Surface Mount Varistors
Multilayer High Speed Transient Voltage Surge Suppressor
MHS Varistor Series
The principal techniques used for the soldering of components in
surface mount technology are infared (IR) re-flow, vapour phase re-flow
and wave soldering. Typical profiles are shown in Figures 5, 6 and 7.
When wave soldering, the MHS 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 a substrate. As the solder paste is heated, it re-flows and solders the
unit to the board.
The recommended solder for the MHS suppressor is a 63/36/2
(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 reflow is recommended.
When using a re-flow process, care should be taken to ensure that the
MHS 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 within 100 degrees of the solder’s peak
temperature is essentail to minimize thermal shock. Examples of the sol-
dering conditions for the MHS 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
50oC before cleaning.
Recommended Pad Outline
TABLE 1: PAD LAYOUT DIMENSIONS
mm in mm in
0402 1.70 0.067 0.510 0.020 0.610 0.024
0603 2.54 0.100 0.760 0.030 0.890 0.035