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PB2016.09 IEC 61000-4-2 ESD Test in Display Down Configurationfor Cell Phones

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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

I.INTRODUCTION

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 [3]. 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.

III.MEASUREMENT RESULTS

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.

IV.SIMULATION

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 [8].

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 [10] 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 [11]. 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.

V.CONCLUSION

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.

VI.ACKNOWLEDGEMENT

This paper is based upon work supported partially by the National Science Foundation under Grant No. IIP-1440110.

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PB2016.09 ESD to the Display Inducing Currents Measured Using a Substitution PC Board

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Abstract— ESD to a display may upset or damage the display or the touch circuit. The ESD may have a visible spark carrying current to the frame of the mobile device, to the connecting flex cable, or into the display by penetrating the glue between the glass layers. It may also be a sparkless ESD that causes corona charging on the surface of the glass have currents as high as 10 A. A measurement technique is presented that allows the measurement of the currents on all traces, including ground of flex cables that connect from the display to the main board of a mobile device. The main board is substituted to a PCB that has the same connections to the body of the mobile device and the same shape, i.e., the same electromagenic affects. However, all connections to the display are terminated in resistive structures that allow measuring the currents in the flex cable individually. Beside measuring ESD currents, the substitution board offers various other applications with respect to the coupling and propagation of self-interference causing signals or EMI problems.

Keywords: flex cable, LCD; mobile device; sparkless ESD; substitution board; touch-screen.

I.INTRODUCTION

Electrostatic discharge (ESD) to portable electronic devices can cause hard and soft errors [1]-[3]. Typically in system level ESD testing, the ESD generator is discharged into the exposed metal parts of the DUT. The ESD generator voltage is incremented in steps and system level errors such as perturbation of the display, loss of touch functionality, system reset, etc. are monitored. Various measurement and modeling techniques have been used to study the effect of ESD discharge directly to different areas of the mobile device such as the main board reference ground, clock traces, power pins, metal chassis of the phone, and the LCD display [4]-[8]. The main limitation is that one notices a disturbance or damage to the ICs that connect to the display, but the currents that caused these effects are not known.

Sparkless ESD on the glass surface is the most likely ESD discharge to displays as cell phone design often provides sufficient isolation at the edges such that a discharge into the phone is not possible. These sparkless discharges [9] into an electronic product may also lead to various types of upsets and damages to ICs via induced current flow using any of the multiple coupling paths within the device, as shown in Fig. 1. Sparkless ESD to glass surfaces was studied in [9] and a method to visualize the surface charges using dust figures on the glass after a discharge event was analyzed in [10]. A discharge event on the glass may cause current coupling to the touch screen matrix and then to the traces of the touch controller IC on the flex cable. Similarly, current coupling to the LCD may induce currents to the LCD driver IC. The severity and type of upset/damage depends on the magnitude of the current flow to the sensitive ICs.

 

The measurement technique presented in this paper allows measuring the induced currents at the flex cable connectors on the substitution board due to an ESD discharge on the mobile phone screen/glass. These current values can help to understand the risk of damage to the driver IC or the display IC or disturbances of the data transfer. In some cases, ICs are mounted on the glass or the flex cable. In these cases the method will only measure the current that flows out of these ICs into the flex cable, as the measurement is done on the main board of the phone.

A substitution board is designed and fabricated to replace the main board of a mobile phone. Well shielded 0.8 mm diameter semi-rigid coaxial cables are used to probe the terminated flex traces individually on the substitution board. It must be noted that using multiple coaxial cables for probing also changes the electromagnetic behavior of the device being tested. These cables offer a path to ground. If ferrite clamps are not used on the coax cables, the cables act like a single ground connection similar to a short USB cable that would connect to a well-grounded system. The distribution of currents on the various I/O lines, flex cable shield, ground and power pins is obtained by this measurement technique. A contact mode discharge into a small copper patch on the glass was selected as the excitation because it approximates the sparkless discharges to the display from a current point of view. In contrast to a real arc it still is a linear system, as no arcing is involved in contact mode. We assume that the currents are strong enough to turn on all ESD protection within the ICs that are placed on the glass, such that only their dynamic resistance is visible. Testing at different voltages has verified the assumption of linear behavior.

