Design of AlN-based micro-channel heat sink in direct bond copper for power electronics packaging

https://doi.org/10.1016/j.applthermaleng.2012.11.014Get rights and content

Abstract

A single-phase, laminar flow, rectangular, and AlN-based micro-channel heat sink (MCHS) with water coolant has been designed and optimized for power electronics packaging. By fabricating micro-channels in the AlN-layer of direct bond copper, the heat conduction path is minimized and high cooling performance of micro-channels is utilized. The scaling effects, including temperature-dependent fluid properties, entrance effect, viscous dissipation and conjugate heat transfer, are considered. Comparison between CFD simulation by ANSYS Fluent and well-established analytical correlations is carried out and importance of entrance effect is emphasized. For the optimal geometry, the total thermal resistance of the AlN-based MCHS is 0.128 K/W at a pressure drop of 66.6 kPa. The conventional packaging structures, in which the Cu-based MCHS is bonded to the direct bond copper by solder or thermal interface material, are investigated to compare with the proposed structure. The proposed structure shows a reduction in thermal resistance by 15% and 80% respectively.

Highlights

► A new high performance integrated power electronics module design is introduced. ► Micro-channel heat sink is directly etched inside direct bond copper. ► Thermal and hydraulic performances are analyzed numerically and analytically. ► The proposed structure shows a reduction in thermal resistance by 80%.

Introduction

With the increasing demands on high power density, high frequency, high efficiency and high reliability in power electronics applications, it is desirable to integrate power semiconductor devices (MOSFTEs, IGBTs and diodes), control circuits and passive devices (capacitors, inductors and transformers) in single module or package. The so-called integrated power electronics module (IPEM) is regarded as one of the driving forces for the modularization and integration of power electronic systems [1]. Packaging technology is the major challenge for IPEM. The standard power electronics packaging normally consists of multiple thermally resistive layers. The long heat conduction path and thermal interface materials (TIM) with low thermal conductivity hinder the cooling performance. In addition, the conventional cooling technologies, including natural convection and forced convection, have become hard to cool the high heat flux (≥100 W/cm2) in high power applications.

Some early works on the integrated power electronics packaging and cooling include: double side cooling with heat pipes [2] or liquid impingement cooling [3], replacement of TIM with solder between the direct bond copper (DBC) and heat sink [4]. Although these works are capable of dissipating heat flux of 100 W/cm2, there is still enough room to further improve the cooling efficiency.

Tuckerman and Pease [5] fabricated the 302 μm × 50 μm micro-channels in a 1 cm × 1 cm silicon wafer for the first time. It cooled a heat flux of 790 W/cm2 at a temperature rise of 71 K. This landmark has activated great interests in application of MCHS for electronics cooling. Kishimoto and Ohsaki [6] developed a new packaging technique by mounting the very large-scale integrated (VLSI) chips on the multi-layered alumina substrate with micro-channels (400 μm × 800 μm) fabricated inside. The allowable power dissipation was 400 W at a flow rate of 1.0 L/min. Schütze et al. [7] integrated the 400 μm × 1000 μm micro-channels in the FR4 printed circuit board (PCB). Heat removal capability of this cooling system was 20 W per channel. Gillot et al. [8] developed a new packaging technique for IGBT module, which was sandwiched by two DBC substrates. The Cu-based MCHSs (2 mm × 0.2 mm) were brazed to the top and bottom DBC substrates by SnPb solder. Zhang et al. [9] investigated application of MCHS for flip chip ball grid array packages (FCBGA), in which the Al-based MCHS (2 mm × 0.21 mm) was assembled to the chip by TIM. Stevanovic et al. [10] developed a new integral MCHS (300 μm × 100 μm) by fabricating micro-channels in the back Cu-layer of DBC substrate.

