As IC process geometries shrink to 90 nm and below and FPGA densities increase, managing power becomes a significant factor in FPGA design. While power traditionally has been a third- or fourth-order concern for most FPGA designs, the dilemma design groups face today is how to provide all the functions the market demands without exceeding power budgets. The more power a device consumes, the more heat it generates. This heat must be dissipated to maintain operating temperatures within specification.
Thermal management is an important design consideration for 90 nm Stratix® II devices. Intel® FPGA device packages are designed to minimize thermal resistance and maximize power dissipation. Some applications dissipate more power and will require external thermal solutions, including heat sinks.
Radiation, conduction, and convection are three ways to dissipate heat from a device. PCB designs use heat sinks to improve heat dissipation. The thermal energy transfer efficiency of heat sinks is due to the low thermal resistance between the heat sink and the ambient air. Thermal resistance is the measure of a substance’s ability to dissipate heat, or the efficiency of heat transfer across the boundary between different media. A heat sink with a large surface area and good air circulation (airflow) gives the best heat dissipation.
A heat sink helps keep a device at a junction temperature below its specified recommended operating temperature. With a heat sink, heat from a device flows from the die junction to the case, then from the case to the heat sink, and lastly from the heat sink to ambient air. Since the goal is to reduce overall thermal resistance, designers can determine whether a device requires a heat sink for thermal management by calculating thermal resistance using thermal circuit models and equations. These thermal circuit models are similar to resistor circuits using Ohm’s law. Figure 1 shows a thermal circuit model for a device with and without a heat sink, reflecting the thermal transfer path via the top of the package.
Figure 1. Thermal Circuit Model
Table 1 defines thermal circuit parameters. The thermal resistance of a device depends on the sum of the thermal resistances from the thermal circuit model shown in Figure 1.