7
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 re ow, 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-323/-23 package, will reach solder re ow
temperatures faster than those with a greater mass.
Avago’s diodes have been quali ed to the time-
temperature pro le shown in Figure 21. This pro le is
representative of an IR re ow 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 evaporating solvents from the solder paste. The
re ow zone brie y elevates the temperature su ciently
to produce a re ow of the solder.
The rates of change of temperature for the ramp-up and
cool-down zones are chosen to be low enough to not
cause deformation of the board or damage to components
due to thermal shock. The maximum temperature in the
re ow zone (T
MAX
) should not exceed 260°C.
These parameters are typical for a surface mount
assembly process for Avago 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 re ow of solder.
Figure 21. Surface Mount Assembly Pro le.
25
Time
Temperature
Tp
T
L
tp
t
L
t 25° C to Peak
Ramp-up
ts
Ts
min
Ramp-down
Preheat
Critical Zone
T
L
to Tp
Ts
max
Lead-Free Re ow Pro le Recommendation (IPC/JEDEC J-STD-020C)
Re ow Parameter Lead-Free Assembly
Average ramp-up rate (Liquidus Temperature (T
S(max)
to Peak) 3°C/ second max
Preheat Temperature Min (T
S(min)
) 150°C
Temperature Max (T
S(max)
) 200°C
Time (min to max) (t
S
) 60-180 seconds
Ts(max) to TL Ramp-up Rate 3°C/second max
Time maintained above: Temperature (T
L
) 217°C
Time (t
L
) 60-150 seconds
Peak Temperature (T
P
) 260 +0/-5°C
Time within 5 °C of actual Peak temperature (t
P
) 20-40 seconds
Ramp-down Rate 6°C/second max
Time 25 °C to Peak Temperature 8 minutes max
Note 1: All temperatures refer to topside of the package, measured on the package body surface
