Nokia_EMC_ESD_Guideline

EMC/ESD GuidelineConceptThe EMC and ESD issues will be prevented by the design of schematics and mechanical structure. There are using the ferrite bead to reduce the noise and the varistor for ESD protection in audio and acoustics circuits. On the other hand, it also uses ASIP chip in keypad and SIM circuits to prevent malfunctions caused by ESD and EMC. The shielding cans are mounted in the PWB grounding, which can provides EMC shielding from PWB to external environment. The mechanical structure, including A, B, C and D covers, are also an important protection for EMC and ESD. Take Yangtze Phone ME structure for explaining, See Picture 1.Picture 1: Yangtze Exploded ViewShielding AssemblyBottom side electronic components are placed under shielding assembly. Topside components are only covered with metal display frame, which acts as an ESD guard but not as a shield for radiation from the components under it.The shielding assembly consists of a shielding can, including shielding frame and lid . The shielding partition performs the inner separation of BB and different RF chambers. The frame is mounted to the PWB grounding and the lid covers tightly to the frame. The shielding can provides EMC shielding from PWB to external environment.The shielding assembly covers most of the bottom side of the PWB and has three closed cans for:∙RF ASIC∙FEM∙Base band circuits.FEMRF_ASICBB_shielding canPicture 2: Shielding cans distributionFollowing bottom side components are not enclosed by the shielding assembly: ∙SIM-Interface∙EasyFlash connector∙DC-Plug∙Power key∙Vibra∙Single speaker∙MicrophoneDisplay and Display FrameAs known from other programs, most LCD is very sensitive to ESD. The known malfunction is a blank LCD caused by a small pulse on the reset line.Yangtze uses 128*128, 65k CSTN LCD. The module consists of an LCD panel, a flex, a driver and components mounted on flex together with LEDs for lighting integrated with light guide into plastic frame. Connection to PWB is done via flex by board-to-board connector. Module is COG type having driver on glass.To prevent ESD, the color LCD has a defined minimum length of the reset pulse. The duration must be longer than 200 ns and is part of the LCD specification. The LCD module is mounted in the metal display frame which acts both as a shield and ESD protection. It should cover all electronic components on the topside of the PWB.The metal frame is fixed by screws. It must be secured that there is a reliable electrical connection between the display frame, the gold tracks on the PWB and the shielding. This altogether prevents malfunctions or damages to LCD caused by ESD.KeypadAll lines coming from the keypad should be prevented by ESD, generally, they should be filtered with a special key-ASIP. This ASIP general be located close to UPP to ensure best filter performance.A direct ESD discharge to keypad lines is not probable, since there is a key dome assembly between the contact and the display frame which has a breakthrough voltage of more than 5kV. Direct discharge would happen directly to the display frame, which should have a good connection to ground.Power-On-KeyIn schemetic circuit, the power-on-key is filtered by an RC combination placed close to the key and not by the key ASIP. The resistor has 27K and it is in series to the UEM input, which is considered enough for ESD protection. Attention has to be paid to the routing of this line since it may be exposed to ESD hits on the side of the board.Backlight and DC/DC-ConverterThe display backlight in Yangtze is done with white LEDs which are integrated in the display module. Keypad illumination is made with white LEDs. As known for us, these LEDs are very sensitive to ESD.To deliver the necessary operating voltage for the white LEDs, a DC/DC-converter is needed. This converter is on the bottom side of PWB and covered by the shielding frame in the BB area.The coil should be placed very near to the converter IC and should be also covered by the frame.The PWB layout in the DC/DC-converter area should be checked very carefully to avoid any current loops, which could cause radiation. The DC/DC-converter has a separate ground potential connected to the general ground in a single point of contact.Audio and Acoustics Parts:MicrophoneFor Microphone, we often add ferrite beads & capacitor to increase the attenuation of high frequencies (HF). The internal MIC is well ESD protected because the metal MIC will be grounded.Picture 3: Microphone electrical interfaceSpeakerSpeaker are always differential in electrical line for audio performance requirement. Ferrite beads and capacitors will increase the attenuation of high frequencies (HF). May be can add the varistors in the circuits for ESD protection.Picture 4: Speaker electrical interfaceEarpieceEarpiece is the most important audio component is phone for our current voice call. ESD & EMC prevention is mandatory. See Picture 5, choke is for differential HF attenuation. Varistor is for ESD prevention. And capacitors is for HF interfering filtering.Picture 5: Earpiece electrical interfaceHeadset(including external Mic and external Earpiece)All external headset audio lines must be symmetrical (length of lines, placing the components). Differential lines minimize noise, but this increases component count compared to a non-differential system.All these lines have ESD suppression and RF filtering components located as near as possible to the connector.Components and layout:XMIC lines:∙For Microphone lines, the common mode choke is used for common mode noise filtering. The location is near the connector.