Spray cooling of IGBT electronic power modules

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1 Thermal Challenges in Next Generation Electronic Systems, Joshi & Garimella (eds) 2002 Millpress, Rotterdam, ISBN G. Mitic, W. Kiffe, G. Lefranc, S. Ramminger Siemens AG, Corporate Technology Department, CT MS 4, Munich, Germany D.E. Tilton, B.A. Smetana, T.D. Weir Isothermal Systems Research, Clarkston, WA, USA Keywords: spray cooling, power electronics, IGBT module ABSTRACT: Removing the dissipated heat directly from the surface of the chip by means of spray cooling increases the efficiency of cooling and reduces the temperature gradients at the chip surface. Both together lead to a significant increase in the reliability of the whole system. A demonstration system with a eupec BSM 150 IGBT module was built to show the efficiency of spray cooling for high power dissipation in electronic power modules. The maximum power dissipation of the module is rated at 1250 W. IGBTs with heat dissipation of almost 200 W/cm 2 were successfully cooled by spray cooling using Fluorinert. The transient electrical measurements showed that the thermal resistance between the junction and the surroundings was K/W in the case of spray cooling in contrast to K/W for direct flow convection between water and baseplate. 1 INTRODUCTION Power semiconductors are used as switches in static converters, for instance to generate a sinusoidal current by means of pulse width modulation (Hierholzer 1996, Sommer 1997). Six such switches can generate a three-phase current whose frequency - and therefore motor speed - is variable. The increasing operating voltages of IGBT modules result in high heat dissipation (up to 200 W/cm 2 ) so that both mechanical and thermo-mechanical requirements must be satisfied. Special attention must also be paid to the increasing thermal resistance caused by thermal cycles. Figure 1 shows the set-up of a typical IGBT module. The waste heat is generated in the cell structure of the IGBTs and has to dissipate through the solder layers, the metallized substrate and the baseplate to the cooling system. In the event of high power losses, water-cooled plates to which the module is attached directly act as a heat sink. A thermally conductive grease or foil is used to reduce the thermal resistance of the interface between the module and the water-cooled plate (Khatri 2001, Rauch 1999). Despite this, the thermal resistance of the interface is comparatively high. To reduce the thermal resistance of the set-up, the Al 2 O 3 ceramic is replaced by AlN with a high thermal conductivity of 180 W/mK (Mitic 1998). For cost reasons, however, this technological step is only feasible for modules with a high blocking voltage of 3.3 kv. The heat generated during operation causes thermally induced stresses between the different materials due to their different coefficients of thermal expansion. Thermal fatigue of the soft solder layer is aggravated by temperature cycles, causing crack propagation which reduces the heat transfer between the IGBT chip and the heat sink and thus the electrical efficiency and operating life of the module (Mitic 1999, Mitic 2000). Isothermal IGBT temperatures are desirable during operation to maintain a degree of high reliability. In a conventional module set-up, the heat has to dissipate from the IGBT through many different thermally resistive layers. Table 1 shows the thermal conductivity and thickness of the different layers of the IGBT module used for the spray cooling measurement demonstrated here. As the module is cooled from the lower side of the baseplate, each layer increases the temperature of the chip. 147

