Thermal Management

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.

Heat Dissipation

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.

Table 1. Thermal Circuit Parameters

ParameterNameUnitsDescription
ΘJAJunction-to-ambient thermal resistanceoC/WSpecified in the data sheet
ΘJCJunction-to-case thermal resistanceoC/W

Specified in the data sheet

ΘCSCase-to-heat-sink thermal resistanceoC/WThermal interface material thermal resistance
ΘCACase-to-ambient thermal resistanceoC/W

ΘSAHeat-sink-to-ambient thermal resistanceoC/WSpecified by the heat sink manufacturer
TJJunction temperatureoCThe junction temperature as specified under Recommended Operating Conditions for the device
TJMAXMaximum junction temperatureoCMaximum junction temperature as specified under Recommended Operating Conditions for the device
TAAmbient temperatureoCTemperature of the local ambient air near the component
TSHeat sink temperatureoC

TCDevice case temperatureoC

PPowerWTotal power from the operating device. Use the estimated value for selecting a heat sink

Thermal Resistances

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.

Table 2. Device Thermal Equations

Device Equation
Without a heat sinkΘJA = ΘJC + ΘCA = (TJ - TA) / P
With a heat sinkΘJA = ΘJC +ΘCS +ΘSA = (TJ - TA) / P

Determining Heat Sink Usage

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

TJ = TA + P × Θ JA

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

ΘJA = ΘJC + ΘCS + ΘSA = (TJMAX - TA) / P

ΘSA = (TJMAX - TA) / P - ΘJC - ΘCS

Example of Determining the Necessity of a Heat Sink

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:

Table 3. Operating Conditions

Parameter
Value
Power
20 W
Maximum TA

50oC

Maximum TJ

85oC

Air flow rate

400 feet per minute

ΘJA under 400 feet-per-minute air flow

4.7oC/W

ΘJC

0.13oC/W

  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 ΘCS of 0.1 °C/W for typical thermal interface material), calculate the required heat-sink-to-ambient thermal resistance:
    ΘSA
    = (TJmax -TA) / P -ΘJC - ΘCS
      = (85 -50) / 20 - 0.13 - 0.1
      = 1.52 °C/W
  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. Intel 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:

TJ

= TA + P × ΘJA

  = TA + P × (ΘJC + ΘCS + ΘSA)
  = 50 + 20 × (0.13 + 0.1 + 1.35)
  = 81.6 °C

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

Heat Sink Evaluations

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

Finite Element Models

The finite element models represent applications where a package contains a heat sink. Intel FPGA tested thermal resistances on two heat sinks from Alpha Novatech using four Intel 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.

Table 4. ΘJA 400 Feet-per-Minute Air Flow

Heat Sink Package ΘJAFrom Modeling (oC/W) ΘJAFrom Datasheet (oC/W)
Z35-12.7B EP2S90 device in a 1,020-pin FineLine BGA® package
2.6
2.2
Z35-12.7B EP2S180 device in a 1,020-pin FineLine BGA package
2.3
2.1
Z40-6.3B EP2S90 device in a 1,020-pin FineLine BGA package
3.3
3
Z40-6.3B EP2S180 device in a 1,020-pin FineLine BGA package
3
2.8

Measurements

Thermal resistance is measured according to the JEDEC Standard JESD51-6. Intel 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 (www.alphanovatech.com). These heat sinks contain pre-attached thermal tape (Chomerics T412).

Four Intel 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.

Table 5. ΘJA 400 Feet-per-Minute Air Flow

Heat Sink Actual ΘJA(oC/W) Datasheet ΘJA(oC/W)
UB35-25B
2.2
2.2
UB35-25B
2.5
2.4

Z35-12.7B

2.8
2.6
Z40-6.3B
3.8
3.4

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

Figure 2. Effect of Airflow Rate on ΘJA

Thermal Interface Material

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.

Grease

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.

Table 6. Greases

Pros Cons
Low thermal resistance
(0.2 to 1 oC cm2/W).

Messy and difficult to apply because of their high viscosity.
Requires mechanical clamping (applying pressure in the 300 kPa range).
In applications with repeated power on/off cycles, "pump-out" occurs, in which the grease is forced from between the silicon die and the heat sink each time the die is heated up and cooled down. This causes degradation in thermal performance over time and potentially contaminates neighboring components.

Gel

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.

Table 7. Gels

Pros

Cons

Low thermal resistance
(0.4 to 0.8 oC cm2/W).
Requires mechanical clamping.

Thermally Conductive Adhesives

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

Table 8. Thermally Conductive Adhesives

Pros

Cons

Low thermal resistance
(0.15 to 1 oC cm2/W).
Not reworkable.

No need for mechanical clamping.

Thermal Tapes

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

Table 9. Thermal Tapes

Pros

Cons

Simple assembly.
High thermal resistance
(1 to 4 oC cm2/W).
No need for mechanical clamping.
Generally not suitable for packages that don't have flat surfaces.

Elastomeric Pads

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.

Table 10. Elastomeric Pads

Pros

Cons

Simple assembly. High thermal resistance
(1 to 3 oC cm2/W).

Requires mechanical clamping.

Needs high pressures (~700 kPa) to achieve an adequate interface.

Phase Change Materials

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.

Table 11. Phase Change Materials

Pros

Cons

Thermal resistance

(0.3 to 0.7 oC cm2/W).

Rework difficult

Requires mechanical clamping (applying pressure in the 300 kPa range).

Heat Sink Vendors

The following is a list of heat sink vendors:

Thermal Interface Material Vendors

The following is a list of thermal interface material vendors: