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ESD 保护 layout指南

Application Report

SLVA680–February 2015

ESD Protection Layout Guide

Guy Yater High Volume Linear

ABSTRACT

Successfully protecting a system against electrostatic discharge (ESD)is largely dependent on the printed circuit board (PCB)design.While selecting the proper transient voltage suppressor (TVS)founds the basis of an ESD protection strategy,its scope is not covered here.ESD selection guides are available in Technical Documents at https://www.sodocs.net/doc/412918271.html,/esd for guidance in choosing the correct type of TVS diode for a particular system.With the proper TVS selected,designing a PCB Layout that leverages the strategies outlined in this ESD Layout Guide will provide the PCB designer with an avenue towards successfully protecting a system against ESD.

Contents 1

Introduction ...................................................................................................................11.1Optimizing Impedance for Dissipating ESD .....................................................................31.2Limiting EMI from ESD .............................................................................................41.3Routing with VIAs ...................................................................................................51.4Optimizing Ground Schemes for ESD (6)

2Conclusion (8)

1Introduction

An ESD event rapidly forces current (see Figure 1),I ESD ,into a system,usually through a user interface such as a cable connection,or a human input device like a key on a keyboard.Protecting a system against ESD using a TVS relies upon the TVS being able to shunt I ESD to ground.Optimizing a PCB Layout for ESD suppression is largely dependant on designing the path to ground for I ESD with as little impedance as possible.During an ESD event,the voltage presented to the protected integrated circuit (Protected IC),V ESD ,is a function of I ESD and the impedance presented to it.Since the designer has no control over I ESD ,lowering the impedance to ground is the primary means available for minimizing V ESD .Lowering the impedance presents several challenges.Mainly,it cannot be of zero impedance,or the signal line being protected would be shorted to ground.In order for the circuit to have a realistic application,the protected line needs to be able to maintain some voltage,usually under a high impedance to ground.This is where the TVS becomes applicable.

Figure 1.IEC 61000-4-2Compliant Level 4(8kV ESD)Waveform

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A TVS is an array of diodes (see Figure 2for a

typical example)arranged to present a very high

impedance to the voltages normally present in

the circuit,but if voltages exceed the design,the

TVS diodes will breakdown and shunt I ESD to

ground before it can damage the system being

protected.The system designer is then

challenged to lower the impedance for I ESD from

the ESD Source through the TVS to ground.Figure 2.A Typical ESD Protection Scheme

The impedance presented to I ESD is a function of any impedance inherent with the TVS (in the diode array and the package of the TVS)and the PCB Layout between the ESD Source and the TVS ground.A TVS is generally designed to offer as low of an impedance to ground for I ESD as its overall design constraints will allow.With the proper TVS selected,a critical phase of the design is to lower the impedance in the PCB Layout between the ESD Source and the TVS ground.

Another concern created by the rapidly changing I ESD is its associated rapidly changing electromagnetic field (EM)causing interference (EMI)to couple onto other circuits of the PCB.This is especially true in the area between the ESD Source and the TVS.Once the TVS shunts I ESD to ground,the trace between the TVS and the Protected IC should be relatively free of EMI.Therefore,unprotected circuits should not be adjacent to an ESD protected circuit's traces between the ESD Source and the TVS.In order to keep EMI emissions at a minimum,circuit traces between the ESD Source and the TVS should have corners which do not exceed 45°or,ideally,which are curved with large radii.

In today's PCB Layout,board space is at a premium.ICs,including TVSs,are designed to be very compact.Also,the density of their placement on the PCB is continually increasing.Multiple layer PCB boards and routing lean heavily upon VIAs for maximizing the density to increase the system's feature set while decreasing the system's size.This PCB architecture,particularly related to layer switching and VIAs,plays an important role in shunting I ESD to ground through the https://www.sodocs.net/doc/412918271.html,rge differences in V ESD at the Protected IC can be induced by the manner in which the circuit is routed to the TVS using VIAs.Generally,placing a VIA between the ESD Source and the TVS is detrimental,but in some circumstances the designer is forced to do so.Even in these circumstance,if properly done,V ESD can still be minimized at the Protected IC.

