AN815

Vishay Siliconix

www.vishay.com

2

Document Number: 71334

12-Dec-03

FIGURE 3.

Front of Board SC70-6

Back of Board SC70-6

vishay.com

THERMAL PERFORMANCE

Junction-to-Foot Thermal Resistance

(Package Performance)

The junction to foot thermal resistance is a useful method of

comparing different packages thermal performance.

A helpful way of presenting the thermal performance of the

6-Pin SC-70 copper leadframe device is to compare it to the

traditional Alloy 42 version.

Thermal performance for the 6-pin SC-70 measured as

junction-to-foot thermal resistance, where the “foot” is the

drain lead of the device at the bottom where it meets the PCB.

The junction-to-foot thermal resistance is typically 40_C/W in

the copper leadframe and 163_C/W in the Alloy 42 leadframe

— a four-fold improvement. This improved performance is

obtained by the enhanced thermal conductivity of copper over

Alloy 42.

Power Dissipation

leadframe is 103_C/W steady-state, compared with 212_C/W

FR4 test board. The following example shows how the thermal

resistance impacts power dissipation for the two different

leadframes at varying ambient temperatures.

ALLOY 42 LEADFRAME

Room Ambient 25 _C

Elevated Ambient 60 _C

COOPER LEADFRAME

Room Ambient 25 _C

Elevated Ambient 60 _C

As can be seen from the calculations above, the compact 6-pin

SC-70 copper leadframe LITTLE FOOT power MOSFET can

handle up to 1 W under the stated conditions.

Testing

To further aid comparison of copper and Alloy 42 leadframes,

Figure 5 illustrates single-channel 6-pin SC-70 thermal

performance on two different board sizes and two different pad

two leadframes are as follows:

LITTLE FOOT 6-PIN SC-70

Alloy 42

Copper

1) Minimum recommended pad pattern on

the EVB board V (see Figure 3.

329.7_C/W

208.5_C/W

maximum copper both sides.

211.8_C/W

103.5_C/W

The results indicate that designers can reduce thermal

leadframe device rather than the Alloy 42 version. In this

example, a 121_C/W reduction was achieved without an

increase in board area. If increasing in board size is feasible,

a further 105_C/W reduction could be obtained by utilizing a

The copper leadframe versions have the following suffix:

Single:

Si14xxEDH

Dual:

Si19xxEDH

Complementary: Si15xxEDH