Urs Kloter

Forced convective interlayer cooling in vertically integrated packages

The heat removal capability of area-interconnect-compatible interlayer cooling in vertically integrated, high-performance chip stacks was characterized with de-ionized water as coolant. Correlation-based predictions and computational fluid dynamic modeling of cross-flow heat-removal structures show that the coolant temperature increase due to sensible heat absorption limits the cooling performance at hydraulic diameters les 200 mum. An experimental investigation with uniform and double-side heat flux at Reynolds numbers les 1000 and heat transfer areas of 1 cm2 was carried out to identify the most efficient interlayer heat-removal structure. Parallel plate, microchannel, pin fin, and their combinations with pins using in-line and staggered configurations with round and drop-like shapes at pitches ranging from 50 to 200 mum and fluid structure heights of 100 to 200 mum were tested. A hydrodynamic flow regime transition responsible for a local junction temperature minimum was observed for pin fin inline structures. The experimental data was extrapolated to predict maximal heat flux in chip stacks with a 4-cm2 heat transfer area. The performance of interlayer cooling strongly depends on this parameter, and drops from >200 W/cm2 at 1 cm2 and >50 mum interconnect pitch to <100 W/cm2 at 4 cm2.

Direct Liquid Jet-Impingment Cooling With Micron-Sized Nozzle Array and Distributed Return Architecture

We demonstrate submerged single-phase direct liquid-jet-impingement cold plates that use arrays of jets with diameters in the range of 31 to 126 mum and cell pitches from 100 to 500 mum for high power-density microprocessor cooling applications. Using parallel inlet and outlet manifolds, a distributed return concept for easy scaling to 40,000 cells on an area of 4 cm was implemented. Pressure drops < 0.1 bar at 2.5 1/min flow rate have been reached with a hierarchical tree-like double-branching manifold. Experiments were carried out with water jets having Reynolds numbers smaller than 900 at nozzle to heater gaps ranging between 3 to 300 mum. We identified four flow regimes, namely, pinch-off, transition, impingement, and separation, with different influences on heat-removal and pressure-drop characteristics. Parametric analysis resulted in an optimal heat-removal rate of 420 W/cm2 using water as a coolant. For a near optimal design with a gap to inlet diameter ratio of 1.2, we measured a heat-transfer coefficient of 8.7 W/cm2 K and a junction to inlet fluid unit thermal resistance of 0.17 Kcm2 /W (720 mum chip), which is equivalent to a 370 W/cm2 cooling performance at a junction to inlet fluid temperature rise of 63 degC, a pressure drop of 0.35 bar, and a flow rate of 2.5 1/min

Hierarchical Nested Surface Channels for Reduced Particle Stacking and Low-Resistance Thermal Interfaces

This paper reports on the improvement of thermal interfaces through the control of particle stacking during bondline formation. Particle stacking occurs in highly filled materials due to pressure gradients developing during squeeze flow over a rectangular surface, resulting in non-uniform interface properties and thick bondlines with a large thermal resistance. Nested surface channel designs are presented to create a uniform pressure drop as interface material flows across a rectangular surface. Reductions in thermal resistance of 2-3times compared with that of flat surfaces are demonstrated with similar reductions in bondline thickness and assembly pressure. We obtained thermal resistances as low as 2 Kmm2/W for thin bondlines (< 5 mum). Comparative power-cycling results also demonstrate improved reliability against paste pump-out with nested channel interfaces.

High-resolution patterning and transfer of thin PDMS films: fabrication of hybrid self-sealing 3D microfluidic systems

This paper describes the fabrication procedure for a hybrid elastomer-Si structure. The procedure comprises embossing and curing a thin film in poly(dimethylsiloxane) (PDMS) with vias in the 30-micrometer regime, followed by a double transfer, first to an intermediate substrate and then, with registration, to the micromachined Si structure. A well-defined adhesion between the PDMS film, the mold, the transfer substrate and the target wafer is key to each successful transfer, and plays a crucial role in the efficient removal of nanometer-thick residual membranes that systematically obstruct vias formed by embossing. Inhibition of the cross-linking of the PDMS pre-polymer in the presence of SU-8 photoresist was observed, and overcome for our case. We fabricated hybrid PDMS-Si microfluidic systems that can be sealed reversibly on any smooth and flat substrate, and filled with different solutions.

Radially Oscillating Flow Hybrid Cooling System for Low Profile Electronics Applications

The radially oscillating flow hybrid cooling system, in the following referred to as RADIOS, provides a thin form factor cold plate with radial spreading of heat to a larger area. A small liquid volume (<10 ml) is hermetically sealed within the system and does not require external hose connections. Four membrane pumps running in a phase-delayed manner induce a constant-speed, oscillating direction fluid flow at the chip source that continuously shuttles heat to an extended periphery and returns cool liquid to the chip. In the peripheral branches, heat is transferred from the liquid to solid structures and finally dissipated to the air. A micro-scale copper mesh enables low-resistance heat transfer (solid-liquid and liquid- solid) in a thin form factor (< 2 mm). Narrow channels between the discrete heat exchanger areas optimize the spreading performance and reduce the fluid volume. Numerical modeling shows an effective conductivity of 20X and 50X over bulk copper for the spreader plates and the interconnecting tubes, respectively. The technology presented here promotes modular liquid cooling units for low-profile computing systems without incurring the risk of flooding associated with conventional liquid cooling circuits.