The primary function of thermal analysis is to predict the temperatures of components and parts within a product. By visualizing heat fluxes, thermal bottlenecks, and missed shortcut opportunities, thermal analysis seeks to eliminate any detected thermal compliance issues.
These temperature predictions are important to other analysis disciplines as well, as many real world engineering materials are known to have temperature-dependent thermo-physical properties. Temperature effects can be critically important, especially in power distribution, signal integrity, and timing signals. Copper’s impedance increases with increased temperature even within common design temperature ranges. Moreover, there may be tradeoffs when deciding what is good for thermal performance and what is good for the rest of the design. This article will discuss how thermal analysis results can influence other forms of analysis and the design tradeoffs that may result.
Thermal Analysis Moves Ahead
For the past 20 years, computational fluid dynamics (CFD) techniques have provided 3D conjugate thermal simulation results that predict and display temperatures in and around electronic product designs. Thermal designers routinely use predicted temperatures to judge thermal compliance, simply by comparing the simulated temperatures to maximum rated operating temperatures. If the operating temperature exceeds the maximum rated value there will be at least a potential degradation in the performance of the packaged IC, and at worst an unacceptable risk of thermo-mechanical failure. These techniques are commonplace today, with widespread adoption all across the electronics sector including heavy usage in semiconductors, telecommunications, automotive, aerospace, and consumer products.
The typical means of visualizing (Figure 1) the predicted temperature field for a printed-circuit board (PCB) provides useful information. However, the latest advances in thermal simulation also offer the calculation and display of thermal bottlenecks and shortcut opportunities (Figure 2). These offer insight into the reasons why certain temperature distributions occur and how best to resolve thermal issues.
Electronic Materials and Temperature
Frequently the variance in thermo-physical properties for a substance is large enough across the expected temperature range to be a first-order design effect. A common example is the thermal conductivity of silicon, which decreases by approximately 20% as temperature increases from 350°K (~77 °C) to 400°K (~127 °C). This of course has the tendency to exacerbate thermal problems at the die level. The hotter the die becomes, the more difficulty heat has in exiting the die due to the lower thermal conductivity value. This effect is often described as a ‘thermal runaway’ scenario.
Copper is used extensively in the electronics industry of course, and it too can have significant thermo-physical property changes over the expected range of operating temperatures. For example, the electrical resistivity of copper increases approximately 4% for every 10°C temperature rise within typical temperature ranges. That equates roughly to a 32% variation in resistivity over an 80°C span of temperatures. This has a substantial effect on the DC resistance of the copper in the board and significantly impacts the voltage drop and current density within the board. As joule heating effects are directly caused by current density and resistivity, temperature impacts the power distribution, and the power distribution impacts temperature.