9
SMT Assembly
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
packages, will reach solder
reflow temperatures faster than
those with a greater mass.
Agilent’s diodes have been
qualified to the time-temperature
profile shown in Figure 17. This
profile is representative of an IR
reflow type of surface mount
assembly process.
After ramping up from room
temperature, the circuit board
with components attached to it
(held in place with solder paste)
passes through one or more
preheat zones. The preheat zones
increase the temperature of the
board and components to prevent
thermal shock and begin evapo-
rating solvents from the solder
paste. The reflow zone briefly
elevates the temperature suffi-
ciently to produce a reflow of the
solder.
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
of the board or damage to compo-
nents due to thermal shock. The
maximum temperature in the
reflow zone (T
MAX
) should not
exceed 235°C.
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 temperatures
and times necessary to achieve a
uniform reflow of solder.
250
T
MAX
200
TEMPERATURE (°C)
150
Reflow
Zone
100
Preheat
Zone
50
0
0
60
120
180
240
300
TIME (seconds)
Cool Down
Zone
Figure 17. Surface Mount Assembly Profile.
HP [ AGILENT(HEWLETT-PACKARD) ]
