IEC 61000-4-2 ESD TEST IN DISPLAY DOWN CONFIGURATION FOR CELL PHONES
Abstract— During the IEC-61000-4-2 test the DUT is placed on a 0.5 mm insulating sheet, which is on the horizontal coupling plane(HCP). For discharge points on the back side of the phone this leads to a situation in which the display is facing the HCP. The phone or tablet forms a capacitance between 50 pF and 300 pF to the HCP. The capacitance value depends on the size of the phone, its screen flatness, and the flatness of the insulator which may deteriorate over time. The discharges to the phone lead to a large displacement current flow through the display. This current has multiple paths to the body of the phone: via the touch electronics, via the display electronics and directly to the body of the phone. As these currents can reach 30A(at 8 kV contact mode), they can lead to upset and damage of both the display and the touch layers. This paper provides analysis of the display down test situation in order to show reproducibiliby problems. The effect of the capacitance variation is shown by the measurement and the PSPICE model. Full-wave model was used to help understand how much of the total current flows through the body of the phone. The Lichtenberg dust figure method was used to show the contribution of the corona discharge.
Keywords— Electrostatic discharge, mobile phone, display, IEC-61000-4-2
Improving display and touch screen technology is often associated with thinner glass and smaller structure size. The displays have to pass ESD testing and test levels of 25 kV are not uncommon, especially if they may be used in an automotive environment. The IEC 61000-4-2 test standard instructs placing the DUT onto a 0.5 mm insulating sheet, which is placed on the horizontal coupling plane . For discharge points on the back side of the phone this leads to a situation in which the display is facing the HCP. The phone forms a capacitance to the HCP in the range of 50-300 pF. This leads to a large displacement current flowing though the display. At 8 kV in contact mode the current reaches 30 A. The current flows through the top glass as displacement current and distributes as conduction current between the touch screen, display, and body of the phone. Improved technologies for displays aim at strengthening the top glass, which increases its dielectric constant, and producing thinner displays which also increases the current for sparkless discharge to the display. Further, smaller touch structures and a higher pixel count in the display increases the likelihood of fused touch traces, and damaged or upset of the display layers or the associated LCD driver ICs or mainboard ICs.
Every immunity test aims at reproducing a real-world situation in the laboratory to ensure robustness against the selected situation. For a cell phone it is an unlikely situation that it would be placed display down on a flat, metallic surface since most surfaces phones are placed on are neither metallic nor flat. However, the large capacitance between the phone and the flat surface leads to the fact that the initial current of the ESD pulse (3.75 A/kV according to the standard for contact mode) is nearly fully injected into the phone and passes on as displacement current through the phone. This geometry is hardly found in reality, but the present implementation of the test inhibits progress in display and touch technology [1-2].
The paper cannot completely describe the current flow and the parameters dependent on this test situation because of the complexity of phones and tablets and the variability in test situations, but it illustrates aspects of the set-up which may be novel.
II.UPSIDE DOWN MEASUREMENT SET-UP
The general set-up situation is shown in Fig. 1. The phone is placed on the HCP. The discharge to the body of the phone will force displacement current from the body of the phone to the HCP, and via the resistive/inductive networks of the display and the touch layers to the HCP. The currents depend strongly on the flatness of the insulating layer. The phone will reach a high voltage relative to the HCP. The final voltage depends on the capacitance ratio of the phone to the HCP, the ESD generator capacitance, and the capacitance value the HCP has to its surroundings. Nevertheless, for high test voltages the voltage between the HCP and the phone is large enough to cause corona discharge on the insulating layer which is explained in section III of this paper.
A. Discharge Current Measurement
The DUT is placed flat on the insulating layer and pressed down during the test to achieve a constant capacitance value, and to ensure that the distance to the HCP is 0.5 mm. In other cases the DUT is placed partially on a spacer (tilt configuration) to change the capacitance between the phone and the HCP, shown in Fig. 2. The measured capacitances for those two cases are 65 pF and 30 pF, respectively; the peak current versus voltage is shown in Fig. 3.
The discharge current is measured using an F65 current clamp, connected to a well-shielded oscilloscope. Both contact discharge mode and the air discharge mode are used for the measurement, shown in Fig. 4 and Fig. 5. The tilt configuration measurement waveform is presented to show the effect of possible corona discharge. In the flat configuration, in which the displacement current is larger, the contribution of the corona current to the total current may be invisible. The difference of the current due to the capacitance variation by the flat/tilt arrangement is shown in section IV. When the peak current is analyzed, there is a linear relationship between the peak current and the charge voltage for contact mode. This may sound obvious as this seems to be a direct result of the contact mode circuit definition. However, if at higher voltages the corona current would significantly contribute to the peak discharge current, the peak current would increase in a nonlinear fashion: It would increase faster than the voltage because of the contribution of the corona current. In measurement we observed a nonlinear current rise around 20 ns after the peak which indicates corona discharge, as shown in Fig. 4. Corona is a process that does not repeat well, this will also lead to the low repeatability of test results if the response of the display is affected by the corona. Thus, test setups which are prone to corona should be avoided. In general, corona caused by surface discharges on the thin plastic layer covering the HCP needs to be considered for higher voltage ESD.
For air discharge the additional parameter of the arc length will complicate the analysis. The arc length will vary from discharge to discharge even if the same voltage is used, thus, the current rise will vary [4-6]. In this case only Lichtenberg dust figures allow to identify the charge deposition caused by the corona, see Fig. 7 and Fig. 8.
B. Corona Discharge
An additional problem arises from surface discharges on the top surface of the insulating material, shown in Fig. 6. These surface discharges will cause additional currents at the edge of the phone and can spread multiple centimeters away from the phone.
