Murata offers two additional types of ESD devices ceramic and silicon.
Todays electronics systems are increasingly susceptible to ESD. ICs are being built at smaller and smaller process nodes, so the transistors are physically smaller. Silicon layers are more likely to rupture, and metal traces are more likely to open or bridge at the smaller process nodes. In addition, the introduction of high speed communications standards has more stringent requirements for signal integrity than ever before. Protection of sensitive electronics from permanent damage therefore requires the incorporation of ESD devices. These devices, which come in a variety of different types, divert the charge away from sensitive parts of the system such as ICs. After an ESD event, the voltage is clamped at a certain level by the ESD device and the current from the event is shunted to ground. To decide what type of ESD device is used in what application, there are various factors that must be considered. As an example, for high speed data transfer, an ESD protection device with capacitance less than 1pF must be used to prevent high capacitance affecting signal integrity. Figure 1 shows a diagram of ESD suppression voltage versus capacitance that is used by Murata to help to select the right device for the application.
Traditional ESD Devices
Varistors are essentially non-linear variable resistors, and suppressors are low capacitance varistors. These devices are relatively inexpensive but they have several disadvantages. Firstly, low performance typical clamping voltage for a suppressor might be 150 to 500V, still way above the limits for most ICs. Secondly, these components have a limited lifespan, typically 10-20 ESD events. Transient voltage suppressors, or Zener diodes, provide a fast response when faced with an ESD event but have a relatively low current capability so are restricted to use in circuits with low current spikes. They have good lifetime and are typically used to protect high speed data lines since they can be used in series to lower capacitance.
1. Ceramic ESD Devices
Ceramic ESD devices feature ultra-low capacitance (0.05pF), meaning they can be used for high speed data lines, and are extremely robust with a long lifetime. Testing Murata ceramic ESD device we applied voltage for an ESD event of 8kV as per IEC61000-4-2 level 4. The peak voltage rose to 300V but clamping voltage was held at 40V. Compare this with the 1pF varistor and the 3pF varistor which feature clamping voltages of 200 and 100V respectively. Ceramic devices also feature extremely low insertion loss (-0.004dB@2.4GHz), giving them another advantage over varistors. A typical Murata ceramic ESD protection device measures 1.0 x 1.5 x 0.33mm.
2. Silicon ESD Devices
Silicon ESD protection devices also have excellent ESD suppression performance, but their capacitance is not typically as low as ceramic devices in the region of 0.25pF. Advantages of silicon ESD protection devices include very fast turn-on time, minimising peak voltage as shown in picture which is a comparison between a part from Muratas silicon ESD range and a TVS diode when subjected to 8kV as per IEC61000-4-2 level 4. In further tests, the silicon device also proved to have a very small on-resistance, just 0.3 Ohms compared to the TVSs 0.8 Ohms. Small on-resistance means clamping voltages can be kept low - in this example, its kept to just 8V, compared to 35V for the TVS. One of the key advantages of using a silicon device versus a ceramic one is that multi-channel versions are available to save board space in todays compact electronics. Figure shows a comparison between using discrete devices and one of Muratas 10-channel silicon ESD devices. Murata multi-channel device, which also incorporates LC filters to protect against EMI simultaneously, measures 2 x 2mm whereas the discrete solution, which features varistors and discrete LC filters, takes more than 10X the board space. There is a large range of silicon parts available with different capacitances, case sizes and number of channels for different applications.
There are many types of ESD protection device available ranging from TVS/ Zener diodes and varistors to the tiny ceramic and silicon devices produced by Murata. Selecting the right component for the application at hand will require an analysis of the performance level required, the board space available and the cost. Ceramic devices are available with ultra-low capacitance, excellent lifetime and low cost. Silicon devices are available with low capacitance, fast response times and the possibility of an array solution in one package, even incorporating EMI protection, which can save board space. All of these devices come in the tiny packaging for which Murata is world-famous.