BY:

JAY PANCHAL

ASWIN BALASUBRAMANIAN

OVERVIEW OF THIS TOUR

WHAT IS PCB…

MATERIALS OF PCB…

TYPES OF PCB...

PARTS OF PCB…

PROS AND CONS...

ISSUES RELATED TO PCB LAYOUT...

EMI IN PCB...

SOLUTION TO EMI...

PRE-TASKS AND DESIGN STEPS…

CONCLUSION…

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PCB

• A printed circuit board

• It supports mechanically and electrically

connects electronic components. Hence

rugged and reliable construction

• Manufacturing circuits with PCBs is

cheaper and faster than with other wiring

methods as components are mounted

and wired with one single part

• A PCB populated with electronic

components is called a printed circuit

assembly (PCA)

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MATERIALS OF PCB The board is coated with a green colour layer called solder mask

The conducting layer is a thin copper foil

Unwanted copper layers are removed by the process of etching leaving

only a thin trace of copper to conduct

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TYPES OF PCB

Single layered PCB

Double or multi layered PCB

Aluminum backed PCB

(Aluminum used as heat sink)

Copper-invar-copper PCB

(Controls expansion in high temp.)

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PARTS OF PCB 1. Pads

The components are connected to this part

Soldering of components is done over the

pads

The pads connects components to traces

2. Traces

Traces connects all the pads consisting of

components

They are the wirings of PCB to conduct

signals

Traces carrying high currents are wide

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PARTS OF PCB

3. Vias

Connects two pads in corresponding

positions on different layers of the board.

4. Top metal layer

Consists of most of the electronic

components and few traces

Components are soldered to the pads on

the top metal layer

5. Bottom metal layer

Carries few components and most of the

traces of PCB

Most soldering is done in this layer

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PARTS OF PCB

Soldering mask

It protects the copper layer from corrosion

Areas that is not to be soldered is also to

be masked by this polymer resist solder.

Prevents solder from binding between

conductors in order to avoid short circuit.

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PROS AND CONS

Pros

Low cost

Widely available

Long life

Compact and hence Easy to Handle

Cons

Reworking on PCB is really difficult

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ISSUES RELATED TO PCB LAYOUT

Component Size

Heat Dissipation

Input and Output

Mounting Points

Parasitic Inductance and Capacitance

Ground Bounce and Rail Collapse

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Component size

Make sure components will actually fit specially for circuit with higher no. of

components

Some components come in multiple sizes. Challenge faced in SMT

Sometimes you can get tall and narrow caps or short and wide capacitors.

Heat dissipation-heat sinks

Heat sink dissipates heat off the component. (Just Moves the Heat)

Components getting heated need large enough heat sink.

Data sheets specify the size of the heat sink

A short circuit may result when two devices sharing the same heat sink

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Input and output

The PCB needs to be mechanically secured to something.

Could be the chassis-consist of metal frame on which the circuit

boards and other electronic components are mounted.

Could be another PCB/socket on PCB.

Could be attachments to a heat-sink.

Mounting points:

Input and Output Pins connected to cable are also sources of

disturbance in circuit.

Noise from power rails are transferred to external cables

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Parasitic Inductance and Capacitance

Critical parameter for High Frequency Signals in Circuit

Series Inductance

Shunt Capacitance

Inductive Coupling

Capacitive Coupling

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Series Inductance:

Not an issue for low frequency circuits(<10 Mhz)

The inductance of a trace may be significant.

Shunt capacitor is added to counter the series inductance of a long trace.

A 100nF capacitor is often used along with a larger capacitor. 100 nF ceramics have very low impedance at higher frequencies.

Shunt Capacitance:

Result of wide wires over a ground plane.

Limits speed of circuits, including digital circuits

Typically insignificant for low performance circuits.

To minimize place a capacitor from voltage to ground

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Inductive Coupling:

Transfer of energy from one circuit component to

another through shared magnetic field

Change in current flow through one device

induces current flow in other device

Current flow in one trace induces current in

another trace

Minimize the long parallel runs of traces

Run traces perpendicular to each other

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Capacitive Coupling

Transfer of energy in electrical n/w due to

capacitance between circuit nodes

Minimizing long traces on adjacent layers will

reduce capacitive coupling

Ground planes are run between the signals

that might affect each other.

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Gound Bounce and Rail Collapse:

Due to high speed digital system. E.g Changing states of Gate

This is the effect due to parasitic of the circuit component

Positive voltage rail experiences reduction in voltage and

grounded rail will have increase in voltage.

Illustration of ground bounce and rail collapse. A CMOS logic gate

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EMI IN PCB:

The strict regulations from CISPR and safety requirements has forced the industry to

pay serious attention to electromagnetic compatibility (EMC) issues.

Emission

Conducted Radiated

Input Power

lines, HF

Signal Flow

Internal and

External Cable

Terminations

Magnetic

Field

Electric

Field

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Coupling paths and parasitic elements are sources of noise and affect EMI filter design.

PCB trace inductance is critical to causing ringing, voltage spikes, switching loss and associated EMI

Ground at Non-zero Potential causes circulating current and lead to EMI

SOLUTION TO EMI:

Segregation of Analog component and Digital

Component which can be Achieved by:

(a) Continuous plane (b) Split plane.

Splitting up Ground and

Power Planes

Break Up Planes into section and

provide common reference point

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(d) Isolated, continuous planes (c) Moated plane

Return planes can also be

completely separate areas Distinct continuous planes

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Power and ground plane stack-up

(a) Coupling between overlapped (b) Coupling between parallel planes

(c) Non overlapping split planes

(d) Shielded isolated planes

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PCB Routing

Placing Parts

Bypass

capacitors

PCB layer

stack-up

Trace Width

and Spacing

width

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PCB design for reducing Fields and EMC 23

Comparison for two PCB Layout 24

1. Generation of film 2. Cut the Raw material

3. Drilling and copper

plating

4. Applying Image

PRE-TASKS AND DESIGN STEPS

5. Etching

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7. Silk Screen on

Component side 6. Solder mask / Solder coat

http://www.youtube.com/watch?v=Q6WJqjVleG0&f

eature=related

PRE-TASKS AND DESIGN STEPS 26

CONCLUSION Components randomly connected randomly to ground points proves POOR PCB DESIGN. It has high

Inductance and leads to EMC issues

Maximizing the ground area reduces the inductance, which in turn reduces electromagnetic

emissions and crosstalk

Continuously move components/change placement during the routing process to minimize track

inductances and capacitances.

Minimize vias and layer changes with critical signal tracks.

Keep High-speed signal tracks MUST be shortest and away from nearby tracks.

Component Segregation – For an EMC-free design i.e group components according to there

functionality such as analog, digital, power supply sections, low-speed circuits, high-speed circuits,

and so on

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References

https://noppa.aalto.fi/noppa/kurssi/s-81.4100/materiaali/S-

81_4100_topic_11.pdf

www.siue.edu/~sumbaug/404PCB_slidesNew09.ppt

PCB Design slides by Chris Stahl

www.wikipedia.org

www.pcbexpress.com

A Practical Guide to high-speed printed circuit board layout

http://www.pcbexpress.com

https://elmatica.zendesk.com/hc/en-us/articles/201043298-Tips-

hints-for-PCB-Via-Design

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