Multilayer Transient Voltage Suppressor

MLE Varistor Series

Surface Mount Varistors

136

www .littelfuse .com

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. When wave soldering, the MLE 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 re-flow, the device is placed in a

solder paste on the substrate. As the solder paste is heated, it reflows

and solders the unit to the board.

The recommended solder for the MLE 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

MLE 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 MLE series of suppressors 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

Termination Options

Littelfuse offers two types of electrode termination finish for the

MLE series:

1. Silver/Platinum (standard) (Not 0402 and 0603 sizes)

2. Silver/Palladium (optional)

FIGURE 6. WAVE SOLDER PROFILE

FIGURE 7. VAPOR PHASE SOLDER PROFILE

TIME (MINUTES)

300

250

200

150

100

50

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

0.5

1.0

1.5

2.0

2.5

3.0

3.5

RAMP RATE

PREHEAT ZONE

MAXIMUM

40-80

SECONDS

FIGURE 5. REFLOW SOLDER PROFILE

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

MAXIMUM

PREHEAT DWELL

PREHEAT ZONE

RAMP RATE

40-80

SECONDS

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

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

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