II.DEVICE UNDER TEST: MOBILE PHONE

A four-layer PCB was designed for a mobile phone to demonstrate the substitution board methodology for measuring individual currents. The features of the phone that influence the electromagnetic behaviour of the phone (such as the main board ground structure, main board-to-phone body contact points, flex cable connector ground contacts, etc.) are identified and reproduced on the substitution board.

The details of the main board, its connections to the LCD and touch flex cables via board-to-board connectors, are shown in Fig. 2. The main board ground structure and its contact points to the mobile phone’s reference ground are shown in Fig. 3.

III.DESIGNED AND FABRICATED SUBSTITUTION BOARD

Based on the dimensions and ESD current flow relevant features of the mobile phone, a four-layer substitution board was designed and fabricated to replace the actual main board of the mobile phone. Fig. 4 shows the layout of the top layer of the substitution board. The board has the following main features:

Similar dimensions and thickness as that of the actual main board.

Similar ground structure and contact points to phone body as that of the actual main board.

Similar ground structure and contact points to phone body as that of the actual main board.

24-pin flex connector receptacle for connection to the LCD flex cable.

10-pin flex connector receptacle for connection to the touchscreen flex cable.

Pads for semi-rigid coax cable probing for all flex connector pins including ground connections.

Pads for resistive terminations for all flex connector pins including ground connections.

The ability to measure the current on the flex shield which is normally connected via a gasket to the main PCB ground.

IV.PROBING CIRCUIT & MEASUREMENT SETUP

Direct probing of the individual traces of the original main board and display flex cables was difficult because of the multilayer board design and thin flex traces. However, the substitution board allowed multiple probing coax cables. The currents in the individual traces of the flex cables were measured using well-shielded 0.8 mm diameter semi-rigid coaxial cables that are soldered on the pads that connect to the flex connector footprint via short (< 1 cm) transmission lines. The currents were measured in two different termination schemes depending on the assumption of the state of the ESD protection diodes on the main board ICs. The signal and I/O lines were loaded with the 50 ohm coax cables when the ESD protection was assumed to be turned off (scenario I). When the ESD protection was assumed to be turned on, the signal lines were terminated with a 1 ohm resistance (scenario II). In both the scenarios, each of the flex cable ground pins/connections was terminated with a 1 ohm resistance. The termination values were chosen as a compromise between the actual loading of the traces, and the ability to measure the currents well. The probing circuit diagram for scenario I and the termination circuit diagram showing all the signal and ground connections of the LCD and touchscreen flex connector are shown in Fig. 5 and Fig.6, respectively.

A photo of the substitution board mounted into the mobile phone’s body including the soldered semi-rigid coaxial cables and the connection of the touchscreen flex cable ground to the substitution board ground via a gasket is shown in Fig. 7.

The measurement set-up used for measuring the currents in the individual lines of the flex cable connectors on the substitution board for a discharge to a copper patch on the glass is shown in Fig. 8. The photo of the measurement set-up is shown in Fig. 9. The mobile phone with the substitution board and soldered semi-rigid coaxial cables is placed, with the display facing upwards, on a reference metal plate using a 1 cm thick Styrofoam spacer. Since the substitution PCB is well grounded by multiple coax cables, the height of the phone above the reference ground is not relevant. The currents were measured for discharges to a 2 cm x 2 cm copper patch on the glass in contact mode for three positive discharge voltages: 1 KV, 2 KV and 3 KV. The different voltages were selected to test the assumption of linearity which is based on the assumption that all ESD protection within the IC on the glass was turned on. As mentioned in the article, the discharge scenario is similar to sparkless discharges to the display [9]. Since there are 29 total coaxial probe outputs, the probe outputs were measured in sets of three. For each set of measurement, three probe outputs were connected to 50 ohm oscilloscope channels using an 8.7 V overvoltage protection device on each channel to limit the maximum voltage on the channel to 8.7 V. The rest of the coax outputs were terminated with 50 ohm each.

