ESD and EOS Solutions for Small-Sized Panels

Abstract:

Small-size panels mainly include mobile phone touch screens, tablet touch screens, and digital camera touch screens. The most common data connection interface between these panels and the motherboard is MIPI (Mobile Industry Processor Interface). By adding TVS protection solution and improving ESD and EOS tolerance level, the chances for the electronic products to be interfered or even destroyed can be greatly reduced in daily life, hence, extending the service life of the product, reducing the return repair rate, allowing consumers to trust the product more as well as improving brand reputation.

Keywords: ESD, MIPI, EOS, Panel   

With the popularization of miniaturized smart products, various consumer electronic products are changing in a rapid pace, and more and more integrated and complicated products have appeared. The panel display technology has also undergone unprecedented changes. As the resolution of display panels increases, data transmission becomes more complex and faster. However, it also causes data transmission to be more susceptible to external interference, and the most common interference factors are ESD and EOS. How to effectively protect the panel and avoid the impact of ESD/EOS attacks on function operation and even damage is an urgent issue that we need to deal with.

1. What is ESD/EOS?

ESD is an electrostatic discharge. It is the phenomenon of Charge transfer occurs when two objects are close to each other or in direct contact with each other due to potential difference caused by friction, inductance, etc. ESD events occur very frequently in real life, especially in environments with lower humidity. For example, if you touch the door handle after rubbing your hands in the winter, you will feel the electric shock, the arc produced when taking off the sweater before going to bed in winter, and the lightning outside the rainy day. These are actually ESD events that happen to us.

The dry environment may cause our body to carry up to hundreds of thousands of volts of static. If you touch the screen of an electronic device such as a mobile phone without protection, the ESD will enter the internal circuit of the electronic device through the FPC of the panel, and instantly release up to tens of amperes of current. Nowadays, the semiconductor process technology is becoming more and more sophisticated, and the most advanced process technology has reached 7nm. However, such expensive high-end IC chips themselves are not as good ESD tolerance as before, which makes the advanced electronic equipment using this chip face great challenges. For minor cases, the ESD events interfere with the normal operation of the electronic device. In severe cases, ESD may directly break down the internal IC chip of the electronic device, causing inside the chip irreparable permanent damage.

EOS is an electrical over stress. When the outside current or voltage exceeds the maximum specifications of the device or electronic product, product performance may be weakened or even damaged. EOS is produced in a variety of situations, including surges caused by unstable power supplies, transient inrush currents due to hot plug, transient induced currents generated by lightning strikes, and wrong wiring operation during tests…….

2. Power Supply ESD/EOS Solution

For system level testing of electronic equipment, the electrostatic test environment used must comply with the IEC 61000-4-2 standard. There are two test methods for contact discharge and air discharge specified in the standard documents. For different environments and electronic products, brand manufacturers can set corresponding test levels according to the actual application environment and needs. For small-sized panel products, it is generally recommended to perform contact discharge ±8kV and air discharge ±15kV tests. Establishing reasonable test standards can improve product reliability and reduce return of product rate.

In addition to ESD testing, many small-size panel products also pass the 8/20μs surge waveform to complete the EOS test. The test is performed according to the IEC 61000-4-5 standard document. The test voltage level and the position to be tested are also performed by the brand manufacturer according to the actual situation. At present, many well-known domestic and international mobile phone brands have developed relevant test standards, mainly for VSP, VSN, VCI (1.8V) Power... Due to space limitations in the FPC component area, it can utilize a single DFN1006 (0402) package or even a smaller size CSP0603 (0201) package TVS product to carry out protection. The following table is recommended by AMAZING Microelectronic Corp. the ESD and EOS protection solutions for power lines.

3. What is MIPI

MIPI [1] is short for mobile industry processor interface and is an open standard specification developed by MIPI Alliance for mobile application processors. Compared with the conventional signal transmission method, the module using MIPI has the advantages of high speed, large amount of transmission data, low power consumption and anti-interference, so it is favored by the mobile industry market.

In the MIPI transmission line used for small-sized panels, the differential lines of the low-speed and high-speed signal transmission are included in the signal transmission. MIPI specifies a differential clock channel with a data channel that can be expanded into four differential pairs. The data rate can be adjusted according to the requirements of processor and external settings. Moreover, the MIPI D-PHY specification only gives a range of data rates and does not specify a specific operating rate. In an application, the available data channels and data rates are determined by the devices at both ends of the interface. The currently available MIPI D-PHY IP core delivers up to 1 Gbps per data channel, which is a feature that has made MIPI fully suitable for today's high performance applications. In addition, in terms of the display driver or other IC-powered power lines, the power supply voltage is related to the panel size, and generally 1.8V, 3.3V, 5V, 8V or 12V, etc. may be used.

4. Best ESD Protection Solution for MIPI

In order to ensure the integrity of the data line signal transmission, the stability of the power supply voltage and the related IC chip not affected by ESD, it is necessary to add an ESD protection device to the line corresponding to the MIPI. For its complex line requirements, the addition of semiconductor TVS (Transient Voltage Suppressor) products is currently the best protection solution. When the TVS tube is subject to an instantaneous high-energy shock at both ends, it can suddenly reduce its impedance at a very fast response speed (<1 ns), directing a large current to the ground, and making clamping voltage between both ends a predetermined value to ensure that the subsequent circuit components are protected from transient high energy shocks.

Semiconductor TVS has the advantages of fast response time, large transient power, low capacitance, low leakage current, small breakdown voltage, low clamping voltage, small size, and easy installation. In the TVS selection, because the signal transmission rate of the MIPI interface data line is fast, it is recommended to use a low-capacity (≤0.5pF) protection solution. In addition, due to the limited space of the FPC component area of the small-sized panel, under the premise of low ESD clamping voltage, it is also necessary to select a small package TVS device as much as possible. The following parts in the table are recommended by AMAZING Microelectronic Corp. to be used for ESD protection solutions on small-sized panel MIPI data line:

Among the solution recommendations in the above table, the most notable product is AZ1743-04F, which uses DFN1308P5Z with package size of only 1.3mmx0.8mmx0.4mm, and the TVS array in a single very small package has integrated 4 I/O protection lines (please refer to the figure 1. It is much smaller than two or four single-channel 0402 package TVS products in PCB space that is more conducive to the design of FPC board in the small space of small-sizes panels. At the same time, AZ1743-04F has lower capacitance value, lower clamping voltage and better product performance than other multi-unit products (please refer to Figure 2). It is the optimal ESD protection solution for MPI data line.

Figure 1. AZ1743-04F Internal circuit and package schematic

Figure 2. Ultra-low electrostatic clamping voltage and ultra-low parasitic capacitance characteristics of AZ1743-04F

In summary, in order to improve the usage experience of electronic products and extend the service life of electronic products, various test requirements will continue to increase as will the testing needs for ESD/EOS. In addition, the current manufacturing process technology of various types of IC chips is becoming more and more advanced, resulting in lower and lower tolerance of the chip. In order to pass the more rigorous ESD/EOS testing, circuits at or near the interface of the sensitive chip need to consider a more efficient protection solution. With the rapid changes in the functions of electronic products, the design technology of TVS products is constantly updated and advanced so as to continue providing more robust protection for electronic products.

Reference

[1].  MIPI official website: https://mipi.org/.