MMBD1000LT1 MMBD2000T1 MMBD3000T1 MMSD1000T1

3

Motorola Small–Signal Transistors, FETs and Diodes Device Data

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.

mm

inches

2.5-3.0

0.039

1.0

0.094

0.8

0.098-0.118

2.4

0.031

0.95

0.037

0.95

0.037

SOT–23

mm

inches

0.037

0.95

0.037

0.95

0.079

2.0

0.035

0.9

0.031

0.8

SC–59

mm

inches

0.035

0.9

0.075

0.7

1.9

0.028

0.65

0.025

0.65

0.025

SC–70/SOT–323

SOD–123

mm

inches

0.91

0.036

1.22

0.048

2.36

0.093

4.19

0.165

POWER DISSIPATION FOR A SURFACE MOUNT DEVICE

The power dissipation for a surface mount device is a

function of the drain/collector pad size. These 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, 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. For example,

for a SOT–23 device, PD is calculated as follows.

PD =

150

°C – 25°C

556

°C/W

= 225 milliwatts

The 556

°C/W for the SOT–23 package assumes the use

of the recommended footprint on a glass epoxy printed circuit

board to achieve a power dissipation of 250 milliwatts. There

are other alternatives to achieving higher power dissipation

from the surface mount packages. One is to increase the

area of the drain/collector pad. By increasing the area of the

drain/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.

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.