The primary objective of analytical study is to achieve the minimum thermal resistance at a fixed pressure drop by geometry optimization, and therefore to provide rules for experiment design. Knight et al. [11] developed an optimization method for rectangular micro-channels with both laminar and turbulent flow. Copeland [12] developed a revised analytical model, which accounted for hydrodynamically and thermally developing laminar flow. Based on the two works, Biswal et al. [13] presented an analytical model for geometry optimization of single-phase, laminar flow, rectangular and liquid cooled MCHS. Effects of various geometry parameters on thermal resistance and pressure drop were evaluated. These analytical models are based on classic fluid dynamics theories with simplified assumptions, such as constant fluid properties, fully developed flow, no viscous dissipation, and idealized boundary conditions. For micro-channels, the so-called scaling effects make these assumptions no longer correct.

Analytical models that take scaling effects into account are proposed and numerical 3D conjugate heat transfer simulation is introduced. Morini [14] and Rosa et al. [15] reviewed the importance of scaling effects for single-phase heat transfer in micro-channels respectively. Although discrepancies were observed within published results, they concluded that classic fluid dynamics theories and correlations can still be used to predict the heat transfer in micro-channels if the scaling effects are carefully considered.

Effect of temperature-dependent thermophysical properties of fluid may become significant, as the MCHS is normally subjected to high heat flux applications. Toh et al. [16] investigated the 3D fluid flow and heat transfer by numerical computation and found that the friction loss lowers at low Reynolds number, as the viscosity decreases with temperature increasing. Herwig and Mahulikar [17] evaluated the importance of temperature-dependent fluid properties and showed a significant difference between actual Nusselt number and that of constant fluid properties for micro-channels. Li et al. [18] conducted 3D numerical simulation for laminar flow in rectangular micro-channels and used the inlet, average, and variable fluid properties. They concluded that the variable properties method is more accurate and superior in engineering applications.

Due to the small size of MCHS, the entrance region will cover a significant fraction. Fully developed flow may not be formed for high Reynolds number. Qu and Mudawar [19] conducted a numerical study on the 3D fluid flow and heat transfer in rectangular micro-channels. They found that the local Nusselt number and heat transfer coefficient at the inlet is much higher and approached zero at the corners. The length of developing region was observed to vary with Reynolds number. Lee et al. [20] experimentally investigated the heat transfer in rectangular Cu-based MCHS with different sizes and compared the results with classic correlations. However, wide discrepancies were observed and they concluded improved correlations for entrance effect are still in need. The numerical simulation was found to be in good agreement with the experimental results. Hence, they recommended the use of numerical simulation to predict the thermal performance of micro-channels. Lee and Garimella [21] investigated the entrance effect in rectangular micro-channels with different aspect ratios by 3D numerical simulation. General correlations for local and average Nusselt number were proposed.

The high velocity gradient in micro-channel also makes viscous dissipation significant. Tso and Mahulikar [22] accounted for the effect of viscous dissipation in circular micro-channels by Brinkman number both experimentally and theoretically. Correlation for the convective heat transfer parameters with Brinkman number was also proposed. Koo and Kleinstreuer [23] numerically investigated the effects of viscous dissipation on temperature field and friction factor in both circular and rectangular micro-channels. They concluded that viscous dissipation is a strong function of aspect ratio and hydraulic diameter. Morini and Spiga [24] analyzed the role of viscous dissipation in heated micro-channels with liquid flow by conventional theory. They presented a criterion to account for the significance of viscous dissipation. A correlation between Brinkman number and average Nusselt number was proposed by their early work [25].

The coupling effect of heat conduction in solid walls and heat convection in fluid will influence the boundary conditions. The wall temperature and heat flux can no longer be approximated as constant. Fedorov and Viskanta [26] developed a 3D numerical model to investigate the fluid flow and conjugate heat transfer in rectangular micro-channels by solving the incompressible laminar Navier–Stokes equations. Extremely large axial and circumferential thermal gradients within the solid walls near the inlet were observed. Maranzana et al. [27] found that for small Reynolds number, the wall heat flux can become strongly non-uniform and most of the heat flux is transferred to the fluid at the entrance. They also proposed a criterion to evaluate the importance of conjugate heat transfer effect. Based on the numerical model with 3D conjugate heat transfer, lots of work has been carried out to investigate the effects of geometry on the thermal performance of MCHS [28], [29], [30], [31].

