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Multilayer Transient Voltage Surge Suppressor

AUML Varistor Series

Soldering Recommendations

Lead (Pb) Soldering Recommendations

The principal techniques used for the soldering of components in surface

mount technology are IR Re-flow & Wave soldering. Typical profiles are

shown in Figures 8 & 9

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.

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 solderís 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 50˚C before

cleaning.

Lead-Free (Pb-free) Soldering Recommendations

Littelfuse offers the Nickel-Barrier termination finish for the optimum Pb-

free solder performance.

The preferred solder is 96.5/3.0/0.5 (SnAgCu) with an RMA flux, but

there is a wide selection of pastes & fluxes available with which the nick-

el barrier parts should be compatible.

The reflow profile must be constrained by maximums shown in Figure10.

For Pb-free Wave soldering, Figure 9 still applies.

Note: the Pb-free paste, flux & profile were used for evaluation purposes

by Littelfuse, based upon industry standards & 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 8. REFLOW SOLDER PROFILE

FIGURE 9. WAVE 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

SECOND PREHEAT

FIRST PREHEAT

230

RoHS

FIGURE 10. LEAD-FREE RE-FLOW SOLDER PROFILE

5.0

6.0

7.0

MAXIMUM TEMPERATURE 260˚C

20 - 40 SECONDS WITHIN 5˚C

PREHEAT ZONE

RAMP RATE

<3˚C/s

60 - 150 SEC

> 217˚C