1 Introduction
As the most widely used and most closely related to people’s lives, mobile phones are the major communication tools for people in China. Therefore, in order to ensure the electromagnetic compatibility of the mobile phone and solve the electromagnetic compatibility problem of the mobile phone, the electromagnetic compatibility test of the mobile phone becomes more and more important. This article introduced the requirements and methods for mobile phone electrostatic discharge testing, summarized and analyzed the main failure phenomena and patterns of mobile phone electrostatic discharge immunity test, which can be used for mobile phone electrostatic discharge immunity test and improve the antistatic performance of mobile phones.
2 Electrostatic Discharge Immunity Method and Requirements
Although different industry standards can be selected when carrying out mobile phone electromagnetic compatibility tests with different standards, the basic standards are the same. Electromagnetic compatibility tests on mobile phones, the main problems are: electrostatic discharge immunity, electrical fast transient burst immunity, conduction disturbances and radiation disturbances. The following mainly discusses the electrostatic discharge immunity test and its problems for the 900/1 800 MHz GSM mobile phone.
Electromagnetic Discharge Immunity (ESD) Immunity in Electromagnetic Compatibility Testing is a key indicator of the product and one of the main unqualified items of the product. When the ESD immunity is low, the product is not suitable for use in an environment with low relative humidity, and it will inevitably affect the performance of the product. The product is more likely to be damaged, and the user's acceptance of the product is reduced. Therefore, the ESD immunity of mobile phones has become one of the main indicators of products.
The mobile phone ESD test experiment layout is shown in Figure 1.
2.1 Mobile Work Status
In the electromagnetic compatibility testing process of mobile phones, there are two typical working states, namely dedicated mode (talking state) and idle mode (standby state).
a) Dedicated mode
The handset under test and the base station simulator establish and maintain the communication link through the space link, and select the RF input signal frequency by setting the ARFCN to an appropriate value. For example: For GSM 900MHz, select a value between 60 and 65. The base station simulator commands the handset under test to operate at the maximum output power level. The mobile phones work in charging and battery-powered states respectively and perform ESD immunity tests.
When the tested handset is required to be in a dedicated mode, the following conditions should be met:
1) The tested handset operates at the maximum transmit power;
2) Monitor the downlink RXQUL;
3) Before the test, the reference levels of the downlink and uplink speech output signals should be recorded in the test equipment (put the volume of the tested mobile phone at the rated volume or medium volume);
4) The level of the voice channel output signal at the ERP of the downlink of the handset under test should be evaluated by measuring the SPL;
5) Measure the decoded signal level of the uplink voice channel output of the mobile phone at the analog output port of the base station simulator (to minimize the external background noise picked up by the microphone of the phone under test).
b) Idle mode
The tested mobile phone and the base station simulator are connected through a space link, the BCCH channel is activated, the tested handset and the base station simulator are kept in synchronization, and are in a standby state. The mobile phones work in the charged state and the battery powered state to perform ESD immunity test.
The ESD immunity test is to simulate whether the performance of a mobile phone will drop or fail when subjected to ESD. The discharge is divided into direct discharge and indirect discharge. Direct contact discharge is applied to the conductive surface, and air discharge is applied to the insulating surface. Contact discharge is the preferred form. Only when the contact discharge cannot be used (for example, the surface is coated with an insulating layer, the cell phone non-metallic keyboard gaps, etc.) can be replaced by air discharge.
2.2 Performance Monitoring During Test
Dedicated mode: The measured mobile phone establishes and maintains the communication connection with the base station simulator. During the scrambling process, it observes whether the tested mobile phone maintains the communication connection. After the entire scrambling process is over, it is observed whether the tested handset can still maintain the communication connection, whether it can work normally, whether there is a user-perceived degradation of communication quality, whether there is a loss of user control function or the loss of stored data.
Idle mode: Observe whether the mobile phone is operating incorrectly. After the test is over, observe whether the tested mobile phone can work normally, whether there is any user-perceived communication quality degradation, whether there is loss of user control function or the loss of stored data.
Specific test points and test conditions are shown in Table 1.
Table 1 Mobile ESD test points and test conditions
2.3 ESD Test Leads to Cell Phone Failure
There are two types of failures caused by ESD testing: one is permanent failure, which results in damage to the phone; the other is soft failure, which means that the function that failed after the end of the test or after restarting the phone can be recovered.
The phenomenon of soft phone failure during ESD testing is:
1) The mobile phone automatically shuts down and resumes work after restart;
2) The mobile phone is disconnected;
3) The phone automatically restarts;
4) The phone crashes and can resume work after rebooting;
5) The screen display is abnormal, such as the screen displaying a white screen, a black screen, the screen display is blurred, the screen appears garbled, or the screen appears stripes;
6) Loss of touch screen function or key function;
7) The software has an error message. If the charger is not plugged in, the screen prompts frequently. "The charger has been removed and the charger is connected."
8) During the test, the quality of the call is reduced, the sound disappears or becomes intermittent, or whistling occurs.
The phenomenon of cell phone damage caused by ESD is:
1) Cannot boot again after automatic shutdown;
2) Some functions of the mobile phone cannot be recovered due to the damage of some devices;
3) The charger is damaged or even exploded when it is in the charge state test.
The author tested 213 batches of mobile phones, of which 41 batches failed the ESD test and the percentage of nonconformities was 19.2%. The test results show that ESD immunity is one of the major unqualified items in EMC testing. In the test, the most deactivated working states were charging and dedicated modes, and the discharge mode was air discharge; the failure points were mostly concentrated on buttons, seams, and LCD screens; the frequency of failures from high to low was: call interruption; call interruption. The mobile phone is in charging state; the mobile phone automatically restarts; crashes, and the work can be resumed after restart; the charger is broken or explode during the test; and the tester cannot resume work after the test is stopped; the white screen, crashes, and tests cannot be resumed after the test.
3 ESD failure measures
The ESD test of mobile phones is unqualified, and most of them are recoverable temporary functional failures. These failures are closely related to the antistatic design of mobile phones. In order to improve the antistatic capacity of mobile phones, improvements can be made from the following aspects:
1) Considering the grounding protection of the structure, it is better to realize multi-point grounding and the clearance is small. The grounding point should be as much as possible, shorten the length of the grounding line, the structure should be sealed as much as possible;
2) Sensitive elements away from gaps or fixed openings;
3) Use a device with a higher antistatic rating;
4) Increase the area of ​​plastic pads, lengthen static interference paths, and reduce the impact of ESD on the lines;
5) Keep ground current away from sensitive circuits and related circuits;
6) For sensitive devices, protection is enhanced through the use of protective devices (such as TVS, ESD protection devices);
7) Keep sensitive devices isolated from static power supply;
8) Reduce the loop area. The power supply should be as close to the ground as possible. The trace should be as short as possible. The signal line should be as close to the ground as possible.
4 Conclusion
This article introduced the mobile phone electromagnetic compatibility test standards and mobile phone ESD test requirements, methods and analysis of major failure phenomena and modes of mobile ESD immunity test, and clarified that ESD immunity in electromagnetic compatibility test is a key indicator affecting mobile phone qualification. It is hoped that this article will help manufacturers and designers who are concerned about the ESD immunity of mobile phones, and at the same time it will also attract the attention of manufacturers and designers who do not have a good understanding of ESD immunity.
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