Intro PCB Design Rev1

8/9/2019 Intro PCB Design Rev1

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PCB Designing

Mr. Shridhar Dudam

Assistant Professor, PICT, Pune

Mr. Lalit Patil

Assistant Professor, PICT, Pune


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Introduction

PCBs are primarily an insulating material used as base, intowhich conductive strips are printed

The base material is generally fiberglass, and the conductive

connections are generally copper A photographic process is used to etch off the copper where it is

not needed to connect components

PCB design defines the electrical pathways betweencomponents

!erived from a schematic representation of the circuit

The PCB layout tool produces the artwor" for the photographic

process


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PCB Design Process

#ystem #pecification

Partitioning system

#chematic $ntry Building component libraries %#chematic and&ootprint'

!etermining PCB si(e and shape

Component Placement

)outing the PCB %Auto )outing or *anual )outing'

Chec"ing !)C and $)C

+enerating *anufacturing &iles


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PCB Design Tools

Computer Based Tools To automate and improve the speed and accuracy

CA$ %Computer aided engineering' Tools

#chematic $ntry, #imulators, Circuit analy(ers, $mulators

CA! %Computer aided design' Tools Placement Tools, )outers, Chec"ing Tools

CA* %Computer aided manufacturing' Tools

Available Integrated PCB !esign tools Protel

Cadstar

-rcad PA!#


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PCB Standards

#tandards for every aspect of PCB design

IPC Specifications

Institute for Interconnecting and Pac"aging $lectronicCircuits %wwwipcorg'

!evelop and maintain specifications governing

All aspects of PCB industry

PCB materials through assembly and test

$.changing design

&abrication and test information

IPC/0001 2 3+eneric #tandard on Printed Board !esign4

*I5 #pecifications

PC*CIA #pecifications

A6#I #tandards


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PCB Board Types

Classified in many different ways according to their various attributes

Classification based on conductors

#ubtractive and Additive

Classification based on substrate -rganic and 6on/organic

Classification based on manufacturing

+raphical and !iscrete/wire

+raphically produced boards

#ingle sided boards

!ouble sided boards

PT7 %Plated through hole', #T7 %#ilver through hole'

*ultilayer boards


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PCB Board Units

PCB Boards are primarily designed in imperial units %inches' as opposed

to metric units %mm'

A thousands of an inch is called mil %not to be confused with mm',

where2

1888 mils 9 1 inch 9 0:; cm

188 mils 9 81 inch 9 0:; mm

18 mils 9 80:; mm

1 mm 9 88


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PCB Construction


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Schematic Entry

=


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PCB Design Layers

A PCB design tools allows the designer to define and design on multiplelayers

*any of these are physical layers such as2 Signal Layer Power plane Layer Mechanical Layer

#ome are special layers such as2 Solder & Paste Mask Layers Silkscreen or op !"erlay Layers Drill guides

#eep$out Layer #ome CA! specific display/only layers2 )atsnest layer !)C $rrors


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Signal Layers

#ignal 5ayers are the trac"s that represent where copperneeds to be placed

They are designed in the positive

There are no conventions for how thic" signal traces canbe The width of your signals depend on

$lectrical nature of the trace %eg power traces aregenerally thic"er than signal traces'

)outing and space constraints

*anufacturing constrains

&or signals use > mil trace widths with > mil spacing

Thic"er traces provide lesser resistance and inductance


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Signal Layer

10


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Internal Layers

Internal 5ayers are generally used for Power Planes

They are drawn in the negative, in other words trac"s

placed on this layer represent the void

A signal name such as ?CC or +6! can be assigned to

an Internal Plane and the CA! tool will automatically

connect pads to the plane, greatly simplifying PCB

routing Internal planes can also be split into sub planes or split

planes


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Mechanical and Keep-out

Layers *echanical 5ayers are special in that they can beadded or viewed in any layer

@sed to provide drill and cut/out guides during the

manufacturing process

eep/out layers generally follow the mechanical layer

to indicate to the CA! tool an area should not be

routed over This is useful as an auto routingconstraint


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Keep-out Layer

1:


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Mechanical Layer

1


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Mechanical and Keep-out

Layers

1


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Layout ith all Layers

1>


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Layout ith Polygon

Plane

1=


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Solder Mas! Layers

#older mas" is usually the green coating on a PCB boardwhich is designed to insulate and protect the underlyingcopper traces from environmental factors, and is also

used to prevent bridging %shorting' traces during wavesoldering

#older mas" usually covers everything on the PCB boarde.cept for pads and vias, though it is good practice to

cover vias, especially if dealing with B+A componentsThis process is called tenting the vias

#older mas" is shown on the CA! tool as a negativeimage ie where there is solder mas" 3shown4 is where

there will be 6- solder mas"


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Solder Mas! Layer

01


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"egati#e o$ Solder Mas!

