, it can be enhanced by insertion of HOPG, as it is for the VME housing in Fig. 4, while closely maintaining the CTE of Al/SiC. The thermoelectric cooler base in Fig. 5 is another good example.

The need for lightweight, high-thermal-conductivity materials in notebook computers has led to the increased use of natural graphite heat spreaders. The lightweight laptop in Fig. 6 has no heat pipes or fans.

In what I consider to be an historic milestone, diamond/SiC CMC heat spreaders are now in IBM servers. Fig. 7 shows a diamond/SiC heat spreader coated with silicon to improve surface roughness.

As previously discussed, the high CTE of pc boards is a key source of thermal stresses and warping. A traditional solution has been the use of Cu/I/Cu constraining layers to reduce CTE. A new lightweight approach uses thermally conductive carbon fibers that, in addition to reducing CTE, increase pc board effective thermal conductivity. The high stiffness of these fibers also reduces warping and increases natural vibration frequency. Thermally conductive carbon fibers also are being used in high-performance TIMs and thermal straps.

Solving Manufacturing Problems

CTE mismatches cause thermal stresses and warping that can result in failures during processing, greatly reducing yield. The increased use of lead-free solders, which have much higher processing temperatures, is exacerbating the situation. I was asked to solve such a problem. In that particular case, the yield of a complex and expensive ceramic package was less than 5%. Modeling the many process steps using finite element analysis enabled definition of the base plate CTE required to produce an acceptable level of warping. This increased the yield to more than 99%, saving more than $60 million.

Cost is a complex issue involving many factors, and component and system costs are both important considerations. For example, an expensive material may well be cheaper than a pumped liquid-cooling system when all costs, including component, manufacturing, servicing and warranty, are included. If we adopt the approach that the proof of the pudding is in the eating, we find that, as discussed previously, several high-performance materials are being used in commercial and aerospace applications, demonstrating their cost-effectiveness.