Grounding schemes are critical in protecting against ESD.Having a chassis ground for the TVS that is separated from the digital and/or analog ground by inductance provides very good protection against ESD related failures.Yet it presents great challenges when routing high speed circuits across multiple ground planes.For this reason,many designs use one common ground for the protected circuits.Ground planes are necessary for the TVS to have success in dissipating I ESD without increasing V ESD .Electrical connections to an earth grounded chassis,like a PCB grounded through-hole for a chassis screw,immediately adjacent to the TVS ground and the ESD Source's ground (i.e.a connector shield)provide a sound methodology in keeping ground shifts at Protected ICs to a minimum.If a system cannot utilize a chassis earth ground,tightly coupled multiple layer ground planes can help keep ground shifts at Protected ICs to a minimum.

To summarize these parameters,successfully protecting a system against ESD includes:

Controlling impedances around the TVS for dissipating ESD current,I ESD ?

Limiting the effects of EMI on unprotected circuits ?

Properly using VIAs to maximize ESD dissipation by the TVS ?Designing a grounding scheme which has very low impedance for the TVS

2ESD Protection Layout Guide

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https://www.sodocs.net/doc/412918271.html, Introduction PCB Layout Guidelines for Optimizing Dissipation of ESD

1.1Optimizing Impedance for Dissipating ESD

Outside of controlled RLC values,PCBs have inherent parasitics which contribute to overall board https://www.sodocs.net/doc/412918271.html,ually these parasitics are detrimental to the functionality of the design.An important parasitic to consider when designing a circuit to dissipate ESD is inductance.Because (see Note 1,below)V ESD =V br_TVS +R DYN(TVS)I ESD +L (dI ESD /dt ),and the term dI ESD /dt is very large,the forced current in an ESD event will cause large voltages to drop across any inductance.For example,in an 8kV ESD event as specified by IEC 61000-4-2,the dI ESD /dt =(30A)/(0.8×10-9s )=4×1010A/s .So even with 0.25n H of inductance an additional 10V is presented to the system.

Note 1:

?V br_TVS is the voltage required for the

TVS to enter its breakdown region and

begin shunting I ESD to ground.

?R DYN(TVS)refers to the resistance

through the TVS diode array while

operating in the breakdown region of

the IV curve.Figure 3.PCB Inductance around a Single-channel TVS

In Figure 3four parasitic inductors are shown:L 1and L 2is the inductance in the circuit between the ESD Source (typically a connector)and the TVS,L 3is the inductance between the TVS and ground,and L 4is the inductance between the TVS and the Protected IC.Not considering VIAs,the inductors L 1and L 4are generally dependant upon design constraints such as impedance controlled signal lines.However,I ESD can still be "steered"towards the TVS by making L 4much larger than L 1.This is accomplished by placing the TVS as near to the ESD Source as the PCB design rules allow while placing the Protected IC far away from the TVS,for example near the middle of the PCB.This effectively creates L 4>>L 1,helping shunt the I ESD to the TVS.Placing the TVS adjacent to the connector also mitigates EMI from radiating into the system.The inductor shown at L 2should not be present in a well designed system.This represents a stub between the TVS and the line being protected.This design practice should be avoided.The Protected Line should run directly from the ESD Source to the protection Pin of the TVS,ideally with no VIAs in the path.The inductor at L 3represents the inductance between the TVS and ground.This value should be reduced as much as possible,and perhaps represents the most predominant parasitic influencing V ESD .The voltage presented to the node "Protected Line"will be V ESD =V br_TVS +I ESD R DYN(TVS)+(L 2+L 3)(dI ESD /dt ).Thus the PCB designer needs to minimize L 3and eliminate L 2.Minimizing L 3is covered in Section 1.4.Minimizing L 1is covered in Section 1.2and Section 1.3.

Summary:

Minimize any inductance between the ESD Source and the path to ground through the TVS ?

Place the TVS as near to the connector as design rules allow ?

Place the Protected IC much further from the TVS than the TVS is to the connector ?Do not use stubs between the TVS and the Protected Line,route directly from the ESD Source to

the TVS

?Minimizing inductance between the TVS and ground is critical

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1.2Limiting EMI from ESD

Fast transients like ESD with high di/dt can cause EMI without proper steps for suppression.For ESD,the primary source of radiation will be in the circuit between the ESD Source and the TVS.For this reason,the PCB designer should consider this region a Keep-Out area for unprotected PCB traces which could damage the system by either having direct contact with an IC,or by carrying the EMI further into the system where it could radiate more EMI.Even with no inductance at L 1(as shown in Figure 3)the rapidly changing electric field during ESD can couple onto nearby circuits,resulting in undesired voltages on unintended circuits.Having any induction at L 1amplifies the EMI.