The charges deposited on the surface can be visualized by using the Lichtenberg dust figure method [6-8]. An example of such charge deposition measured at 25 kV is shown in Fig. 7 and Fig. 8. The surface discharge begins at around 20 ns after the initial pulse and, according to the measured data, adds tens of Ampere which can increase the likelihood of fused bridges in the touch screen layer and other damages or upsets in the touch and the display.
The corona discharges form tree-like structures. Their propagation velocity is about 1 mm/ns and they can contribute to multiple amperes of additional currents. A detailed photo is shown in Fig. 8.
A. Full-Wave Model of the Upside Down Test
The full-wave model of the upside down test helps to understand the current flow in a more detailed LCD structure. Here, the final goal is to model the display, including its inner structure, the flex cable connections, and the first level of I/O on the phone. However, this publication only shows the first steps, which also include the touch structure. A general overview of the simulation domain is shown in Fig. 9. The HCP size and its distance to the ground plane (PEC plane) are proportionally reduced to maintain the HCP to ground capacitances. This proportional reduction maintains the capacitance ratios, but it does change the wave propagation on the HCP during the first nanoseconds of the discharge, such that this effect needs to be investigate. A variety of simplification, such as modeling the ITO layer by an equivalent thin sheet are verified using multiple simulations.
For the first 5 ns, until the reflected wave reaches the ESD generator, the HCP acts as infinite large ground plane. This is also verified by the measurement of currents for discharges to the ground and HCP, shown in Fig. 10. The downsized HCP cannot model the with respect to the wave propagation on top the HCP during the first nanoseconds. Although the difference is not large, it is better to use simulation data that is obtained by using an infinite large ground plane during the first nanoseconds, and then later use the simulation data that is dominated by the capacitance of the HCP, thus data obtained from a proportionally reduced HCP size.
The detailed phone model is shown in Fig. 11, the touch screen patches are connected by thin bridges for one orientation, and each array of the touch screen layer is connected to the main board using a 10 ohm resistor (30 in total), the total current through the touch screen is the summation of the current through each array.
An S-parameter voltage port substitutes the relay contact of the Noiseken ESD generator. The excitation signal at this port is a step function with fast rise which represents the rapid voltage collapse of the discharge process .
The simulated current distribution is shown in Fig. 12. This illustration compares the current at the tip and the current in all touch layer connections. The result is valid for this specific case, however, it is influenced by the glass thickness, touch structure, ITO layer conductivity, etc. Here, the full-wave model provides a numerical way to investigate the effect of those parameters and it allows simulating the current in the bridges which are prone to fusing due to the high current density. The numerical model had 9 billion cells before the lumping process  and the number of cells was reduced to 7 million which led to a simulation time of six hours for 40 ns on a computer having two Intel(R) Xeon(R) CPUs and 64 GB RAM installed. A GPU-based system will reduce the simulation time further, allowing more details of the display and the flex connection to be introduced.
The current distribution clearly indicates that the initial peak current is mainly flowing as displacement current through the glass directly into the body of the phone, while the later current is flowing into the touch layers. This is a result of the resistivity of the touch layers. It is known that the current flow in the touch layers can fuse the bridges (see Fig. 11). The simulation allows capturing the current in the bridges. The current in one of these currents path is shown in Fig13.
B. PSPICE Model of the Upside Down Test
The flatness of the insulating layer can have a significant effect on the discharge current. Two effects need to be considered: (1) because of the variation of the capacitance between the phone and the HCP, the total current can change;
(2) the current distribution can change as the areas with locally larger capacitance values will carry higher current. The effect of the capacitance has been investigated in both PSPICE simulation and measurements. Fig. 14 shows a circuit model that allows investigating the effect of capacitance variations between the phone and the HCP. The numerical modeling of the ESD generator is explained by . For the first 5 ns, the HCP is shorted to ground, as the HCP acts as infinite large ground plane during this time frame. Later, the capacitance begins to dominate. Varying the value of the capacitance between the phone and the HCP (C4) from 40 pF to 300 pF affects the discharge current. With higher capacitance, the peak current reaches higher value and decays slower, as shown in Fig. 15. Section III indicated a similar behavior.
C. Comparison with the Measurement
The measured waveform (DUT pressed onto the insulator on the HCP, 8 kV) is compared with the full-wave simulation and the PSPICE result, shown in Fig.16. Both the full-wave and PSPICE simulation results are able to model the discharge current with respect to the peak current, rise time and the 30ns value within 20% tolerance. Thus the model can be used to investigate the aspects in the test setup which can lead to the variation of the current.
This paper analyzes aspects of the display down configuration for a cell phone in the IEC 61000-4-2 test. The paper shows measurement of linear and nonlinear behavior, including the surface corona which can increase the currents by multiple Ampere. It uses a long known (since 1776) method for visualization of surface discharges to demonstrate for the first time the effect of these surface discharges during upside down ESD testing.
It is further shown that the capacitance between the DUT and the HCP strongly affects the discharge current. This capacitance depends on the flatness of the insulator surface and is often not well controlled. Finally, the simulation of the current distribution in a touch layer obtained by full-wave simulation including the ESD generator, test set-up, cell phone and details of the touch layer, was presented.
The upside down test situation is somewhat unrealistic, as few phones or tablets will be placed on a flat metal surface in a display down position. Besides being unrealistic, the test set-up may also lead to difficult to reproduce results as the capacitance depends strongly on the thickness and flatness of the insulator used on the HCP.
This paper is based upon work supported partially by the National Science Foundation under Grant No. IIP-1440110.