The DBC with a sandwich structure of Cu-ceramic-Cu plays a key role in the modularization and integration of power electronic systems. It serves as the mechanical support, electrical isolation and heat removal path for the power module. As the thick top Cu-layer allows large current capability, circuit diagram can be patterned in this layer. Introduce of DBC in the IPEM can simplify packaging structure and increase power density. Currently, Al2O3 is most widely used as the ceramic material of DBC due to the low cost and mature mass production. However, due to the low thermal conductivity, Al2O3 is limited to low and middle power applications. AlN (aluminum nitride) has been regarded as the most promising candidate for the ceramic material due to high thermal conductivity and high mechanical strength. Besides, it is especially attractive for application in SiC (silicon carbide) power electronics packaging, as its coefficient of thermal expansion (CTE) is very close to that of SiC.

For the conventional packaging design, the heat sink is usually bonded to the DBC by TIM to eliminate air gap. The heat generated by the power module needs to pass through DBC, TIM and finally be dissipated to the ambient by the fluid inside the MCHS. However, the low thermal conductivity of TIM (typically 1–3 W/K m) is a main impediment of cooling performance. Hence, high melting point solder is proposed to substitute the role of TIM in high power and high temperature packaging. Au80Sn20 is a promising solder material due to the high thermal conductivity, high melting point (280 °C) and CTE close to that of Cu. However, replacement of TIM with solder will increase the packaging cost. The standard packaging structure with multiple thermally resistive layers hinders the cooling performance.

Recent progress in the AlN processing technologies makes it possible to fabricate micro-channels inside the AlN-layer of DBC by wet chemical etching, high density plasma etching, diamond-tipped dicing saw and laser cutting. Hence, the DBC can even serve as the heat sink. By minimizing heat conduction path and eliminating TIM or solder, the AlN-based MCHS promises best cooling performance. In addition, the ultra-light weight and compact volume make it attractive for the hybrid electric vehicles (HEVs) applications. The US army research laboratory first proposed and fabricated the AlN-based MCHS in DBC for application in the military HEVs [32]. However, the mechanism and geometry optimization are still lacked to further improve the performance. In present work, heat transfer and fluid dynamics mechanisms of the AlN-based MCHS are studied for the first time with commercial CFD solver ANSYS Fluent. The scaling effects, including temperature-dependent fluid properties, entrance effect, viscous dissipation and conjugate heat transfer, are considered. The importance of these scaling effects for present work is assessed. Although the classical fluid dynamics theories for macro-channel are not precise enough to predict the performances of micro-channel, it can still be used to explain the phenomenon and mechanism.

Section snippets

Packaging structure

Fig. 1(a) shows the proposed packaging structure with the AlN-based MCHS. The micro-channels can be fabricated by the methods aforementioned. The AlN-based MCHS is sandwiched between two thick Cu films by the maturely developed AlN–Cu bonding process. Fig. 1(b) shows the conventional structure, in which the Cu-based MCHS is bonded to the backside of DBC by the TIM or solder. For both structures, the power module is soldered on the top side of DBC and then sealed with encapsulant and metal case

Results and discussion

The experimental results of Ref. [32] are studied as a comparison with the results of CFD simulation. Computational region of the AlN-based MCHS are shown by Fig. 3(a), with quadrilateral mesh of 181 × 41 × 41 grid lines. Water is used as the working fluid with the inlet temperature 298 K. Constant heat flux of 100 W/cm2 is assumed at the top of computational region. Boundary conditions of pressure inlet (P = Pi) and pressure outlet (P = P0) are adopted. The pressure at the inlet varies and the

Conclusion

By fabricating micro-channels inside the AlN-layer of DBC, the multiple thermally resistive layers of standard packaging technology is eliminated and heat conduction path is minimized. The AlN-based MCHS promises high cooling efficiency as well as ultra-light weight and compact volume, which is especially attractive in HEVs application. The heat transfer and fluid dynamics mechanisms of AlN-based MCHS for power electronics packaging are investigated for the first time both analytically and

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