Figure 4shows an unprotected line

running adjacent to a protected line

between the ESD Source and the TVS.

This practice should be avoided.During

an ESD event there will be a large

dI ESD /dt between the ESD Source and

the TVS.The traces on this path will

radiate EMI and any nearby traces could

have a current induced in them by the

EMI.If these traces have no TVS

protecting them,the induced current in

the unprotected line can cause system

damage.

If there are any VIAs on the protected

line between the ESD Source and the

TVS,these same principles apply to any

layer the VIA crosses,no unprotected

Figure 4.EMI Coupling onto an Adjacent Unprotected lines should be ran adjacent to the VIA.Trace

Another aspect of PCB Layout to consider is the style of the corners between the ESD Source and the TVS.Corners tend to radiate EMI during I ESD .The best method of routing from the ESD Source to the TVS is using straight paths which are as short as possible.Beyond lowering the impedance in the path to ground for I ESD ,shortening the length of this path also reduces the EMI being radiated inside the system.If corners are necessary,they should be curved with the largest radii possible,with 45°corners being the maximum angle if the PCB technology does not allow curved traces.

Figure 5.Electric Field During an 8kV ESD Event for Three Different Corner Types

In Figure 5note that for a 90°corner,the corner is a strong source of EMI.The electric field at the corner is at least 7kV.This will lead to an electric arc (ionization)for any radius less than about 2.6mm (in air).The EMI for the 45°and curve are much less pronounced.To further show the effects of corner styles,Figure 6plots the crosstalk between parallel traces with these three corner types.The 90°corner has much higher coupling than the others,especially in the ESD frequency content region.

4ESD Protection Layout Guide

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PCB

PCB Protected IC Protected IC TVS

TVS VIA VIA ESD Source

ESD Source

Case 1: Best

Case 2: Worst 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz 90° 45° Curved

Frequency [Log]

M a g n i t u d e (d B )0

-10

-20

-30

-40-50-60

-70

-80

-90-100ESD Frequency

content region https://www.sodocs.net/doc/412918271.html, Introduction

Figure 6.Inter-trace Crosstalk with 45°,90°,and Curved Corners

Summary:

Do not route unprotected circuits in the area between the ESD Source and the TVS ?

Place the TVS as near to the connector as design rules allow ?

Route with straight traces between the ESD Source and the TVS if possible ?If corners must be used,curves are preferred and a maximum of 45°is acceptable

1.3Routing with VIAs

It is best to route traces on the PCB from the ESD Source to the TVS without switching layers by VIA.Figure 7shows two examples.In Case 1,there is no VIA between the ESD Source and the TVS,so that I ESD is forced to the TVS protection pin before the VIA in the path to the Protected IC.In this case the VIA represents L 4shown in Figure 3.In Case 2,I ESD branches between the Protected IC and the VIA to the TVS protection pin.In this case the VIA represents L 2in Figure 3.This practice should be avoided.The inductance of the VIA is between the TVS and the path from the ESD Source to the Protected IC.This has two detrimental effects:Since current seeks the path to ground with the least impedance,the Protected IC may take the brunt of the current in I ESD and any current that does pass through the VIA will increase the voltage presented to the Protected IC by L VIA (dI ESD /dt ).

Figure 7.Routing with VIAs

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PCB Protected IC TVS

VIAs ESD Source

Case 3

Introduction https://www.sodocs.net/doc/412918271.html,

There may be cases where the designer has no

choice but to place the TVS on a different layer

than the ESD Source.Figure 8shows Case 3,

a variation to Case 2.In Case 3,I ESD is forced to

the protection pin of the TVS before I ESD has a

path to the Protected IC.This is an acceptable

compromise to Case 2.Figure 8.Routing with VIAs

These three cases represent examples when VIAs are used between the ESD Source and the Protected IC.It is best to avoid this practice,but if necessary Case 1is the preferred method,Case 2should be avoided,and Case 3is acceptable if there is no alternative.

Summary:

?Avoid VIAs between the ESD Source and TVS if possible

?If a VIA is required between the ESD Source and the Protected IC,route directly from the ESD

Source to the TVS before using the VIA

1.4Optimizing Ground Schemes for ESD

Successfully eliminating all the parasitic inductance between the ESD Source and the TVS will not be effective without a very low impedance path to ground for the TVS.The TVS ground pin should connect to a same layer ground plane that is coupled with another ground plane on an immediately adjacent layer.These ground planes should be stitched together with VIAs,with one VIA immediately adjacent to the ground pin of the TVS (see Figure 10).

