10
SMT Assembly
passes through one or more
Assembly Instructions
SOT-3x3 PCB Footprint
Reliable assembly of surface
mount components is a complex
process that involves many
material, process, and equipment
factors, including: method of
heating (e.g., IR or vapor phase
reflow, wave soldering, etc.)
circuit board material, conductor
thickness and pattern, type of
solder alloy, and the thermal
conductivity and thermal mass of
components. Components with a
low mass, such as the SOT
preheat zones. The preheat zones
increase the temperature of the
board and components to prevent
thermal shock and begin evaporat-
ing solvents from the solder paste.
The reflow zone briefly elevates
the temperature sufficiently to
produce a reflow of the solder.
Recommended PCB pad layouts
for the miniature SOT-3x3 (SC-70)
packages are shown in Figures 26
and 27 (dimensions are in inches).
These layouts provide ample
allowance for package placement
by automated assembly equipment
without adding parasitics that
could impair the performance.
The rates of change of tempera-
ture for the ramp-up and cool-
down zones are chosen to be low
enough to not cause deformation
0.026
packages, will reach solder reflow of the board or damage to compo-
temperatures faster than those
with a greater mass.
nents due to thermal shock. The
maximum temperature in the
0.07
reflow zone (T
) should not
MAX
Agilent’s diodes have been
qualified to the time-temperature
profile shown in Figure 28. This
profile is representative of an IR
reflow type of surface mount
assembly process.
exceed 235°C.
0.035
These parameters are typical for a
surface mount assembly process
for Agilent diodes. As a general
guideline, the circuit board and
components should be exposed
only to the minimum tempera-
tures and times necessary to
achieve a uniform reflow of
solder.
0.016
Figure 26. PCB Pad Layout, SOT-323
(dimensions in inches).
After ramping up from room
temperature, the circuit board
with components attached to it
(held in place with solder paste)
0.026
0.075
250
200
TMAX
0.035
0.016
150
Reflow
Zone
Figure 27. PCB Pad Layout, SOT-363
(dimensions in inches).
100
Preheat
Zone
Cool Down
Zone
50
0
0
60
120
180
240
300
TIME (seconds)
Figure 28. Surface Mount Assembly Profile.
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
