DAN222
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
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 remembers these profiles from one operating
session to the next. Figure 4 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”
200°C
STEP 2
STEP 3
VENT
HEATING
“SOAK” ZONES 2 & 5
“RAMP”
STEP 5
STEP 4
HEATING
HEATING
ZONES 4 & 7
ZONES 3 & 6
“SPIKE”
“SOAK”
170°C
160°C
STEP 6 STEP 7
VENT COOLING
205° TO 219°C
PEAK AT
SOLDER JOINT
DESIRED CURVE FOR HIGH
MASS ASSEMBLIES
150°C
150°C
140°C
SOLDER IS LIQUID FOR
40 TO 80 SECONDS
(DEPENDING ON
MASS OF ASSEMBLY)
100°C
100°C
DESIRED CURVE FOR LOW
MASS ASSEMBLIES
50°C
TIME (3 TO 7 MINUTES TOTAL)
T
MAX
Figure 4. Typical Solder Heating Profile
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