The design of anisotropic thermal conductivity in multilayer printed circuit boards (PCB) is studied, where the flow of heat is manipulated through the informed layout of circuit board electrothermal traces. Three representative circuit board configurations are considered to illustrate the basic method. A baseline circuit board (Configuration 1) comprising a heat-generating device in thermal communication with a heat sensitive device by the way of a standard electrical trace connection is studied first to characterize heat flow due to conduction and convection. Building off of this baseline structure, the thermal management advantages and disadvantages of a closed heat shield for anisotropic PCB thermal conductivity are explored using the second circuit board (Configuration 2). The design of the third multilayer circuit board (Configuration 3) with optimized anisotropic PCB thermal conductivity for enhanced heat flow control is simultaneously explored. It is experimentally shown that, for PCBs subject to free convection and power densities on the order of 1–10 W/cm2, the design technique allows for noticeable (−10 °C) reduction in the temperature of the heat sensitive device without a significant (<3 °C) increase in the heat-generating device temperature. The experimental heat transfer results are verified through simulations that further confirm and explain the functionality and limitations of the heat flow control concept. The effect of Joule heating is also investigated numerically. A conceptual framework for electrothermal codesign of multilayer PCBs with integrated heat flow control and more device-dense packaging is proposed.

Design of Anisotropic Thermal Conductivity in Multilayer Printed Circuit Boards

Application of flexible printed circuit board (FPCB) in personal computer motherboards: Focusing on mechanical performance

The modelling of rotating parts, such as axial fans, is one of the main challenges of current CFD simulations of industrial applications. Different methods are available, but the most commonly used is the multiple reference frame (MRF) method. This paper investigates how different flow properties, such as temperature, pressure and velocity, develop when passing through the MRF domain. The results are compared to the more physical rigid body motion (RBM) approach. It is found that the MRF method transports the upstream properties with the streamlines of the relative velocity from the upstream to the downstream interface. This leads to a non-physical rotation by an angle that is dependent on the length of the domain and the ratio between axial and tangential velocity in the MRF region. The temperature field is more affected than the flow field, since wake structures from upstream obstacles are destroyed due to the wake of the blades. Downstream structures affect the flow in the upstream region by an increase in static pressure, which causes the streamlines in the MRF zone to slow down. Depending on the size of the obstacle, this can cause substantial distortions in the upstream and downstream flow field.

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Evaluation of the Multiple Reference Frame Approach for the Modelling of an Axial Cooling Fan

A modeling methodology for thermal analysis of the PCB structure

Simple multiple reference frame for high-order solution of hovering rotors with and without ground effect

Electronic enclosures commonly use fans, including axial fans, impellers and centrifugal blowers, when cooling by forced convection is required. Computational fluid dynamics (CFD) models of electronic enclosures have traditionally employed abstract fan models. These abstract fan models possess highly simplified fan geometries and numerical flow characteristics. The fan geometry is usually a rectangular or circular planar face with or without an inner concentric no-flow region representing the hub. The flow characteristics are typically summarized by a vendor supplied “pressure head-flow rate” (P-Q) curve, which, along with the continuity equation, is applied between the inlet and exhaust faces of the fan. It is generally expected that these fan curves are obtained through careful experimentation on laboratory apparatus conforming to test codes published by societies such as ASME and AMCA.

Validation of an advanced fan model with multiple reference frame approach

In numerical computations, axial fans are typically abstracted as two-dimensional surfaces, and this forms the basis for the “Lumped Fan” (LF) model. The LF-model relies on experimentally derived P–Q (fan) curves which should conform to the published test codes. Despite its simplicity, the LF-model’s accuracy depends on the application and acceptable error margin. Therefore, with decreasing error margins in thermal engineering, there has been an interest in accurate fan modeling techniques such as “Multiple Reference Frame” (MRF) model. The current effort provides a two-part validation of the MRF model results for an axial fan against the relevant experiments: comparison of the (i) P–Q curves, and (ii) temperature distribution in an electronic enclosure. In the former, both fan models exhibit good agreement with the experiments, however the flow structures determined by the two models exhibit substantial differences. Given these flow structures’ strong influence on the temperature field, the MRF m...