∙Two varistors are placed on each microphone line for ESD protection∙10nF capacitors, (not part of ESD/EMC protection), are needed to suppress the GSM signals.XEAR lines:∙For audio speaker lines, the common mode choke is used for common mode noise filtering. The location is near the connector.∙Two varistors are placed on each speaker line for ESD protection∙Inductor and Varistors increase the attenuation of high frequencies (HF).∙10nF capacitors, (not part of ESD/EMC protection), are needed to suppress the GSM signals.Picture 6: XMIC and XEAR electrical interfaceSIM InterfaceThe SIM reader is surrounded by the BB shielding can. If the battery is removed and SIM-flap is opened the SIM connector is visible and touchable. A good ground connection under and beside the SIM connector is needed.All lines coming from the SIM interface are filtered with a special SIM ASIP to prevent malfunctions caused by ESD or EMC. This ASIP should be located close to SIM connector to ensure best filter performance.The SIM card is fixed by a SIM-flap. This SIM-flap covers most of the SIM area and is mode of metal. To use this flap for shielding and ESD protection it should have a reliable electrical connection to ground at all 4 edges.The slot in the B-cover is needed to assemble the SIM-flap. It must be secured that a discharge happens to the grounded metal flap and not to the components on the PWB.Picture 7: SIM Interface electrical interfaceEasyFlash ConnectorAll lines coming from the EasyFlash connector are filtered with a special ASIP to prevent malfunctions caused by ESD or EMC. This ASIP should be located close to EasyFlash connector to ensure best filter performance.Picture 8: EasyFlash electrical interfaceRF EMC strategyEMC consists of three major disciplines (see Picture 9): Signal integrity,emission and susceptibility. Signal integrity contains all problems of crosstalk,reflection, delay and all its effects on rise and fall times. Signal integrity effectson the stable functionality of an electronic device. If any of these effectsmentioned above occur, the device would not work correctly. Signal integrity isrelevant for high-speed boards. Emission is a problem mostly caused by digitaldevices and high harmonics of non-linear effect due to source devices. At leastby electromagnetic susceptibility a device is influenced by electromagneticfields of another device. But it is mostly the analog part of a device which isinfluenced by susceptibility.Picture 9: Three disciplines for EMCElectromagnetic compliance (EMC) must not simply be added to a device in apost-PCB-design process. This is due to the fact that the most post-designmeasures do not fight the reasons but the results of electromagneticincompliance. As Picture 10 shows the latter measures to improve EMC aremade the smaller the choice of measures becomes and −what is mostimportant −the more expensive the measures and thus the product itselfbecomes. In other words, EMC has to be designed with PCB layout and RFcomponent specifications or even the whole mechanic structure.Picture 10: EMC-Facilities and their costs as a function of the developmentprocessRadiated spurious emissionsRadiated TX harmonic emissions straight from antenna are prevented. TX harmonics radiation from FEM or TX components itself is prevented with SMD soldered metal shielding can. Small bypass capacitors in supply voltage lines prevent the conducting of TX harmonics to other parts of the PWB and then radiating from there. Those capacitors are placed as near the FEM as possible in PWB.RF layout routingThe route between battery connector and FEM (PA) as wide and short as possible, so it can reduce the routing resistor and TDMA noise. The RF route of PWB surface will be covered by shielding can and avoid the TX harmonics radiation of RF signal. The RF route of PWB inner will be protected by grounding and avoid the interference from other signalSARSince antenna location and product's mechanical design determines the amount of radiation, SAR has been taken into account as a very critical parameter in antenna and mechanics design from very early phases of the project.FM RadioThe most sensitive line here is probably the antenna input. For protection a second order band pass filter is implemented keeping undesired frequencies out and reducing the energy of ESD discharges.PWB LayoutTo address possible issues with EMC/ESD, special care must be taken in the layout.The partitioning of the PWB is based on functional blocks, space requirements, shape and design.Functional blocks (including EMC/ESD protection blocks) are normally grouped together. However, due to the design and functionality of phone, there are following exceptions: ∙Power key∙Single speakerThe battery connector is located near the FEM. UPP and Flash are also located close to each other to keep the address, data and clock lines short.The PWB layout will be created under supervision of a BB engineer who has special knowledge on EMC requirements.In general, the unused space on the PWB top- and bottom-side should be filled with copper.。