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.
SC-59/SOT-23 POWER DISSIPATION
The power dissipation of the SC-59/SOT-23 is a function of
the drain 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, RθJA, 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.
TJ(max) – TA
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.
150°C – 25°C
= 225 milliwatts
The 556°C/W assumes the use of the recommended
footprint on a glass epoxy printed circuit board to achieve a
power dissipation of 225 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, the power dissipation can be doubled using
the same footprint.
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
• Always preheat the device.
• The delta temperature between the preheat and soldering
should be 100°C or less.*
• When preheating and soldering, the temperature of the
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.
• The soldering temperature and time should not exceed
260°C for more than 10 seconds.
• When shifting from preheating to soldering, the maximum
temperature gradient should be 5°C or less.
• After soldering has been completed, the device should 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.
• Mechanical stress or shock should not be applied during
* Soldering a device without preheating can cause excessive
thermal shock and stress which can result in damage to the
2 Motorola Small–Signal Transistors, FETs and Diodes Device Data
Free Datasheet http://www.datasheet4u.com/