NSL35TT1 Datasheet, PDF (6/7 Pages) ON Semiconductor – High Current Surface Mount PNP Silicon Mount PNP Silicon Transistor

NSL35TT1
SOLDER STENCIL GUIDELINES
Prior to placing surface mount components onto a printed
circuit board, solder paste must be applied to the pads. A
solder stencil is required to screen the optimum amount of
solder paste onto the footprint. The stencil is made of brass
or stainless steel with a typical thickness of 0.008 inches.
The stencil opening size for the surface mounted package
should be the same as the pad size on the printed circuit
board, i.e., a 1:1 registration.
TYPICAL SOLDER HEATING PROFILE
For any given circuit board, there will be a group of con-
trol settings that will give the desired heat pattern. The op-
erator must set temperatures for several heating zones, and
a figure for belt speed. Taken together, these control set-
tings make up a heating “profile” for that particular circuit
board. On machines controlled by a computer, the comput-
er remembers these profiles from one operating session to
the next. Figure 11 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 pro-
file 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 re-
flow work, the circuit boards and solder joints tend to heat
first. The components on the board are then heated by con-
duction. The circuit board, because it has a large surface
area, absorbs the thermal energy more efficiently, then dis-
tributes this energy to the components. Because of this ef-
fect, the main body of a component may be up to 30 degrees
cooler than the adjacent solder joints.
200°C
150°C
100°C
STEP 1
PREHEAT
ZONE 1
RAMP"
STEP 2
VENT
SOAK"
STEP 3
HEATING
ZONES 2 & 5
RAMP"
STEP 4
STEP 5
HEATING ^HEATING
ZONES 3 & 6 ^{ZONES 4 & 7}
SOAK"
SPIKE"
DESIRED CURVE FOR HIGH
MASS ASSEMBLIES
160°C
170°C
150°C
STEP 6 STEP 7
VENT COOLING
205° TO 219°C
PEAK AT
SOLDER JOINT
100°C
140°C
SOLDER IS LIQUID FOR
40 TO 80 SECONDS
(DEPENDING ON
MASS OF ASSEMBLY)
DESIRED CURVE FOR LOW
MASS ASSEMBLIES
50°C
TIME (3 TO 7 MINUTES TOTAL)
T_MAX
Figure 11. Typical Solder Heating Profile
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