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Addressing difficult thermal analysis problems

Posted: 10 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:application specific integrated circuits? ASICs? ASSPs? Thermal test chips? TTC?

In a world where application specific integrated circuits (ASICs) and application specific standard products (ASSPs) are dominating every conceivable application, greater attention is being applied to their long term reliability. These chips are being built on smaller lithographies, running at higher speeds, dissipating more power and to make things worse, they are being encapsulated in ever decreasing package sizes. What could possibly go wrong?

Plenty! Higher device performance comes at a price; higher temperatures. And with higher temperatures comes lower reliability if thermal considerations aren't carefully controlled. Semiconductor manufacturers have long been aware of the problems associated with heat.

Most have application notes and white papers plastered across their web sites espousing the benefits of careful calculation of power management using their values of ΘJA and ΘJC (Junction-to-Ambient and Junction-to-Case thermal resistance, respectively) often with sidebars suggesting various heat sinks to use in marginal situations. This puts the burden of solving temperature related problems on the backs of the user.

Recent technology advances and the proliferation of the use of thermal test chips like those developed by JVD Inc. for Thermal Engineering Associates of Santa Clara, California in the United States is allowing semiconductor manufacturers and companies designing their own ASIC/ASSP devices to get ahead of the curve by thermally engineering their silicon before going to production.

Thermally engineer your own ICs
Today, almost anyone can design a thermally engineered IC. Whether you're a designer at a commercial semiconductor company or you're crafting your own ASIC, the tools are readily available to physically simulate the thermal effects of your design, well in advance of spending any money to produce your first silicon prototypes. Thermal test chips (TTCs) allow system designers to fully model, measure and modify their designs before committing to costly silicon.

TTCs are special silicon die (yes, they are analogue ASICs) that are used to model and measure the thermal performance of your chip design in situ before you commit those tooling dollars for masks and wafers. Modelling allows you to create multiple individual heat sources on the TTC die, identical to the heat sources that will occur on your final IC. Temperature sensors, strategically located throughout the TTC give you precise measurement of the temperature of the die at multiple locations simultaneously.

The heat sources can be modulated to replicate various portions of your IC being power on, off or in an intermediate mode. By tracking the absolute or changes in temperature at any point on the TTC, you can determine if one or more heat sources combine to exceed safe operating temperatures of the intended IC design. If temperatures are problematic, you can go back to your IC design and modify the chip's layout to isolate the heat sources and alleviate the potential problem.

How it's designed
TTCs are produced like any other analogue ASIC. In this example, the individual Unit Cell is 2.5mm x 2.5mm and each cell contains two heat sources (metal film resistors) and four strategically located temperature sensors (diodes) (figure 1a and 1b).

Figure 1a: A basic TTC Unit Cell schematic. Note the strategic placement of the temperature sensing diodes.

Figure 1b: A basic TTC Unit Cell layout. Each cell has two heat source resistors (show in orange) that occupy 86% of the die area contained within the electrical contact pads. Four temperature sensing diodes shown in green.

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