2 Table 1. Thermal conductivity of the different layers in the IGBT module BSM 150 GT 120 DN2 Econo Module Thermal Conductivity [W/m.K] Thickness Silicium µm Solder µm Cu (DCB) mm Al 2 O mm Cu (DCB) mm Solder µm Cu (Baseplate) mm 30 P/A = 100 W/cm 2 T [Kelvin] IGBT Solder Al 2 O 3 ceramic Cu (DCB) Solder Cu baseplate Vertical position [mm] Figure 1. Temperature distribution in the IGBT module BSM 150 GT 120 DN2 with water cooler Removing the dissipated heat directly from the surface of the chip by means of spray cooling increases the efficiency of cooling and reduces the temperature gradients at the chip surface. Both together lead to a significant increase in the reliability of the whole system. A demonstration system with a eupec BSM 150 IGBT module was built to show the efficiency of spray cooling for high power dissipation in electronic power modules. The three-phase fullbridge has a nominal DC collector current of 150 A and the collector-emitter voltage is specified as 1200 V. At a gate voltage of 15 V, the maximum power dissipation of the module is 1250 W. 2 SPRAY COOLING BASICS A very efficient technique to extract heat from a surface is to utilize the latent heat of evaporation of a liquid. A technique called spray cooling, in which liquid droplets sprayed onto hot electronic components, evaporates to remove excess heat. The hot vapor is recovered by rejecting the waste 148 G.Mitic, W.Kiffe, G.Lefranc, S.Ramminger, D.E.Tilton, B.A.Smetana & T.D.Weir

3 heat to the ambient in a heat exchanger where it is condensed back into a liquid. Thus, the fluid is continually recycled for reuse within a closed cycle system. This mechanically driven evaporation/condensation process is nearly isothermal, or constant temperature, and can effectively maintain components at quite uniform temperatures to improve reliability. All surfaces exposed to the liquid/vapor environment remain very close to the saturation temperature of the fluid. This effectively reduces both hot spots and thermal cycling; these thermal mechanical stresses are the primary causes of failure in electronic systems. Spray cooling may therefore play a pivotal role in the design of high reliability, high performance electronic systems. The vaporization process at the device level depends on the degree of superheat of the heated surface at temperature T s. The superheat is defined by the temperature difference ( T) between the heated surface temperature, T s, and the interface temperature, T f, required to drive the flow of energy (heat) from the sprayed surface to the liquid/vapor interface where evaporation occurs. The efficiency of this process depends on many factors, including the thickness of the liquid film covering the surface, the spray characteristics, and the conditions of the surface itself. Figure 2 below illustrates a typical relationship between the heat flux (q ) removed from the surface versus the superheat. ) 2 q / A (W /c m q /A (W/cm 2 ) h II III CHF I T s T f ( o C ) Figure 2. Typical Spray Cooling Performance Curve. There are three distinct superheat regimes involved with heat from hot surfaces. These three regimes comprise the low, intermediate, and high superheat regimes and are illustrated in Figure 2. The portion of the curve labeled Region I, is the low superheat case. Here single phase forced convection is the dominant heat transfer mode. There is insufficient temperature difference to drive evaporation, or phase change. The portion of the curve labeled Region II has increased evaporation, resulting in an increase in the heat transfer efficiency. This region is characterized by large increases in heat flux achieved with only small increases in temperature difference. Depending on the fluid properties, surface characteristics, spray characteristics, and liquid film thickness, nucleation of vapor bubbles at the solid surface or within the liquid film may occur. The portion of the curve labeled Region III is where the surface begins to partially dry out, and any increase in heat flux results in larger increases in superheat. This region is considered relatively unstable since minor perturbations in heat flux can go beyond the Critical Heat Flux (CHF) point and result in a surface burnout condition. The preferred spray cooling system design will operate in the Region II and avoide operation in the unstable Region III. If CHF is exceeded, the surface temperature instantly jumps to a very high superheat regime. In this very high superheat regime, a vapor layer is formed between the droplet and the heated surface, reducing the heat transfer coefficient. This mechanism is most often referred to as the Leidenfrost phenomena. The above discussion is useful for understanding the performance of spray cooling at the device level, as it relates to heat removal from the wetted surface. Of course, reliability and Mean Time 149

However, it should be noted that spray cooling improves heat transfer from all wetted surfaces, where in conventional aircooling, only the surface with the attached heat sink is effective.

