BF720T1

3

Motorola Small–Signal Transistors, FETs and Diodes Device Data

INFORMATION FOR USING THE SOT-223 SURFACE MOUNT PACKAGE

MINIMUM RECOMMENDED FOOTPRINT FOR SURFACE MOUNTED APPLICATIONS

Surface mount board layout is a critical portion of the total

design. The footprint for the semiconductor packages must

be the correct size to insure proper solder connection

interface between the board and the package. With the

correct pad geometry, the packages will self align when

subjected to a solder reflow process.

SOT-223

0.079

2.0

0.15

3.8

0.248

6.3

0.079

2.0

0.059

1.5

0.059

1.5

0.059

1.5

0.091

2.3

0.091

2.3

mm

inches

SOT-223 POWER DISSIPATION

The power dissipation of the SOT-223 is a function of the

pad size. This can vary from the minimum pad size for

soldering to a pad size given for maximum power dissipation.

Power dissipation for a surface mount device is determined

by TJ(max), the maximum rated junction temperature of the

die, R

θJA, the thermal resistance from the device junction to

ambient, and the operating temperature, TA. Using the

values provided on the data sheet for the SOT-223 package,

PD can be calculated as follows:

PD =

TJ(max) – TA

R

θJA

The values for the equation are found in the maximum

ratings table on the data sheet. Substituting these values into

the equation for an ambient temperature TA of 25°C, one can

calculate the power dissipation of the device which in this

case is 1.5 watts.

PD =

150

°C – 25°C

= 1.5 watts

83.3

°C/W

The 83.3

°C/W for the SOT-223 package assumes the use

of the recommended footprint on a glass epoxy printed circuit

board to achieve a power dissipation of 1.5 watts. There are

other alternatives to achieving higher power dissipation from

the SOT-223 package. One is to increase the area of the

collector pad. By increasing the area of the collector pad, the

power dissipation can be increased. Although the power

dissipation can almost be doubled with this method, area is

taken up on the printed circuit board which can defeat the

purpose of using surface mount technology. A graph of R

θJA

versus collector pad area is shown in Figure 6.

0.8 Watts

1.25 Watts*

1.5 Watts

°

A, Area (square inches)

0.0

0.2

0.4

0.6

0.8

1.0

160

140

120

100

80

Figure 1. Thermal Resistance versus Collector

Pad Area for the SOT-223 Package (Typical)

Board Material = 0.0625

″

G-10/FR-4, 2 oz Copper

TA = 25°C

*Mounted on the DPAK footprint

Another alternative would be to use a ceramic substrate or

an aluminum core board such as Thermal Clad

™. Using a

board material such as Thermal Clad, an aluminum core board,

the power dissipation can be doubled using the same footprint.