Getting EMC Design Right First Time

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Getting EMC Design Right First Time;Getting EMC Design Right First Time;
EMC》 FASTPASS foreword EMC FastPass was created with a sole mission To get electronic design com panies through onerous EMC testing requirements faster and cheaper Our aim is to assist the electronics design industry to focus on making products that make money, and minimize the impact of regulatory red tape wherever possible. to achieve this, we offer valuable advice on how to design products that pass testing first time around, such as this ebook and our industry leading Emc Auto CorrectDesignReview(visithttp://www.emcfastpass.comformoredetailsaswellasregulatory guidance to help clients avoid or minimize 3rd party testing wherever possible. We also actively lobby government agencies such as the FCC for sensible de-regulation opportunities that lower the testing burden on electronics manufacturers As a senior hardware design engineer and former founder owner of an EMc test lab, i can appreciate the industry's frustration with the EMc/rf testing process. It is expensive and can lead to product launch delays of weeks or even months. Over the 3 years that I managed a test lab, I noticed several areas that many manufacturers consistently struggled with. Most companies i came across only released 1-3 new products per year and as such, did not encounter the testing process regularly. test labs on the other hand see tens or hundreds of products per year and therefore have a much better understanding of typical failure modes and EMC FastPass closes this loop in knowledge between test labs and the electronics industry utilizing a step by step support process. the process regularly saves clients $10,000+ in testing costs and 6weeks time to market Enjoy Andy eadie Tips and Tricks Email sign-O If you haven 't already joined the 100's of companies around the world that have signed up to our free newsletter where we regularly disclose little known design and regulatory tips that can be worth as much as $5000 to your company then please do so here Disclaimer The information provided within this document is based upon industry standard best practices. However, implementing any or all of the recommended changes contained within this report offers no guarantee that the product will be functional or pass EMc emissions or immunity test standards. this document or any portion of its contents may not be modified, copied, reproduced or distributed in any form for profit or otherwise, without express consent of 'EMc FastPass. Contact info emcfastpass com for publishing requests Page 3 of 76 Copyright o 2014 EMC FastPass EMC》 FASTPASS Table of Contents Grounding and Planes 1.1 Filters 1.2 Routing Plane slots and boundaries Decoupling capacitors ,10 1.5 Power Distribution Network Impedance .11 High Speed Considerations 13 2.1 Identifying High Speed signals .13 2.2 Impedance .::.::::a:.....::::::·::::::::.....:::::.:..:::::::...:: 14 2.2.1 Single ended characteristic Impedance .15 2,2.2 Differential Impedance 16 2.3 Terminations .18 2.3.1 Single ended Terminations 18 2.3.2 Differential Terminations 20 2.4 Placement 22 2.4.1 Partitioning… 22 2.4,2 Zoning.…,…,…,…,…,…,…,…,…,,…,…,……,……,………22 2.4.3 Analog Buffers 23 2.5 Routing 24 2.5.1 Topology 24 2.5,2 Bus routing… 26 2.5.3 Corners∴… 28 2.5,4 Vias∴ 28 2.5.5 Differential Routing 28 2.5.6 Proximity to Board Edge. ......................................30 2.6 Signa| Timing… 31 2.6.1 dentifying time-critical signals.,…………31 2.6,2 Propagation time 32 2.6.3 Eye diagram analysis…..,,… 32 2.7 Oscillators/Crystals 33 Crosstalk 35 3.1 dentifying victims and aggressors……… 35 3.2 Layer Assignment.....,…,… 35 Orthogonality……… 36 3.4 Parallelism .36 Guarding… 37 3.6 Return paths.......... 38 3.7 Receiver placement 39 EMC Considerations 42 4.1 General Considerations .. 4.2 Components 42 4.2.1 Component selection ,43 4.2.2 High speed filters .43 4.2.3 Unused pins…., 44 Page 4 of 76 Copyright 2014 EMC FastPass EMC》 FASTPASS 4.2,4 Heatsinks w.wo..,44 4.2,5 Shielded components…,,,…,… 244 4.3 Power Supply....... 