4 Between Failure (MTBF) is dependent on the junction temperature of the device. Thus, in applications where spray is applied to packaged devices, traditional methods of evaluating the conduction heat transfer from the junction to the package surface may be applied. However, it should be noted that spray cooling improves heat transfer from all wetted surfaces, where in conventional aircooling, only the surface with the attached heat sink is effective. By directly spray cooing the device junction at the surface, the waste heat is removed directly from the heat source. Thus, thermal resistances may be substantially lowered with spray cooling. In addition to understanding the heat transfer between the semiconductor junctions and the wetted surfaces, the heat transfer between the fluid and the environment (in the condenser or heat exchanger) will also impact the overall operating temperatures. Thus, spray cooling requires full system level modeling of the complete heat transfer paths. 3 SPRAY COOLING DEMONSTRATION SYSTEM A demonstration system with an IGBT module designated as BSM 150 GT 120 DN2 was built to determine the thermal properties of spray cooling (Fig. 3b). The module contains six IGBTs each with dimensions of 11 x 11 mm and six free-wheeling diodes (Fig. 3a). To utilize direct die contact spray cooling, the module was packaged without silicone gel. PF5060 Fluorinert (3M) with a boiling temperature of 56 C was used for this spray cooling demonstration. Because a calibration has to be performed at temperatures up to 90 C, (Galden) D02 with a boiling temperature of 180 C was used for this purpose. The temperature in the baseplate was measured at two positions directly beneath the IGBTs as shown in Fig. 3a. Holes were therefore drilled in the baseplate and NiCr/NiAl thermocouples were fitted into them using conductive thermal grease. The nozzles for spray cooling were positioned on both sides of the module directly above the IGBTs and beneath the baseplate at the IGBT positions. Transient measurements were performed with spray cooling on both sides as well as with spray cooling only on the chip side. Thermocouple 2 Thermocouple 1 (a) (b) Figure 3. Spray cooling demonstration system (b) with BSM 150 GT 120 DN2 IGBT module (a) 150 G.Mitic, W.Kiffe, G.Lefranc, S.Ramminger, D.E.Tilton, B.A.Smetana & T.D.Weir

5 4 MEASUREMENT OF THERMAL RESISTANCE Transient electrical or infrared measurements can be used to measure the chip temperature and the thermal resistance of power modules, including the cooling system. IR imaging allows the chip temperature to be measured during operation with an accuracy of one degree Kelvin. However, the surface of the unpackaged chip must not be covered by media which is optically absorbent in the infrared region. This means that IR imaging is not feasible when silicone gel is used for packaging. 80 Thermostat PC 60 Module + Power - supply T [K] 40 + V G = 15V - I mess A D ,3 0,4 0,5 U CE [V] (a) (b) Figure 4. Set-up for transient measurements of the collector-emitter voltage (a) and temperature dependent collector-emitter voltage for calibration (b) T [K] ,0E-05 1,0E-03 1,0E-01 1,0E+01 1,0E+03 Time [s] Figure 5. Measured transient temperature relative to the ambient temperature after switching off the IGBT module To measure the chip temperature, we performed transient electrical measurements of the temperature-dependent collector-emitter voltage U CE. A few microseconds after the device is switched off by a computer-controlled switch, a current I CE = 100 ma is applied and U CE is measured with an oscilloscope. The measured voltage can be assigned to a specific chip temperature after calibration at three different temperatures using a thermostat. From a comparison with infrared measurements, we know that the measured temperature corresponds to a median chip temperature of the active IGBT area. Figure 4 and 5 shows the set-up and basic results of transient electrical measurements. 151

