A computer circuit board with a high clock speed processor chip was analyzed in an effort to establish the effectiveness of the cooling system.   Processor chips can run too hot and their performance can deteriorate as a result.   Different geometries for heat sinks and cooling fan locations can be analyzed in order to optimize chip cooling performance.  The particular design analyzed incorporated a high capacity cooling fan that draws air over an aluminum heat sink attached to the chip.  The heat sink was a multi-tower design.  The power of the processor chip could not be dissipated in the circuit board it was attached to, and all of it needed to be transferred efficiently to the air currents produced by the fan.

The CFD capabilities of the ANSYS Multiphysics program were used to calculate simultaneously the fluid flow velocities throughout the computer case and the steady-state temperature distribution throughout the electronics. The fluid and non-fluid regions were solved together as a conjugate heat transfer problem.  A finite element mesh was generated of the entire system including a section of the motherboard, the processor chip, and the heat sink, as well as several of the attached electrical components and connectors.   Each electrical component was assigned the appropriate thermal material properties, and the properties of dry air were assigned to the fluid.  A heat source was distributed realistically within the processor chip, and a velocity was imposed on the fluid nodes at the location of the fan blades. 

An on/off transient solution was then calculated, following which the flow and temperature results were animated.  Flow was indeed encouraged through the fins of the heat sink.  Streamlines of the particle flow are shown in Figure 1 (animation).   The temperature distribution throughout the heat sink once it had reached steady-state is shown in Figure 2.  The optimized cooling system was found to be efficient enough to keep the hottest point in the processor chip below 136°F surface temperature, slightly below the 140°F desired.

 Back to Top

Shock and Vibration Finite Element Analysis of a Printed Circuit Board

A commercial-off-the-shelf power supply was to be modified for military use.  It was not obvious whether the existing design was robust enough to pass the military shock (MIL-S-901D) and vibration (MIL-S-167) test protocols.  Before expensive prototypes were manufactured and tested, the system was to be analytically evaluated in an effort to identify weak points so that they could be redesigned.

MSI was able to simulate the standardized military shock and vibration test protocols using finite element techniques. 3D models of the circuit boards were first created.  These models included all the major electrical components, as well as the board itself and its mounting screws.  The component wires and connections were included explicitly in the model.  Particular attention was paid to the solder joint connections.

The appropriate random vibration loading conditions were then applied to the board through the mounting points on the rack simulating the Environmental Stress Screening (ESS) protocol.  In addition, shock and vibration loads were applied separately in all three orthogonal directions as per the specifications. The deflections and stresses within each of the components were surveyed for each run.  Animated plots were made of the natural frequency mode shapes and the transient response to the shockload.  These were put on CD-Rom for viewing by the customer’s engineers.

Certain component leads and their associated solder connections exhibited stresses high enough to consider them potential failure sites during the shock and vibration testing.  Based on the animated modal and transient (animation) deflection shapes of the computer models, it became obvious how to modify the designs to make them more robust.   In general, additional support was needed to prevent “tipping” motions of the components.  Upon analysis of the modified boards, the stresses were found to decrease significantly. 

Often, complicated 3D analyses such as these take too long to be useful to design engineers faced with aggressive product development timelines.  This was not the case here. By combining the time efficiency of Pro/Engineer and the analytical power of ANSYS Multiphysics, the analyses were performed quickly, yet still with the high level of detail needed to identify problem areas and solutions.

Back to Top