Thermal Simulation Cuts Cost.
BY JOHN PARRY, Mentor Graphics, Corp.
Package choice has traditionally been made more-or-less independently of PCB design, except on rare occasions where a specific package was chosen because physical or other factors were critical. Now, thermal simulation tools provide detailed thermal package specifications that can aid package choice.
Every IC package and IC function has a different thermal signature. These thermal characteristics can influence the entire system design. Providing adequate cooling for particularly hot parts requires both correct placement of the IC on the board and adequate heat extraction through the mechanical system. The increased accuracy and availability of thermal data aids in simulation and proper thermal control for the entire system.
Rather than just looking at thermal simulation from a theoretical view, an actual application of thermal simulation provides a more empirical look at the process.
Early in the development process of a dual-port 10GB ethernet physical layer, IC engineers at one company∗ had to decide between a flip chip package, which provides the optimal thermal performance, or a wire bond package, which is less expensive. They used thermal simulation software to evaluate the thermal performance of the chip with package style.
Using Simulation Early in the Process
The performance and integration of this device presented greater than normal cooling challenges, so packaging alternatives were evaluated at an early stage of the design process.
In the flip chip interconnection method, the active area of the chip is flipped downward so that any part of the surface area can be used for interconnection. This method allows for a large number of shorter interconnections, which generally improves electrical and thermal performance. In the wire bond method, the die faces upward and small pads of metal near the edges are attached to wires that lead to pins on the outside of the package. Wire bond interconnection is a lower cost alternative in most applications.
The default way to evaluate the effect of different packages on the thermal performance of a new chip has long been to build a package prototype and chip, and perform physical testing. This means that thermal performance evaluations cannot be initiated until prototypes are available, which sometimes delays the product introduction and nearly always limits the ability to optimize thermal performance by considering alternative packages.
Figure 1. A surface temperature plot of the package as mounted on a thermal test board.
Using thermal simulation software makes it possible to produce detailed models that reconstruct the physical geometry of the package and accurately predict the temperature of the various elements within the package for a variety of conditions. The simulation results are used to evaluate the thermal performance of the device and provide thermal design guidelines to customers, such as what volume and temperature of airflow is required to maintain junction temperatures at safe levels. Figure 1 shows the surface temperature of the IC mounted on a test board.
Reducing Chip Modeling Time
Originally, maintaining the accuracy of the simulation required an intricate manual process that involved replicating the geometry of the die and package as well as lengthy simulations that were related to the complexity of the chip geometry. A new online simulation tool greatly reduces the time required to model the thermal performance of chip designs. Users enter data describing the IC through forms using a web browser and the website then creates a model of the device that can be incorporated into a thermal simulation. The user has the choice of creating a full-detail geometric model or a behavioral compact model. Compact models can predict the temperature of electronic component packages at critical points such as the junction, case, and board in less time than required with conventional models.
Flip Chip vs. Wire Bond
Determining the thermal package signature begins by creating a model of the device using both package types with the online simulation tool. Key parameters of the chip are defined; including the package type, package size, die size, number of balls, and number of metal layers in the package and power dissipation. The web site then creates detailed thermal models of each type of package. This enables creation of the two models in a small fraction of the time that would have been required to model the geometry from scratch.
Figure 2. Side temperature profile through the package centerline.
Next, a reference design is used to test new ICs and validate the simulation data. The reference design passes a steady stream of air over the chip. Temperature and airflow of the airflow are varied, and the simulation results predict the junction temperatures of the device. Each device is tested at a range of 100 to 300 linear ft/min. of airflow and an ambient temperature of 85°F. In this case, the simulation results showed neither interconnection method would exceed the junction temperature limits throughout the entire range of airflows.
By using thermal simulation, the cost of the chip was kept as low as possible. In addition, thermal simulation provides a data set that can then be used by systems designers, PCB designers, and cabinet/cooling engineers.
Using IC Thermal Data in Systems Design
Many IC manufacturers create compact product models to predict the component’s response to changes in airflow, temperature, and pressure. The models provide basic thermal performance parameters under JEDEC standard boundary conditions that designers can incorporate into complete system models to predict thermal performance. This helps ensure the accuracy of system-level models and reduces time spent on analysis.
Through use of standardized EDA tools, simulation data collected as early as IC design can be assimilated further into the design process, enabling better designs in less time for all aspects of system design.
* Applied Micro Circuits Corp.
JOHN PARRY, CEng, CITP, MBCS may be contacted at Mentor Graphics Corp. www.Mentor.com/mechanical.