Electronics Protection - Winter 2014 - (Page 8)

Feature Thermal-Fluid Modeling for Flat Thin Heat Pipes/Vapor Chambers Mohammed T. Ababneh & Pete Ritt Advanced Cooling Technologies Frank M. Gerner, University of Cincinnati Rapid and continuous improvement in electronic systems technology has necessitated improved thermal management products and solutions. With these thermal management product advances also comes the challenge of developing more sophisticated modeling techniques to identify and understand key aspects that affect device performance. In the product area, Advanced Cooling Technologies has developed an advanced Vapor Chamber Product, called the CTE matched Vapor Chamber, for laser and other high power electronics cooling (See Figure 1). To better understand the performance of these and other heat pipe devices, there have also been advances in the understanding of the importance of non-condensable gas build on the internal wick structures. Discussion of the high performance TGPs and the importance of wick level NCGs on device performance are briefly reviewed here. ACT's CTE matched vapor chamber, also referred to as flat heat pipes or Thermal Ground Planes (TGPs), offer high heat loading, with heat fluxes above 700 W/cm² over a 1 cm² area and total power up to 2,000 W at a heat flux of 500 W/cm². These new Vapor Chambers/ TGPs are constructed with aluminum nitride ceramic plates covered with direct bond copper (DBC). This structure enables direct attachment to high powered silicon, gallium arsenide and gallium nitride microelectronic chips. The new vapor chamber is suited for thermal management in laser and other high powered electronic cooling applications. Vapor chamber sizes are customizable. To date, sizes up to 10 cm by 10 cm have been manufactured. The TGPs have exceptional thermal transport and enable increased power density in defense electronic systems by utilizing lightweight, thin, heat spreaders for single chip packages and multi-chip modules (MCM) utilizing micro- and nanostructured materials. The heat transferred to the evaporator section by an external source is conducted through the TGP wall and wick structure, and then vaporizes the working fluid in the wick. As vapor is formed, its pressure increases, which drives the vapor to the condenser, where the vapor releases its latent heat of vaporization to the heat sink in the condenser. The condensed fluid returns to the evaporator due to a pressure difference. Thus, the vapor chamber is able to transport the latent heat of vaporization in the TGP. This process will continue as long as there is sufficient capillary pressure to pull the condensed liquid from Figure 1. Vapor Chamber/TGP the condenser into the evaporator with etched electrical circuitry, by the surface tension. gold plated with gold-tin solder pads ready for direct attach Generally, to study the thermal performance of a vapor chamber or of 1cm² vertical cavity surface emitting laser (VCSEL) chips. A similar heat pipe device, it is necesrepresentative sample of the sary to determine the liquid and converging wick structure is vapor pressure losses inside the heat shown lower right. pipe. Normally, momentum and energy equations are solved in the vapor and liquid regions, together with heat conduction in the solid wall. In order to analyze the performance of heat pipes properly the heat and mass transport at the vapor/liquid interface become more significant as heat pipes decrease in size, as in our case. Numerous experimental, analytical, and numerical 8 Winter 2014 * www.ElectronicsProtectionMagazine.com models have been developed to study small diameter, flat heat pipes. However, explaining behavior at the vapor/liquid interface layer has shown some deficiencies. A significant characteristic of this model is that it depends on empirical interfacial heat transfer coefficient data to very precisely model the interfacial energy balance at the vapor/ liquid saturated wick interface. Specific areas addressed here are the impact of non-condensable gas inside the vapor chamber/TGP and their impact of g-forces is discussed here. For the current TGPs the ratio of solid to liquid thermal conductivities (ksintered copper / kwater ≈275) is very large, local thermal equilibrium does not always exist between the solid and liquid phases in the porous wick. Therefore one may not utilize a porous media energy equation for the current TGP. For the TGPs investigated, which utilize water as the working fluid, Jacob number<<1, and convection in the liquid can be neglected. Therefore, the energy transport within the fluid saturated wick is purely by diffusion. Just as important as not having a convection term in the energy equation (u dT/dx≈0), is the assumption that the evaporative heat transfer coefficient (hevap) is only a function of temperature. A mass transport experiment (MTE) is utilized to find hevap experimentally in order to estimate the performance of the TGP as shown in Figure 2. Figure 2. Left: Schematic of the MTE experimental setup to measure evaporation heat transfer coefficient of wicks. Right: Heat transfer coefficient as a function of ∆T. For our case, the thin film resistance is much larger than the vertical wick and substrate thermal resistances where the energy transport within the substrate and the vertical wick by conduction. For conventional heat pipes the Biot number (Bi>>1) which is a dimensionless quantity to compare the conduction resistance (t/k.A) within a solid body to the external convection resistance (1/h.A) to that body. For the TGP, the conduction resistance was reduced by decreasing the thickness of TGP and by using substrate and wick materials that has relatively high thermal conductivity so Bi~1 that means the convection resistance or the thin film resistance become more significant. Macro-scale vapor chamber models that are including a thermal resistance model and a pressure drop model are developed which capture the major physics governing fluid flow and heat transfer inside the TGP. The thermal resistance model contains a simple pure conduction model and pressure drop models that can consider non-condensable gases (NCGs), which typically accumulate in the condenser section of the TGP. NCGs have a significant effect on the fluid movement and condensation rate in the heat pipes. So novel experimental tools is developed for evacuating the device to remove all NCGs and filling the TGP with a working fluid to ensure proper performance of the device. Variations from the optimal charge can adversely affect the performance of the TGP [2]. Temperatures and pressures through the system are calculated after solving a system of linear equations. The thin film resistances at the evaporator and the condenser were calculated based on the heat transfer coefficient versus ∆T curve (Figure 2). Results highlight the importance of such factors (NCGs, gfactor); Figure 3 shows the effect of NCGs on the TGP thermal conductivity. It is clear that the TGP's thermal conductivity is proportional inversely to the mole fraction of NCGs. For the present testing arrangement, http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Winter 2014

Editor's Choice
EMI Compliance: Choosing the Right Shielding and Gasketing
Thermal-Fluid Modeling for Flat Thin Heat Pipes/Vapor Chambers
Increase Rack Cooling Efficiency and Solve Heat-Related Problems
Seven Essential Cabinet Design Considerations for Protecting 19 Inch Electronics
A Better Alternative to Heat Pipes: Integrating Vapor Chambers Into Heat Sinks
Common IP Testing Failures and How to Avoid Them
Industry News

Electronics Protection - Winter 2014