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Surface Mount InformationMotorola Small–Signal Transistors, FETs and Diodes Device Data
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 mini-
mize the thermal stress to which the devices are subjected.
•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 shall 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 since the use of forced
cooling will increase the temperature gradient and will
result in latent failure due to mechanical stress.
•Mechanical stress or shock should not be applied during
cooling.
* Soldering a device without preheating can cause excessive
thermal shock and stress which can result in damage to the
device.
TYPICAL SOLDER HEATING PROFILE
For any given circuit board, there will be a group of control
settings that will give the desired heat pattern. The operator
must set temperatures for several heating zones and a figure
for belt speed. Taken together, these control settings make
up a heating “profile” for that particular circuit board. On
machines controlled by a computer, the computer remem-
bers these profiles from one operating session to the next.
Figure 2 shows a typical heating profile for use when
soldering a surface mount device to a printed circuit board.
This profile will vary among soldering systems, but it is a
good starting point. Factors that can affect the profile include
the type of soldering system in use, density and types of
components on the board, type of solder used, and the type
of board or substrate material being used. This profile shows
temperature versus time. The line on the graph shows the
actual temperature that might be experienced on the surface
of a test board at or near a central solder joint. The two
profiles are based on a high density and a low density board.
The Vitronics SMD310 convection/infrared reflow soldering
system was used to generate this profile. The type of solder
used was 62/36/2 Tin Lead Silver with a melting point
between 177–189°C. When this type of furnace is used for
solder reflow work, the circuit boards and solder joints tend to
heat first. The components on the board are then heated by
conduction. The circuit board, because it has a large surface
area, absorbs the thermal energy more efficiently, then
distributes this energy to the components. Because of this
effect, the main body of a component may be up to 30
degrees cooler than the adjacent solder joints.
STEP 1
PREHEAT
ZONE 1
“RAMP”
STEP 2
VENT
“SOAK”
STEP 3
HEATING
ZONES 2 & 5
“RAMP”
STEP 4
HEATING
ZONES 3 & 6
“SOAK”
STEP 5
HEATING
ZONES 4 & 7
“SPIKE”
STEP 6
VENT STEP 7
COOLING
200°C
150°C
100°C
50°C
TIME (3 TO 7 MINUTES TOTAL) TMAX
SOLDER IS LIQUID FOR
40 TO 80 SECONDS
(DEPENDING ON
MASS OF ASSEMBLY)
205° TO 219°C
PEAK AT
SOLDER JOINT
DESIRED CUR VE FOR LOW
MASS ASSEMBLIES
100°C
150°C
160°C170°C
140°C
Figure 2. Typical Solder Heating Profile
DESIRED CURVE FOR HIGH
MASS ASSEMBLIES