Soldering Recommendations

The principal techniques used for the soldering of components in surface

mount technology are Infrared (IR) re-flow, vapour phase re-flow and wave

soldering. Typical profiles are shown in Figures 14, 15 and 16. When wave

soldering, the ML suppressor is attached to the circuit board by means of an

adhesive. The assembly is then placed on a conveyor and run through the

soldering process to contact the wave. With IR and vapour phase reflow; the

device is placed in a solder paste on the substrate. As the solder paste is

heated, it re-flows and solders the unit to the board.

The recommended solder for the ML suppressor is a 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. For 0402 size devices, IR

re-flow is recommended.

When using a reflow process, care should be taken to ensure that the ML

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. Examples of the

soldering conditions for the ML suppressor are given in the tables below.

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

Termination Options

Littelfuse offers three types of electrode termination finish for the

Multilayer product series:

1. Silver/Platinum (standard, not available for 0402)

2. Silver/Palladium (optional)

(The ordering information section describes how to designate them.)

FIGURE 14. REFLOW SOLDER PROFILE

FIGURE 15. WAVE SOLDER PROFILE

FIGURE 16. VAPOR PHASE 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

SECOND PREHEAT

FIRST PREHEAT

TIME (MINUTES)

250

200

150

100

50

0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

RAMP RATE

PREHEAT ZONE

MAXIMUM

40-80

SECONDS

TIME (MINUTES)

250

200

150

100

50

0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

MAXIMUM

PREHEAT DWELL

PREHEAT ZONE

RAMP RATE

40-80

SECONDS

ML Varistor Series

Surface Mount Varistors

Multilayer Transient Voltage Surge Suppressors

147

www .littelfuse .com

3

3. Nickel Barrier (optional for 0402-1210 package size)

(The ordering information section describes how to designate them.)

Next

Previous