Publication date: 16 February 2010
Careful precautions must be taken to ensure that transient surges do not destroy sensitive CMOS circuitry. Typically, designers must harden circuitry against transient surges that range from sub nanosecond ESD pulses to slow speed, higher power transients such as automotive alternator load dump or power line transients. Multilayer Varistors (MLVs) are an effective solution to suppress transient voltages, offering electrical and physical advantages over other means of transient protection – such as zener diodes. Devices are available in a wide range of voltages - from 3.3 to 385V. Energy ratings vary by case size but typically range from ESD-only protection levels (50mJ) to load dump values up to 50J.
Early radial-leaded MOVs were bulky devices that had inconsistent breakdown voltages, broad peak current distributions and early wear-out characteristics. Even with these disadvantages radial MOVs were often chosen as transient voltage suppressors (TVS) because of a lack of cost-effective alternatives and the fact that they delivered a performance similar to devices such as Gas Discharge Tubes (GDTs). Improvements in process control and optimization of materials allowed manufacturers to greatly minimize the wear out performance of radial leaded MOVs and tighten the distribution of key component performance, resulting in higher power radial devices and ultimately surface mount power line MOVs. New performance characteristics such as ‘off state capacitance’ were added allowing designers to use surface mount MOVs as secondary effect EMI filters.
Further developments allowed single layer SMT MOVs to transition into miniature multilayer structures that exhibit significant performance advantages over diodes. Effectively, in an MLV every grain of doped zinc oxide is a Schottky diode. The structure between the plates produces series and parallel diode connections and the entire volume dissipates energy (see Fig 1 – MLV vs Diodes).
MLVs have physical and electrical advantages over traditional protection schemes such as diodes. These advantages include small size, fast ‘turn on’ time, and repetitive strike capability.
MLVs are approximately 90% smaller than other protection schemes as a single MLV can replace a back-to-back diode plus an EMC capacitor. An equivalent circuit model of a MLV is shown in figure 2 (Varistor do’s and don’ts). The forward transmission characteristic is shown in figure 3 (Varistor do’s and don’ts) for low capacitance MLVs. In its ‘off’ state, the network acts like an EMI filter. In its ‘on’ state the MLV works like back-to-back zener diodes.
MLVs feature a lower package inductance than diodes. So much lower, in fact, that MLVs exhibit a significantly faster ‘turn on’ time than diode suppressors. Typically, MLVs turn on in 300 to 700ps (exact speed depends upon case size). Conversely, diodes turn on much slower than fast transients. Whereas an ESD pulse has a rise time of less than 1ns, a diode turns on somewhere between 1.2ns and 1.5ns (again dependent upon case size). ‘Turn on’ comparison speeds are shown in figure 4 (Varistor do’s and don’ts). Therefore, MLVs provide superior protection for transistors and ICs.
In addition, MLVs have repetitive strike capability, allowing them to withstand tens of thousands of ESD strikes without degradation. Of even more importance, MLVs can withstand large amounts of 8x20µs in rush current without degradation – see figure 5 (Varistor do’s and don’ts).
Developments at MLV manufacturers such as AVX have shrunk parts to 0201 case size discretes, and also produced 0805 x4 element arrays and 0805 x4 element varistor filter arrays configured in an LC-T manner. Application specific MLVs have been created for CAN bus, LIN, Flexray, USB port and a variety of other requirements. MLVs can exhibit capacitances as high as 10nF down to <1pF. Additionally MLVs are now available with capacitance tolerances of +/- 10%.
Historically, the solderability of Multilayer Varistors (MLVs) was a problem for electronics manufacturers. The basic construction of an unplated MLV features an external metal termination that connects the internal electrodes to the circuitry of the assembly using the MLV. The external termination 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 assembly 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 content was chosen. However, this alloy is prone to leaching during assembly, so an additional metal was added to act as an inhibitor and to improve the leach resistance. While this improves the leach resistance, this addition makes the termination less solderable. The result is that either the terminations leach away, or they do not solder well.
A plated termination system, typical of other electronic components such as capacitors and resistors, produces a much better assembled product (figure 6) (Varistor do’s and don’ts). In the plated termination, the base termination layer is still used (it provides contact from the electrodes to the circuitry). 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 soldering so that the thick film material does not leach away. It must also be thick enough to prevent the inter-metallic layer between the thick film termination and the nickel layer from affecting the solderability.
In order to protect the nickel, a layer of solder is plated on top of the nickel which preserves the solderability of the nickel layer. It must be thick and dense to keep oxygen and water from reaching the nickel layer.
However, zinc oxide varistors are semi-conductive in nature – this is what allows them to ‘turn on’ and divert a damaging transient away from sensitive electronic circuitry and safely to ground – which poses a major problem for the manufacturer that needs to plate the terminations, as the ceramic also plates! This condition, called over-plating, must be controlled, since it is both cosmetically undesirable and could result in an unwanted path of conduction across the chip.
AVX has developed a proprietary process that passivates the ceramic surface of the MLV. This allows the company to plate parts for a longer time without the over-plate effect. This results in significantly thicker layers of nickel and alloy plated onto the base termination. These thicker layers translate into bond strengths that are typically twice that of competitor’s parts, and solder fillets and parts that pass all measured solderability tests.