45 4.3.1 Mains… 45 4.3.2 Voltage regulators 45 4.3.3 Switching mode power supplies. ,46 4.4 Routing 47 4.4.1 Clock distribution 47 4.4.2 Guarding 48 4.4.3 Stubs 49 4.5 System design 4.5,1 Chassis grounding… 49 4.5,2 Cables 50 4.5,3 Connectors∴ 51 4.6 ESD Safety 52 4.6.1 Connectors 52 4.6.2 nsitive pins 54 4.6.3 High Voltage...............,,.154 sign for Test 55 5.1 Test points 55 5.2 Tooling pins 5.3 56 5.4 Sealing .. 56 5.5 AOI 56 Design for Manufacture 57 6.1 Stackup...........,,… 57 6.1.1 et 6,1.2 Drill pairs......... 57 6.1.3 Copper balance 58 6.2 Design Rules Check… 59 6.2.1 Clearance 59 6,2.2 Width ::.::a:::·:: 160 6.2.3 Annular Ring… .60 6.24 Slivers.…………………160 6.2.5 Acid Traps 62.6 Silkscreen over pads 2 6.3 Mechanical 63.1 Mechanical drawings 62 63,2 ravings 63.3 Slots 6.3.4 Mounting holes 64 63.5 Pane|izatⅰo∩, …64 6.3.6 PCB manufacturing specifications 65 6.4 Assembly Considerations. 66 6,4.1 Assembly flc ,66 64.2 Stencils 66 64.3 Panelized Assembly… 644 Decals 67 64.5 Fiducials ∴68 Page 5 of 76 Copyright o 2014 EMC FastPass EMC》 FASTPASS 64.6 Thermal relief……….69 64.7 Assembly drawings and bom 648 PCB Assembly specifications..................................70 6.5 ng files 71 6.5.1 Gerber ∴…71 65,2 NC Drill 71 653 Pick pla 71 65,4 Test Point Report……,…,… 71 7. Thermal considerations 72 7.1 Power Dissipation…,…,,…,,…… 72 7.2 7.3 Cooling Areas 73 7.4 Current Carrying capacity ∴73 Helpful Utilities 75 8.1 IPC7351 Land Pattern calculator 75 8.2 PCB Fabrication wall! hart.,…,……,…………75 8.3 PCB Calculators 75 8.4 Filterselectionsimulatorwwwwwwwwwwww.76 8.5 Circuit simulation tools 76 Page 6 of 76 Copyright 2014 EMC FastPass EMC》 FASTPASS 1. Grounding and planes 1.1 Filters Hint on filtering Use a global filter for each power supply. Use a local filter for each block sensitive to power supply noise. Each power supply must have a filter located in close proximity either to the voltage regulator if it is located on board or near the pcb entry point if regulator is external this filter should be designed in accordance with the ripple characteristics of the regulator and the power supply requirements of the integrated circuits, and should include at least two capacitors One large capacitor (uF )for low-frequency filtering One small capacitor (nF) for high-frequency filtering Integrated circuits that require a clean power supply should be provided with an additional L-C filter to avoid noise coupling through from other blocks of the circuit An example of appropriate filtering for such a case is illustrated by figure 3. 1 High PSRR Block P。Wer Supply PSRR Block Fig 3. 1 Filtering of high and low psrr blocks supply Further details on the filter placement and routing are provided in the emc chapter, paragraph 6.3.2 Page 7 of 76 Copyright o 2014 EMC FastPass EMC》 FASTPASS 1.2 Routing An absolute MUST for routing power /ground traces Power/ground trace should be as wide as possible and close to each other. The power distribution network (PDN) should provide a low-impedance path between the voltage regulator and the integrated circuits the best way to achieve this is by using power planes both for the supply voltage and ground, as planes provide an inter-plane capacitance and a low inductance. If multiple supply voltages and ground nets are used, they should not be placed on parallel planes as the capacitive coupling between them will allow high frequency currents to flow between planes for such a case the plane layers should be split between multiple nets. Figure 3.2 illustrates an example of poor and proper separation of signal and rF groun POOR GOOD Digital Digital RF RE GND GND|● Fig 3. 