MSD1819A-RT1 MSD1819A-ST1

4

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

0.035

0.9

0.075

0.7

1.9

0.028

0.65

0.025

0.65

0.025

SC–70/SOT–323 POWER DISSIPATION

The power dissipation of the SC–70/SOT–323 is a function

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

soldering to the pad size given for maximum power

dissipation. Power dissipation for a surface mount device is

determined by TJ(max), the maximum rated junction tempera-

ture 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 which in this

case is 150 milliwatts.

PD =

150

°C – 25°C

833

°C/W

= 150 milliwatts

The 833

°C/W assumes the use of the recommended

footprint on a glass epoxy printed circuit board to achieve a

power dissipation of 150 milliwatts. 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, a power dissipation of 300 milliwatts can be

achieved 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 should be a maximum of 10

°C.

260

°C for more than 10 seconds.

maximum temperature gradient should 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.