Computations with the multiple reference frame technique: Flow and temperature fields downstream of an axial fan

A printed circuit board (PCB) comprises alternating layers of dielectric material and current carrying traces and vias. As performing system-level simulations of PCB's with detailed trace and via geometries is very costly, the present approach considers the effects of the trace and via geometry in the physical model by importing ECAD data consisting of the trace and via layout of the board and determines locally varying orthotropic conductivity (kx, ky and kz ) on the printed circuit board based on the ECAD data. In addition, the present approach considers the effects of Joule heating in the current carrying traces by utilizing multiple 2-D sources where the powermap is determined by solving the governing electric field equations on the trace. In this paper, the effects of both trace layer orthotropic thermal conductivity and Joule heating are studied on a sample PCB. Comparisons are made with earlier studies and conventional models when possible. It is shown that location of the hot spots and temperature values differ substantially if different methods are used.

Orthotropic thermal conductivity and Joule heating effects on the temperature distribution of printed circuit boards

Application of multi level approach with exact boundary condition import for performing accurate and time efficient CFD design calculations for a populated system cooled by forced air flow is studied. The approach involves a coarse system level model for determining appropriate boundary condition profiles for sub-systems, and a detailed sub-system level model that utilizes the boundary condition determined from the system level model. The advantages of the proposed approach are quantified by comparison with a detailed full system model. The size of the models in terms of mesh count and the accuracy of predicted component temperatures were the quantities compared. The acceptable errors of the multi level modeling approach with profile boundary conditions were in contrast with the multi-level modeling in which the boundary conditions were uniform nominal values or mean values of velocities and temperatures.

A multi level modeling approach with boundary condition import for system, sub-system or board level thermal characterization

Forced convection air-cooled electronic systems utilize fans to sustain air flow through the enclosure. These fans are typically axial flow fans, radial impellers, and centrifugal blowers. When computing flow fields in electronic enclosures, axial fans have traditionally been abstracted as lumped fan models which may or may not be able to capture the necessary details. Under certain conditions, such lumped models may also capture some flow characteristics in the case of impellers and centrifugal blowers. These lumped models comprise a significantly simplified fan geometry, i.e. usually a planar (2-D) rectangular or circular surface with/without an inner (hub) concentric no-flow region for an axial fan or a rectangular prism/cylinder with a planar inlet for blowers/impellers, and a “pressure head-flow rate” (P-Q) curve, which may be supplied by the fan vendor or experimentally derived by the thermal designer. Irrespective of the source, the P-Q curve is obtained from laboratory experiments that conform to the test codes published by societies such as ASME and AMCA. Convenience and accuracy of lumped fan models are dependent on the specific application, cooling method and also the acceptable error margin. The acceptable error margin of the thermal design has shrunk significantly in the last decade. This has caused an interest in more accurate and robust fan modeling techniques such as Multiple Reference Frame (MRF) model which has already been commonly and successfully used in many different industries for a while. In this paper, an attempt was made to provide a validation of the MRF fan modeling applied to different types of fans. The computational fluid dynamics (CFD) model of an AMCA standard wind tunnel was used for each of the fans investigated. The P-Q curve obtained from the MRF model is benchmarked against the corresponding experimentally derived P-Q curve. Benefits and limitations of the MRF model are also discussed.Copyright © 2011 by ASME

Gokul Shankaran

Papers

Validation of an advanced fan model with multiple reference frame approach

Computations with the multiple reference frame technique: Flow and temperature fields downstream of an axial fan

Orthotropic thermal conductivity and Joule heating effects on the temperature distribution of printed circuit boards

A multi level modeling approach with boundary condition import for system, sub-system or board level thermal characterization

Advances in Fan Modeling: Using Multiple Reference Frame (MRF) Approach on Blowers