Layer

00


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Sil!screen Layers

The sil"screen layer is also "nown as -verlay Top-verlay refers to the sil"screen on top of the board, andBottom -verlay refers to sil"screen on bottom

This is the layer onto which the component designatorsare printed %)1, )0, D' so as to identify individualcomponents during component placement of the board

They are also used during the PCB routing process to

indicate the outlines of your components This helpsyou in placing %or not placing' components too close toone another, or too close to the edge of the board

*a"e sure your sil"screen doesnt run over any

e.posed copper %such as pads'E


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Sil!screen Layer

0;


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%ther Layers

Paste *as" Paste mas" is similar to solder mas", e.cept that it is used to

create solder paste screens which can then be used to solder#*!s in a hot re/flow soldering process

*ulti/5ayer *ultilayer is a simple way of adding an obFect such as

mechanical drawing or a pad to A55 the PCB layers AnyobFect on this layer will automatically be added to all the PCBlayers

!rill 5ayer There are two drill layers The !rill +uide 5ayer is used for legacy eGuipment and is

generally not in use today

The !rill !rawing 5ayer provides coded plots of board hole

locations


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Drill Draing Layer

0


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Multi-Layered Boards

*ulti/layer PCB boards are the norm for comple.designs, in particular those which have si(e constraints

The simplest of multi/layer boards is a ; layer board

which provides two internal power planes and two outersignal layers

*ore comple. multi/layer boards can reach up to 0;layersE

#ome PCB material limit the number of layers, such asfle.ible PCB boards

By using dedicated power and ground planes, youdrastically reduce the comple.ity of routing, so thatreducing inductance and impedance across the board

Prevent ground loops from happening


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Dou&le Sided PCB


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Component Placement

Component placement is an e.tremely importantfunction of the designer

Components should be placed according to their

connections to other components, thermalconsiderations, mechanical reGuirements, as well assignal integrity and routability

Components which have connections to each othershould be placed in the same vicinity

&or e.ample, a processor should be placed very close tothe )A* and &lash ICs on which it relies

Components should also be placed on a grid, usually a188mil grid, in order to provide for a symmetric flow of

routing where trac"s and components are lined up


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'uto router

CA! tools provide auto/router and board wi(ardfunctionality

In reality, PCB designers dont use an auto/router

The technology behind an auto/router has a science ofits own, drawing from disciplines such as artificialintelligence, heuristic algorithms, and ultimatelyattempting to solve the traveling salesman problem

The irony is, only a very e.perienced PCB designer canta"e advantage of an auto/router, and even then, it isonly used for a fraction of the board


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Pads

Pads come in all sorts of shapes and si(es They can bethrough/hole or #*! but they all follow the samegeneral guidelines

Through/hole pads are used for leaded components li"eresistors and capacitors are round, with about a 8 milinner diameter

!ual in/line Pac"ages %!IP' components li"e ICs fairbetter with an oval shaped through/hole pad, with 8mils in one dimension and 188 mils in another

#urface mount %#*!' components adhere to the chipspecs and are generally rectangular

Pin 1 of any chip footprint should always be a different

shape2 rectangular for !IPs and oval for #*!s


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(ias

?ias are special pads which connect electrical signalsfrom one side of your board to another

In special circumstances, from one layer to another

without crossing all layers %blind or buried vias' Blind and buried vias are to be avoided at all cost, theyare difficult to debug and rewor"

?ias are made of conductive material which are called

Plated Through/hole There is really no difference between ?ias and Padse.cept that the CA! tools manage them differently toallow more comple. operations on vias

?ias are generally much smaller than pads


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)er&er *iles

+erber, otherwise "nown as )#/0;/!, is the industrystandard photo plotting language

It is used by photo plotter eGuipment which use light to

3draw4 a line It was developed in the 1=>8s, and has since been

modified in many ways

It is an A#CII format file, which instructs the photo

plotter to perform one of four basic functions2 *ove to location H,

#elect the correct aperture tool

-pen, close, or flash shutter

$nd of line instruction %the JK character'


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Manu$acturing )uidelines

Internal layers should be designed at least 1:4 awayfrom the edge of the PCB board This is to ensure thatthe manufacturer of the board does not cut into thecopper or short the planes ou can employ the "eepout

layer for this purpose $ach manufacturing house has their own manufacturing

guidelines for minimum space width, copper/to/edgedistance, via and hole si(es, annular ring, etc

*any assembly houses reGuire that you provide yourPCB boards in a paneli(ed format Paneli(ing a designmeans fitting multiple boards on a standard panel -ncethe assembly is done, they are cut into individual PCB

boards


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Manu$acturer

re+uirements In order for a board manufacturer to build the PCB boards, theyneed a set of gerber files

The PCB CA! tool is capable of generating gerber files for every

PCB layer 7owever, an aperture library and sometimes a drill file must be

generated separately

As discussed, gerber files instruct the eGuipment where to go and

what to do The aperture list specifies which tool to use Aperture files come with different resolutions, it is best to send

the highest resolution aperture list possible to the manufacturer

$.cellon drill files which may also be sent separately are used to

tell the eGuipment what si(e holes to drill and where


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Manu$acturing Process

#tep 12 &ilm +eneration

The film is generated from the design files %gerber files' whichare sent to the manufacturing house -ne film is generated perlayer

#tep 02 )aw *aterial Industry standard 88:=4 thic", copper clad panel

#tep


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Manu$acturing Process

#tep 2 Pattern Plate

$lectrochemical process to build copper in the holes and on thetrace areas Apply tin to surface

#tep 2 #trip and $tch

)emove the dry film, and then etch the e.posed copper The tinprotects the copper circuitry from being etched away

#tep >2 #older *as"

Apply a solder mas" area to the entire board with the e.ceptionof solder pads

#tep =2 #older Coat

Apply solder to pads by immersing into tan" of solder 7ot air"nives level the solder when removed from the tan"

#tep 182 6omenclature

Apply white letter mar"ings using screen printing process


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Documents

Intro PCB Design Rev1