6 5 RESULTS OF THE TRANSIENT ELECTRICAL MEASUREMENTS The spray cooling experiment was performed at a maximum power dissipation of up to 1400 W with operation of all six IGBTs. This corresponds to a power dissipation of 190 W/cm 2. Table 2 summarizes the measured thermal data and compares it with that of an open water cooler. The PF5060 Fluorinert had a temperature of approximately 40 C at the inlet and is sprayed onto the chip surface and onto the baseplate beneath the chips. Under these conditions, we found a temperature of 92.8 C (thermocouple 1) and 94.0 C (thermocouple 2) respectively in the baseplate beneath the chips as shown in Figure 3. The transient measurement yielded a chip temperature of 127 C. Because a large part of the IGBT surface is covered by a 15-µm thick polyimide layer with a low thermal conductivity of 0.25 W/mK, we anticipate high temperature gradients in that layer. When the polyimide is removed chemically from the IGBTs, the junction temperature decreases to C and the temperatures in the baseplate are consequently reduced to 82.8 C and 82.2 C respectively. These results from spray cooling are compared to those obtained by using a water-cooled plate. The best cooling efficiency is obtained by an open water cooler which avoids any interfaces with thermal conductive grease. To ensure similar thermodynamic conditions, the water flow was chosen to be equal to the Fluorinert flow of the spray cooling experiment, namely around 1.7 l/min. In the open water cooler, a rhombus structure of fins across the baseplate produces highly turbulent flow which increases the heat transfer. The ambient temperature of the water-cooled plate is 16.4 C and the transient electrical measurement yielded a junction temperature of 106 C. The temperature rise was thus 90 C in contrast to 71 C in the case of spray cooling. Table 2. Comparison of the temperatures and the thermal resistance of spray cooling and water-cooled plates at the IGBT module BSM 150 GT 120 DN2 Type of cooling Spray Cooling Spray cooling without polyimide on chips Pressure [psi] Direct liquid baseplate flow convection Power consumption [W] Flow rate Fluorinert / H 2 O [l/min] P v power dissipation [W] T c baseplate (thermocouple 1) [ C] T c baseplate (thermocouple 2) [ C] T a Fluorinert/H 2 O (inlet) [ C] T j junction [ C] R th(j-c) [K/W] R th(j-a) [K/W] G.Mitic, W.Kiffe, G.Lefranc, S.Ramminger, D.E.Tilton, B.A.Smetana & T.D.Weir

7 6 DISCUSSION The transient electrical measurements showed that the thermal resistance between the junction and the fluid was K/W in the case of spray cooling in contrast to K/W for direct flow convection between water and module baseplate. Although the experiment was performed with similar liquid flow rates, the thermal properties of water and Fluorinert are quite different. Under the demonstrated conditions, IGBTs with heat dissipation of almost 200 W/cm 2 were successfully cooled by spray cooling using Fluorinert with a low heat of evaporation of 89 KJ/kg compared with that of water, namely 2250 KJ/kg. In addition, the specific heat of water is four times higher than that of Fluorinert. The tested module package is not optimized for spray cooling because of its many different material layers. These different layers in the IGBT module cause heat to spread across the baseplate. Direct water flow thus cools the module efficiently across the whole baseplate. Because the module is only sprayed with Fluorinert directly at the IGBTs and the corresponding area of the baseplate, further improvements in cooling efficiency could be achieved by spray cooling the whole module. Finally, the benefits of spray cooling could be increased even more by using an optimized package without a baseplate and ceramics made of aluminum nitride with a high thermal conductivity up to 180 W/mK instead of aluminum oxide with a typical value of 30 W/mK. LITERATURE Hierholzer, M Application of high-power IGBT Modules. Proc. PCIM 96, 26 Khatri, P. & Ziemski, J Dry-To-The-Touch Thermal Grease International Symposium on Advanced Packaging Materials: Mitic, G. et al The thermal impedance of new power semiconductor modules using AlN subtrates. IAS, Industry Applications Society: Mitic, G. et al Reliability of AlN substrates and their solder joints in IGBT power modules. IEEE- Electron Device Society, ESREF 99: Mitic, G. et al AlSiC Composite Materials in IGBT Power Modules. IAS, Industry Applications Society: Rauch, R.A Test methods for characterizing the thermal transmission properties of phase-change thermal interface materials. Electronics Cooling, May Sommer, K.H. et al Multichip high-power IGBT modules for traction and industrial applications. Proc. EPE97 153

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