Thermal Management

Parameter

Name

Units

Description

ΘJA

Junction-to-ambient thermal resistance

oC/W

Specified in the data sheet

ΘJC

Junction-to-case thermal resistance

oC/W

Specified in the data sheet

ΘCS

Case-to-heat-sink thermal resistance

oC/W

Thermal interface material thermal resistance

ΘCA

Case-to-ambient thermal resistance

oC/W

 

ΘSA

Heat-sink-to-ambient thermal resistance

oC/W

Specified by the heat sink manufacturer

TJ

Junction temperature

oC

The junction temperature as specified under Recommended Operating Conditions for the device

TJMAX

Maximum junction temperature

oC

Maximum junction temperature as specified under Recommended Operating Conditions for the device

TA

Ambient temperature

oC

Temperature of the local ambient air near the component

TS

Heat sink temperature

oC

 

TC

Device case temperature

oC

 

P

Power

W

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

Device

Equation

Without a heat sink

ΘJA = ΘJC + ΘCA = (TJ - TA) / P

With a heat sink

ΘJA = ΘJC +ΘCS +ΘSA = (TJ - TA) / P

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

Parameter

Equation

ΘSA = (TJmax -TA) / P - ΘJC - ΘCS

 

= (85 -50) / 20 - 0.13 - 0.1

 

= 1.52 °C/W

Parameter

Equation

TJ

= TA + P × ΘJA

 

= TA + P × (ΘJC + ΘCS + ΘSA)

 

= 50 + 20 × (0.13 + 0.1 + 1.35)

 

= 81.6 °C

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

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

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.

Pros

Cons

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

Requires mechanical clamping.

Pros

Cons

Low thermal resistance
(0.15 to 1 oC cm2/W).

Not reworkable.

No need for mechanical clamping.

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.

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.

Pros

Cons

Thermal resistance

(0.3 to 0.7 oC cm2/W).

Rework difficult

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