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 3 & 6 ZONES 4 & 7
“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)
TMAX
Figure 4. Typical Solder Heating Profile
4
Motorola Small–Signal Transistors, FETs and Diodes Device Data
MOTOROLA [ MOTOROLA, INC ]
