Littelfuse SM20 Varistor Series - Manual (Page 3)

© 2014 Littelfuse, Inc. 83
Revised: February 6, 2014
Varistor Products
SM20 Series Varistor
Surface Mount Varistors > SM20 Series
Specifications are subject to change without notice.
Please refer to www.littelfuse.com/series/sm20.html for current information.
SM20 S
eries
Lead (Pb) Soldering Recommendations
The principal techniques used for the soldering of
components in surface mount technology are IR Re-flow
and Wave soldering. Typical profiles are shown on the right.
The terminals of SM20 series devices are tin plated copper,
and the recommended solder is 62/36/2 (Sn/Pb/Ag), 60/40
(Sn/Pb) or 63/37 (Sn/Pb). Littelfuse also recommends an
RMA solder flux.
Wave soldering is the most strenuous of the processes.
To avoid the possibility of generating stresses due to
thermal shock, a preheat stage in the soldering process
is recommended, and the peak temperature of the solder
process should be rigidly controlled.
When using a reflow process, care should be taken to
ensure that the SM20 chip is not subjected to a thermal
gradient steeper than 4 degrees per second; the ideal
gradient being 2 degrees per second. During the soldering
process, preheating to within 100 degrees of the solder's
peak temperature is essential to minimize thermal shock.
Once the soldering process has been completed, it is
still necessary to ensure that any further thermal shocks
are avoided. One possible cause of thermal shock is hot
printed circuit boards being removed from the solder
process and subjected to cleaning solvents at room
temperature. The boards must be allowed to cool gradually
to less than 50ºC before cleaning.
Lead–free (Pb-free) Soldering Recommendations
The terminals of SM20 series devices are tin plated copper,
and the recommended Lead-free solder is 96.5/3.0/0.5
(SnAgCu) with an RMA flux, though there is a wide
selection of pastes and fluxes available that should be
compatible.
The reflow profile must be constrained by the maximums
in the Lead–free Reflow Profile. For Lead–free Wave
soldering, the Wave Solder Profile still applies.
Note: the Lead–free paste, flux and profile were used for
evaluation purposes by Littelfuse, based upon industry
standards and practices. There are multiple choices of all
three available, it is advised that the customer explores the
optimum combination for their process as processes vary
considerably from site to site.
FIGURE 14. REFLOW SOLDER PROFILE
FIGURE 15. WAVE SOLDER PROFILE
FIGURE 16. LEAD-FREE RE-FLOW SOLDER PROFILE
TIME (MINUTES)
300
250
200
150
100
50
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
MAXIMUM WAVE 260°C
SECOND PREHEAT
FIRST PREHEAT
MAXIMUM TEMPERATURE 250˚C,
TIME WITHIN 5˚C OF PEAK
20 SECONDS MAXIMUM
PREHEAT ZONE
RAMP RATE
<3˚C/s
60 - 150 SEC
> 217˚C
PREHEAT ZONE
PREHEAT DWELL
RAMP RATE
<2°C/s
MAXIMUM TEMPERATURE
230°C
TEMPER
A
TURE °C
TIME (MINUTES)
250
200
150
100
50
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
40-80
SECONDS
ABOVE 183°C
TEMPER
A
TURE °C
TEMPER
A
TURE °C
TIME (MINUTES)
300
250
200
150
100
50
0
0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
FIGURE 14. REFLOW SOLDER PROFILE
FIGURE 15. WAVE SOLDER PROFILE
FIGURE 16. LEAD-FREE RE-FLOW SOLDER PROFILE
TIME (MINUTES)
300
250
200
150
100
50
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
MAXIMUM WAVE 260°C
SECOND PREHEAT
FIRST PREHEAT
MAXIMUM TEMPERATURE 250˚C,
TIME WITHIN 5˚C OF PEAK
20 SECONDS MAXIMUM
PREHEAT ZONE
RAMP RATE
<3˚C/s
60 - 150 SEC
> 217˚C
PREHEAT ZONE
PREHEAT DWELL
RAMP RATE
<2°C/s
MAXIMUM TEMPERATURE
230°C
TEMPER
A
TURE °C
TIME (MINUTES)
250
200
150
100
50
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
40-80
SECONDS
ABOVE 183°C
TEMPER
A
TURE °C
TEMPER
A
TURE °C
TIME (MINUTES)
300
250
200
150
100
50
0
0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
FIGURE 14. REFLOW SOLDER PROFILE
FIGURE 15. WAVE SOLDER PROFILE
FIGURE 16. LEAD-FREE RE-FLOW SOLDER PROFILE
TIME (MINUTES)
300
250
200
150
100
50
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
MAXIMUM WAVE 260°C
SECOND PREHEAT
FIRST PREHEAT
MAXIMUM TEMPERATURE 250˚C,
TIME WITHIN 5˚C OF PEAK
20 SECONDS MAXIMUM
PREHEAT ZONE
RAMP RATE
<3˚C/s
60 - 150 SEC
> 217˚C
PREHEAT ZONE
PREHEAT DWELL
RAMP RATE
<2°C/s
MAXIMUM TEMPERATURE
230°C
TEMPER
A
TURE °C
TIME (MINUTES)
250
200
150
100
50
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
40-80
SECONDS
ABOVE 183°C
TEMPER
A
TURE °C
TEMPER
A
TURE °C
TIME (MINUTES)
300
250
200
150
100
50
0
0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
Reflow Solder Profile
Wave Solder Profile
Lead–free Re-flow Solder Profile