MMBTH81LT1

2

Motorola Small–Signal Transistors, FETs and Diodes Device Data

INFORMATION FOR USING THE SOT–23 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–23

mm

inches

0.037

0.95

0.037

0.95

0.079

2.0

0.035

0.9

0.031

0.8

SOT–23 POWER DISSIPATION

The power dissipation of the SOT–23 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–23 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 225 milliwatts.

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 225 milliwatts. There

are other alternatives to achieving higher power dissipation

from the SOT–23 package. 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.

SOLDERING PRECAUTIONS

The melting temperature of solder is higher than the rated

temperature of the device. When the entire device is heated

to a high temperature, failure to complete soldering within a

short time could result in device failure. Therefore, the

following items should always be observed in order to

minimize the thermal stress to which the devices are

subjected.

soldering should be 100

°C or less.*

leads and the case must not exceed the maximum

temperature ratings as shown on the data sheet. When

using infrared heating with the reflow soldering method,

the difference shall be a maximum of 10

°C.

260

°C for more than 10 seconds.

maximum temperature gradient shall be 5

°C or less.

be allowed to cool naturally for at least three minutes.

Gradual cooling should be used as the use of forced

cooling will increase the temperature gradient and result

in latent failure due to mechanical stress.

cooling.

* Soldering a device without preheating can cause excessive

thermal shock and stress which can result in damage to the

device.