Thermal and Stress Simulations
09/01/2003
Saving time in new chip designs
By Sherman Ikemoto
To simplify the thermal management task facing customers, every chip released must meet tough internal performance specifications. But it is difficult to determine the thermal performance of a new design without actually building and testing it, which normally does not occur until relatively late in the design cycle. A variety of different tests are run because of the need to evaluate thermal performance under many different conditions such as with different types of heatsinks and packaging options. Finding a problem at this point involves a time-consuming troubleshooting process and also makes it necessary to repeat the physical tests. Another critical issue is the design of the test equipment is required to validate thermal performance. A typical test stand includes both heating and cooling systems as well as a control system designed to cycle the microprocessor through a number of different temperatures, while the chip runs performance tests. Designing the control algorithm used to run the test stand is a major challenge because it is based on junction temperatures that are difficult to predict accurately in advance.
Creating and Solving CFD and FEA Models
Bao-Min Liu, a Singapore-based senior engineer at Advanced Micro Devices (AMD), used a microprocessor as a platform to evaluate a new generation of software that performs component and system-level thermal analysis with the idea of resolving thermal issues in advance of first silicon. The idea was not to replace physical testing, which is required for validation purposes, but rather to resolve thermal issues in both the silicon and the test stand prior to physical testing to avoid additional design iterations. An advantage of the software he selected* is that its developer offers a thermo-mechanical stress module that uses the temperature field calculated by the computational fluid dynamics (CFD) analysis to provide a fast, first-order estimate of thermo-mechanical stresses within components, boards, thermal interface layers, solder joints, etc. This module requires little additional setup time because it uses the geometry and temperatures already created for CFD to drive the stress analysis.
Liu modeled the microprocessor as cuboids and then added the model to a model of a board specified in Electronic Industries Association/Joint Electronic Devices Engineering Council (EIA/JEDEC) standard EIA/JESD51-3. The boards specified in this standard have specific requirements for stock material, board outline and trace design. The model was solved to generate thermal performance parameters, including junction-to-ambient thermal resistance, junction-to-board thermal resistance and junction-to-case thermal resistance, as well as temperature profiles within the package under various conditions (Figure 1). Liu tried a variety of different heat sinks and also evaluated lid and lidless options. He concluded that the thermal resistance from the junction to the heat sink was higher than desirable in the initial concept design largely because new process rules made it possible to shrink die size. He modified the model to add a metal lid to reduce the junction temperature and improve the mechanical shock resistance of the package. He then re-ran the analysis and confirmed that all thermal performance parameters were within design specifications with the new packaging design.
 Figure 1. The software predicts the temperature profile of the microprocessor package under specfied test conditions. |
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Next, Liu created a new systems-level design based on the test stand that was intended primarily to evaluate the thermal stress on the heater plate. The test stand is an insulated box with a heat exchanger at the top, holding heating and refrigeration coils. Air is drawn into the heat exchanger through a circulating fan, passed over coils, and returns to the chamber through vents in the left and right side. A thermocouple measuring air temperature is used by the control system to determine how much heat to add or subtract from the air passing through the heat exchanger. The control system is programmable to produce almost any desired temperature/ time profile.
He first created a CFD model of the test stand that also provided the model and mesh needed for thermal stress analysis (Figure 2). A CFD simulation provides fluid velocity, pressure, temperature and other variables, as appropriate, throughout the solution domain for problems with complex geometries and boundary conditions. As part of the analysis, an engineer may change the geometry of the system or the boundary conditions, and observe the effect of the changes on fluid flow patterns or distributions of other variables. CFD also can provide detailed parametric studies that can significantly reduce the amount of experimentation necessary to develop new or modified equipment.
 Figure 2. The stress profile was seamlessly generated based on the same thermal analysis illustrated in Figure 1. |
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The domain was broken down into small elements known as "the mesh." Boundary conditions, such as the heating and cooling system, were applied to this geometry. The governing partial differential equations were then discretized over this mesh and the resulting algebraic equations were solved for variables such as fluid velocity and temperature. Solving the model generated the temperature results required as input for the thermal stress analysis. Liu used graphic techniques to investigate the results and relate these predictions to the performance of specific design alternatives. The geometry, grid and temperature information were seamlessly passed from the CFD solver to the finite element analysis (FEA) solver at the end of the CFD calculations.
The software extracted the component of interest, the microprocessor, from the full CFD model and removed all mesh elements that represent air. Liu then entered the appropriate material properties and boundary conditions to apply to the package for the stress analysis. No new model build was required, as the software used the model from the CFD analysis. The only extra input needed was relevant materials data and boundary conditions. No extra model building or meshing was required. The software then performed an FEA analysis on the package and predicted the stress magnitudes. The analysis assumes that solid materials behave elastically and can have both isotropic and orthotropic material properties. This integration between analysis routes was seamless; this allowed the ability to undertake stress calculations at the end of the thermal calculation.
The solid areas in the model were discretized using traditional finite element procedures, and the change in temperature predicted by the CFD analysis was used to calculate the solid deformation. Small changes were made to the model to investigate the effects of a design change on performance. It only took about one week to create the package-level and system-level models and to perform thermal and mechanical stress analysis. The results showed there were no problems with the test stand. In the past, traditional finite element analysis software was used to model test stands. The integrated approach saves a considerable amount of time and also provides more accurate results because it does a better job of modeling convective heat transfer.
Based on the results, Liu concluded that the stress levels of the heater plate were within acceptable levels. The results of the CFD analysis also were used to develop the control algorithm for the test stand. Liu was unable to evaluate the thermal conditions accurately within the package until the prototype had been built and tested. More recently, he has been able to predict junction temperatures accurately long before parts were available for testing. In this application he took a further step forward by using stand-alone finite element software to analyze mechanical stresses on the test stand.
References
*Flomerics Inc.'s FLOTHERM
SHERMAN IKEMOTO, director of marketing, may be contacted at Flomerics Inc., 4699 Old Ironsides Drive #390, Santa Clara, CA 95054-1860; (408) 562-9100, ext 802; Fax: (408) 562-9101; E-mail: [email protected].