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. 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.

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.

Table 1. Thermal Circuit Parameters

Finite element models were used to predict thermal resistance of packaged devices, the values of which closely match the thermal resistance values provided in the Stratix II Device Handbook. Table 2 shows the thermal resistance equations for a device with and without a heat sink.

To determine the necessity of a heat sink, designers can calculate the junction temperature using the following equation:

T_J = T_A + P × Θ _JA

If the calculated junction temperature (T_J) is more than the specified maximum allowable junction temperature (T_JMAX), an external thermal solution (heat sink, added air flow, or both) is required. Reworking the equation in Table 2 above:

Θ_JA = Θ_JC + Θ_CS + Θ_SA = (T_JMAX - T_A) / P

Θ_SA = (T_JMAX - T_A) / P - Θ_JC - Θ_CS

The following procedure provides a method one can use to determine whether a heat sink is required. This example uses an EP2S180F1508 Stratix II device, with the conditions listed below in Table 3:

1. Using the junction temperature equation, calculate the junction temperature under the listed operational conditions: TJ = TA + P × ΘJA = 50 + 20 × 4.7 = 144 °C

The junction temperature of 144 °C is higher than the specified maximum junction temperature of 85 °C, so a heat sink is absolutely required to guarantee proper operation.

2. Using the heat-sink-to-ambient equation (and a Θ_CS of 0.1 °C/W for typical thermal interface material), calculate the required heat-sink-to-ambient thermal resistance:

3. Select a heat sink that meets the thermal resistance requirement of 1.52 °C/W. The heat sink must also physically fit onto the device. FPGA reviewed heat sinks from several suppliers, and references a heat sink from Alpha Novatech (Z40-12.7B) for this example.

The thermal resistance of Z40-12.7B at an air flow of 400 feet per minute is 1.35 °C/W. Therefore, this heat sink will work since the published thermal resistance ΘSA is less than the required 1.52 °C/W.

Using this heat sink, and re-verifying:

81.6 °C is under the specified maximum junction temperature of 85 °C, verifying that the Z40-12.7B heat sink solution will work.

The accuracy of heat sink thermal resistances provided by heat sink suppliers is critical in selecting an appropriate heat sink. FPGAs uses both finite element models and actual measurements to verify that the vendor supplied data is accurate.

The finite element models represent applications where a package contains a heat sink. FPGAs tested thermal resistances on two heat sinks from Alpha Novatech using four FPGA devices. Table 4 shows that the thermal resistances predicted by the models and the thermal resistances calculated from the supplier's datasheets are a close match.

Thermal resistance is measured according to the JEDEC Standard JESD51-6. FPGA measured thermal resistances of the following heat sinks from Alpha Novatech: UB35-25B, UB35-20B, Z35-12.7B, and Z40-6.3B. Detailed information on these heat sinks is available at the Alpha Novatech website (https://www.alphanovatech.com/en/index.html). These heat sinks contain pre-attached thermal tape (Chomerics T412).

Four FPGA devices were used to measure the heat sinks shown in Table 5, which shows a good correlation between the obtained measurements and the thermal resistances obtained from the supplier's datasheets.

The following graph in Figure 2 shows the effect of airflow rate on Θ_JA.

Thermal interface material (TIM) is the medium used to attach a heat sink onto a package surface. It functions to provide a minimal thermal resistance path from the package to the heat sink. The following sections describe the major categories of TIM.

The grease used to bond heat sinks to packages is a silicone or hydrocarbon oil that contains various fillers. Grease is the oldest class of materials and the most widely used material used to attach heat sinks.

Gels are a recently developed TIM. Gels are dispensed like grease and are then cured to a partially cross-linked structure, which eliminates the pump-out issue.

Thermally conductive adhesives are usually epoxy or silicone based formulations containing fillers, offering a superior mechanical bond.

Thermal tapes are filled pressure sensitive adhesives (PSAs) coated on a support matrix such as polyimide film, fiberglass mat, or aluminum foil.

Elastomeric pads are polymerized silicone rubbers in the form of easy-to-handle solids. With a typical thickness of 0.25 mm, most pads incorporate a woven fiberglass carrier to improve handling and contain inorganic fillers as the greases do. They are supplied as die-cut performs in the precise shape needed for the application.

Phase change materials are low temperature thermoplastic adhesives (predominantly waxes) that typically melt in the 50 to 80 °C range. When operating above the melting point they are not effective as an adhesive and need mechanical support, so they are always used with a clamp applying about 300 kPa of pressure.

The following is a list of heat sink vendors:

• Alpha Novatech (www.alphanovatech.com)
• Malico Inc. (www.malico.com.tw)
• Aavid Thermalloy (www.aavidthermalloy.com)
• Wakefield Thermal Solutions (www.wakefield.com)
• Radian Heatsinks (www.radianheatsinks.com)
• Cool Innovations (www.coolinnovations.com)
• Heat Technology, Inc. (www.heattechnologiesinc.com)

The following is a list of thermal interface material vendors:

• Shin-Etsu MicroSi (www.microsi.com)
• Lord Corporation (www.lord.com)
• Thermagon Inc. (www.thermagon.com)
• Chomerics (www.chomerics.com)
• Henkel (www.henkel-adhesives.com)