Figure 9shows the PCB Inductance around a single-channel TVS (as shown earlier in Figure 3).This section considers only the inductance at L 3.Recall that,with L 2eliminated,the voltage presented to the Protected IC during an ESD event will be V ESD =V br_TVS +I ESD R DYN(TVS)+L 3(dI ESD /dt)and for 8kV,dI ESD /dt =4×1010.Clearly,L 3must be lowered as much as possible.

Figure 9.PCB Inductance around a Single-channel TVS

6ESD Protection Layout Guide

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VIA in SMD GND VIA:0.254 mm (10 mil) pad,

0.152 mm (6 mil) drill.Epoxy filled and plated.GND Plane Detail 0.1 mm (4 mil) clearance VIA to copper Copper pour Stitching

VIAs (x 4)

https://www.sodocs.net/doc/412918271.html, Introduction

To lower L 3,the TVS ground pin would ideally connect directly to a coupled ground plane.Figure 10shows the ground pad of a TVS connected to the top layer ground plane.There are four stitching VIAs connecting the top layer ground plane to an internal ground plane.These could connect more than one ground plane layer depending on the layer count and board design.A grounded chassis screw is located very near to the TVS ground pad as well.A grounding scheme resembling this yields a very low impedance to ground for L 3.

Figure 10.Two Layer PCB -Top Ground Plane Stitched to a Mid-Layer Ground Plane

Figure 10is not relevant for some types of TVSs due to package types.Those with BGA packages which have the ground pin internal ground plane,preferably to multiple,coupled ground a ground pin.

Figure 11.Grounding an Isolated Ground BGA Package

VIAs need to be constructed to offer as little impedance as possible.Due to the "skin effect,"maximizing the surface area of the GND VIA minimizes the impedance of the path to ground.For this reason make both the VIA pad diameter and the VIA drill diameter as large as possible,thus maximizing the surface area of the outside of the VIA surface and the inside of the VIA surface.The ground plane should not be broken in the vicinity of the GND VIA.If possible,attaching the GND VIA to a ground plane on multiple layers minimizes the impedance.The GND VIA should be filled with a non-conductive filler (like epoxy)as opposed to a conductive filler,in order to keep the surface area of the inside of the VIA created by the drill.The GND VIA should be plated over at the SMD pad.Clearances between the GND VIA and non-ground planes (i.e.power planes)should be kept at a minimum.This increases capacitance which lowers impedance.

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Conclusion https://www.sodocs.net/doc/412918271.html, 2Conclusion

Designing ESD protection into a system can be successful with the proper techniques applied.Following these ESD layout guide outlines will ensure the TVS has optimum conditions for dissipating the ESD.

In summary:

?Control Impedances around the TVS for dissipating ESD current,I

ESD

–Minimize any inductance between the ESD Source and the path to ground through the TVS

–Place the TVS as near to the connector as design rules allow

–Place the Protected IC much further from the TVS than the TVS is to the connector

–Do not use stubs between the TVS and the Protected Line,route directly from the ESD Source to the TVS

–Minimizing inductance between the TVS and ground is critical

?Limit the effects of EMI on unprotected circuits:

–Do not route unprotected circuits in the area between the ESD Source and the TVS

–Place the TVS as near to the connector as design rules allow

–Route with straight traces between the ESD Source and the TVS if possible

–If corners must be used curves are preferred and a maximum of45°is acceptable ?Properly use VIAs to maximize ESD dissipation by the TVS:

–Avoid VIAs between the ESD Source and the TVS if possible

–If a VIA is required between the ESD Source and the Protected IC,route directly from the ESD Source to the TVS before using the VIA

?Use a grounding scheme that has very low impedance:

–Connect the TVS Ground Pin directly to a same layer ground plane that has nearby VIAs stitching to an adjacent internal ground plane

–Use multiple ground planes when possible

–Use a chassis screw,connected to PCB ground,near to the TVS and ESD Source(i.e.the connector ground shield)

–Use VIAs of large diameter with a large drill,which lowers impedance

8ESD Protection Layout Guide SLVA680–February2015

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ESD 保护 layout指南

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