The discharge current into the patch was also measured simultaneously using an F65 current clamp, as shown in Fig. 10. The discharge current into the patch in each set of measurement was practically identical. The current clamp output was connected to the oscilloscope channel using a 20 dB attenuator and an overvoltage protection circuit clamping at 8.7 V.

V.RESULTS AND ANALYSIS

The main goal of the current measurements was to understand the relative distribution of currents within the mobile phone and on the various signals and ground connections of the flex cable for a discharge to the glass. The measured peak current magnitudes for scenario I and II on each of the different pins of the LCD and touchscreen flex cable connectors for all three discharge voltages are plotted in Fig. 11-Fig. 12 and Fig. 13- Fig. 14, respectively. As an example, the measured current waveforms for a set of measurements, in scenario I (50 Ohm termination for I/O), at the LCD and touchscreen flex cable connector for +1 KV discharge is shown in Fig. 17. The peak current plots in Fig. 11-Fig. 13 show that the currents in the LCD and touchscreen flex cable ground structures are more than 10 times larger than the currents that flow in the I/O lines while they are about 2-4 times larger for the case when each of the signal lines are terminated with a 1 ohm resistance.

Fig. 14 shows that most (1 A) of the peak patch current flows in the I/O (pin-9) on the touch flex connector when the patch is in the center of the glass. To investigate the dependence of the peak current on touch pin-9, the peak currents were measured for different positions of the patch, as shown in Fig. 15, effectively scanning the discharge location on the glass. The plot of the peak currents on touch pin-9 for different discharge positions on the glass, Fig. 16, indicates strong dependency of the peak current on the discharge location. The peak current magnitudes on the I/O lines might also cause various upset events in the actual system. Correlation of the currents measured using the substitution board to the upsets events on the actual mobile phone was not part of this study. However, the authors hope that the substitution board methodology can be used for such an investigation.

From the current comparisons, for scenario I and scenario II (Fig. 11 – Fig. 14), it can be inferred that the ESD protection turn on can significantly change the current paths within the phone and the current distribution between the I/O and ground pins. To obtain an approximate distribution of currents that flow into the flex cable and to the body of the phone via capacitive coupling after a discharge to the glass, all the measured currents on both the LCD and touchscreen flex cables were summed up and compared with the discharge current into the copper patch. The comparison between the patch current and summation of individual currents and the summation of the absolute magnitude of individual currents for scenario-I is shown in Fig. 18. The peak current magnitude of the discharge current is about 1.6 A and that of the summed-up currents is 1.2 A in scenario I.

VI.CONCLUSION

A measurement technique is presented that allows measuring the induced currents at the LCD and touchscreen flex cable connectors on a substitution board due to an ESD to the glass/screen of a mobile device. The design of a substitution board based on the EMC relevant features of the mobile device and the measurement set-up for measuring the currents on various I/O and ground connections individually was demonstrated. The use of multiple coaxial cables that are well connected to the reference plane presents a similar scenario as that of a mobile device connected by a single wide, short USB or charging cable. The resistive terminations for different lines are chosen so as to emulate the impedance of the actual main board terminations as close as possible. Here, one can change the values depending on the assumed status of the ESD protection at the IC I/Os. Preliminary measurements using the substitution board indicated that a large part of the discharge current flows to the PCB via the flex cables and a smaller part via capacitive coupling to the phone body. The current distribution may be different for different products. In addition, observations of the measured peak current magnitudes at various flex connector pins show that the turn on of the ESD protection circuits at the ICs I/O can significantly change the current paths within the phone. These ratios may be influenced by the status of the ESD protection circuits at the ICs I/O. The current values measured using the substitution board can help to understand damage to the driver IC or the display IC or disturbances of the data transfer and can be used to correlate the current magnitudes to system level effects. The applicability of the substitution board methodology is not limited to mobile devices and can be applied to other electronic products as well.

Besides measuring ESD currents the substitution board offers various other applications with respect to the coupling and propagation of self-interference causing signals or EMI problems.

VII.ACKNOWLEDGEMENT

This paper is based upon work supported partially by the National Science Foundation under Grant No. IIP-1440110.