2 Example of poor and good ground separation The first solution is poor because RF currents must travel to the digital ground in order to reach the gnd point at supply The small clearance between digital and rf ground will generate capacitive coupling The second solution is good because RF return path does not overlap with the digital return path The large clearance between planes minimizes the coupling capacitance Additional inductors increase the coupling impedance, to prevent RF currents to flow from one plane to another If planes cannot be used for each supply voltage, the routing should be done with respect to the following recommendations Power and ground traces should be as wide as possible i Power and ground traces should not create large loops as this will drastically increase the self inductance iii Any available areas on PCb should be filled with ground island (iv If a layer is used for both routing and as a ground plane, caution should be taken when routing through the plane in order to avoid creating large return loops. If a trace must penetrate the plane then this should be bridged, as illustrated by fig 3.3 Page 8 of 76 Copyright 2014 EMC FastPass EMC》 FASTPASS Fig 3. 3 Un-bridged(a) and bridged (b) ground plane (v) If a star distribution topology is required for crosstalk or EMC considerations, such as the pdn illustrated in fig. 3. 1, each supply trace must have its own return path in order to fulfill the"small loop area"requirement (vi) If multiple isolated ground levels are used, such as a common ground and a rF ground, they should not be routed in proximity to avoid capacitive coupling 1.3 Plane slots and boundaries Hint on routing over plane slots and boundaries Don't!!! Plane slots are created when a plane is cut to accommodate routing traces. this should be avoided when possible, but sometimes must be done due to routing space restrictions. Plane slots are also created by thd components such as dual in- line ics or row connectors. Figure 3.4 illustrates such examples of plane slots Fig 3.4 Plane slots When a signal trace passes over a plane slot, the return current will have to travel around the slot and so create a large loop which will add an impedance discontinuity increase its self inductance and radiated emission. to prevent this, high speed signal traces should not pass over plane slots. If this cannot be avoided than another return path should be created close to the signal path, either using a ground trace or a partial plane on another layer a similar thing happens when signals pass over the boundary between separated planes this should also be avoided Page9 of 76 Copyright o 2014 EMC FastPass EMC》 FASTPASS 1.4 Decoupling capacitors Hint on decoupling Use multiple decoupling capacitors per pin pair for /Cs expected to generate a lot of noise(high-speed bus drivers, DSPs, processors) Hint on routing decoupling capacitors Make sure the supply current goes to capacitor first, and only then to the /c Each integrated circuit should have at least a decoupling capacitor placed in close proximity to the supply pins. the decoupling capacitors are required to provide the transient currents the iC needs during switching to counteract the effect of the power supply output inductance and supply-lC interconnection inductance For the decoupling capacitors to be effective, their placement and routing should be done with respect to the following recommendations: The decoupling capacitors should be placed as closed as possible to the supply pins of the ic (i If the IC has multiple vCC-GND pairs, each require its own decoupling capacitor. See manufacturer's datasheet and application notes for further details If the power and ground pins are far apart, it is better to place the capacitor closer to the ground pin as signals are referenced to ground (iv the power and ground pins should not be connected directly to the power and ground nets but through the decoupling capacitor otherwise its effectiveness will be limited the traces connecting the decoupling capacitor to the ic pins should be as wide and closely spaces as possible, to provide a low impedance between them (vi) The traces connecting the decoupling capacitor to the power and supply nets should be narrow and loosely spaced to provide a high impedance to the potential noisy power and ground nets Figure 3. 5 illustrates the appropriate placement and routing of the decoupling capacitors CC plan VCC GND Via to GND plar Fig 3.5 Placement /routing of the decoupling capacitors Page 10 of 76 Copyright o 2014 EMC FastPass

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