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AVX Multilayer Varistors – Application Notes

REWORKING ASSEMBLIES

Thermal shock is common in MLVs that are manuallyattached or reworked with a soldering iron. Rework should be performed byapplying the solder iron tip to the pad and not directly contacting any part of the component. AVX stronglyrecommends that any reworking of MLVs be done with hot air reflow rather than soldering irons. Direct contact by the soldering iron tip often causes thermalcracks that may fail at a later date. If rework by soldering iron is absolutely necessary, it is recommended that the wattage of the iron be less than 30 watts and the tip temperature be <300°C.

VARISTOR SOLDERABILITY

Historically, the solderability of Multilayer Varistors (MLVs)has been a problem for the electronics manufacturer. He was faced with a device that either did not wet as well as other electronic components, or had its termination material leached away during the assembly process. However, by utilizing proprietary procedures, AVX Corporation provides the market with a MLV that has solderability comparable to that of other electronic components, and resists leaching during assembly.

BACKGROUND

The basic construction of an unplated MLV is presented in Figure 1. The external termination is a metal that connects the internal electrodes to the circuitry of the assembly usingthe MLV. The external industrial electrode must accomplish two goals. First, it must be sufficiently solderable to allow the solder used in assembly to wet the end of the chip and make a reliable connection to the traces on the circuit board. Second, it must be robust enough to withstand the assem- bly process. This is particularly important if wave soldering is used. Unfortunately these two goals are competing. In order to achieve good solderability, an alloy high in silver contentis chosen. However, this alloy is prone to leaching duringassembly, so an additional metal is added to improve the leach resistance. While this improves the leach resistance, this addition makes the termination less solderable. The results are either terminations that leach away, or do not solder well (see the photographs in Figure 2). Clearly, a plated termination system (as seen in Figure 3) isdesired. This system, which is typical of other electronic components such as capacitors suppliers and resistors, produces a much better assembled product. In the plated termination, the base termination layer is stillused (it provides contact from the electrodes to the circuit- ry). On top of the base termination is a layer of nickel. This is the surface to which the solder bonds during assembly. It must be thick enough to stay intact during IR reflow or wave
Figure 1Unplated MLV Figure 2Leaching of Unplated Terminations Non-Wetting of Unplating Terminations Figure 3Plated MLV

Electrodes Thick Film Material

Ceramic ElectrodesSolder LayerNickel Layer Ceramic ThickFilm Material

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