Chapter 1

1.1 Introduction:

Printed Circuit Board (PCB) provides both the physical structures for mounted and

holding of electronic components as well as the electrical interconnection between

components. That means a PCB is a platform upon which electronic components such

as integrated circuit chips and chips are mounted. A PCB consist of non conducting

substrate upon which a conductive pattern or circuitry is formed. Copper is the most

prevalent conductor, although nickel, silver, tin, tin-lead and gold may also be used as

top level metal. There are different types of PCBs: single sided, double sided, multi

layer. Single sided PCBs have conductive pattern on one side only, double sided have

conducting pattern on both sides, multi layer consist of alternating layers of conductor

and insulating material bounded together. The conductive layers are connected by

plated through holes, which are also used to mount and electrically connect

components. PCB may also be rigid, flexible, or a combination of two. The

combination of PCB and components is an electronic assembly, also called printed

circuit board assembly.

1.2 Functions of PCB:

PCB are dielectric substrates with metallic circuitry formed on that. The are some

times referred to as the base line in electronic packaging. It serves as a wide range of

functions.

The functions of PCB are: 1. To interconnect the singular electronic components electrically.

2. To give physical strength and placement for all the components.

3. In some cases to replace some passive components themselves.

4. Now it is more and more also used to conduct away the heat generated by the

components.

1.3 Classifications of PCBs:

Number of layers:

1. Single sided board.

2. Double sided board

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3. Multi layer board.

1.4 Different dielectric material:

1. Ceramic.

2. Organic with rigid, flex or rigid-flex combined material.

1.5 Fabrication technique:

1. Subtractive technique starts from copper-clad dielectric material materials and

later on the unwanted copper will be etched away.

2. Additive technique as well as semi additives technique starts from un coated

dielectric materials and the following process of coating a metal layer

3. involves either chemically plating or vaporizing of copper in a high vacuum.

1.6 India’s position in the PCB-World

The PCB industry made a beginning in India almost at par with other Nations in the

early 1950’s. Till India opened its economy in the 90’s, the policies of the government

has had a hard in such a way, that the industry as such had no need to go in to open

competition. Even afterwards, the PCB industry didn’t change very fast towards

serious efforts in the direction of improving its technical strength to face serious

scenario in electronics. PCB industry was also an organized sector prior to 1985. The

annual growth of 7% is expected up to the year 2010.the top 15 countries manufacture

93% of world’s production.

In 2000 Japan produced 27%, US 25%, Taiwan 11%, and China produced 9% of the

total PCB produced. We like it or not, India is still struggling to be a part of this rapid

growing PCB market. It produces only .5% of the global PCB production.

1.7 About the base material:

A large variety of copper clad are available these days. Some of them can be punched

at low cost. Some of them are advantageous due to their high tg value (tg value or

glass transition temperature is the typical mechanical property of the base material up

to which temperature no delaminating take place), excellent mechanical strength or

chemical properties. The designer has to select out the right material according to the

requirements that means the designer should have the fundamental knowledge of the

materials used in end product.

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FR-2 Laminates are composed of paper and phenol-resin and are cheap. But the

absorb water and should not be used in producing PTH PCBs. These are useful in

applications where tight dimensional stability is not required.

FR-4 Laminates are made of epoxy resin impregnated with woven glass fiber. The

have very good thermal, mechanical and electrical properties and are mostly

extensively used material in the PCB industry.

General Components of the Base-Material in the manufacturing of PCB are

comprised of three basic building blocks: Resin, Reinforcement& metal cladding.

Resins are used to impregnate the selected reinforcement. The selection of a

particular resin is dependent on electrical, temperature and process parameters. The

selection of particular resin is dependent on electrical, temperature and process

parameters. Epoxy resins, which are most commonly used are modified with additives

to achieve higher thermal properties or improve chemical resistance. The resins used

in the manufacture of base material are thermosetting resins which are highly cross

linked polymeric structures and do not melt on heating. Phenol, Polyester, Epoxy,

Polyimide are some of the resins in use.[1]

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Chapter 2

2.1 Metal cladding:

Metal cladding as conductive layer can be made of copper, nickel, stainless steel or

beryllium copper. But the mostly used substrate is copper due to its availability, cost

and functionality. Copper foil can be adhered on one side or on both sides of the

composite. There are two different types of copper foils: Electro-Deposited (ED)

copper foils are produced by depositing copper on stainless drum that rotates in a

plating tank with anodes of very pure copper. The finished copper foil is then

removed from the drum and wound on a roll. The finished copper is very smooth and

shiny on the drum side and has a matte and rough side on the other side. The rough

side gives the strong adhesion to the dielectric material. HTE or Rolled copper is

produced by annealing Electro-Deposited copper at a higher temperature.

2.2 Reinforcement

Reinforcement provides mechanical strength, stability and rigidity to the

lamination. Paper sheet, Cotton fabric, Asbestos sheet, Glass cloth, Ceramic materials

are the some of the reinforcement. Fiber glass has gained popularity because of high

tensile strength and dimensional stability, and resistance to temperature variation,

moisture and corrosion. [5]

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2.3 Single-Sided Technology

Typical Flow Chart of Single Sided PCB is given below:

BLANK CUTTING

BRUSHING

IMAGE TRANSFER

EATCHING

DRILLING

MASKING

LEGEND PRINTING

ROUTING

BARE BOARD TEST

Fig:1 Basic steps of PCB designing[1]

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2.4 Technologies for Microvias :

New electronics products are required to be smaller, faster, lighter and cheaper in

order to complete in today’s market. Achieving these requirements, fine pitch area

array packaging, fine pitch ball grid array and flip flop on board assembly

technologies are being implemented. The rate these packaging technologies can be

adopted is largely being dictated by the availability of higher density PCB-

Technologies with significant reduction in conductor line width and via size at

relative low cost.

The real benefit of HDI is in the small holes, identified as “microvias”. These holes

are very small, defined by the institute of interconnecting and packaging electronic

circuit (IPC), as equal to or less then 150 micrometer (um) in diameter. The small size

allows additional room for conductor routing. Microvias normally connect only two

layers. The term capture land is used to define the start of microvia and target land is

used to describe the bottom of microvia. Via are mostly created by photo imaging,

laser ablation, plasma etching or by filling a conductive ink in to via.

2.5 PCB Design using Hand Tapping:

1. PCB design using hand taping is the process of technical drawing.

2. In hand taping method layout should be prepared on grid paper.

3. In hand taping, components pads can be prepared by using black pads.

4. Routing can be done by tapes with different widths.

5. In hand taping method lay out should be prepared in 2:1 or 4:1 scale

depending on art reduction scale chosen.

6. Each layer has to be prepared separately.

2.6 CAD System as a substitute:

Disadvantages of hand taping:

1. Each layer has to be designed separately.

2. We can’t generate NCD files for CNC drilling.

3. Designer can’t design the lay out in 1:1 scale.

4. Difficulty in modification of design.

5. Time consuming.

6. Art work can’t be stored for long period because adhesive power of tapes

decreases with time.[4]

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Chapter 3

3.1 PCB Design Softwares:

1. OrCAD.

2. TANGO.

3. Eagle.

4. PADS.

5. REDCAD.

6. P-CAD.

7. Mentor.

3.2 Advantages of CAD:

1. Auto placement.

2. Auto routing.

3. Modification is easy with CAD.

4. We have ECR and DCR.

5. CAD systems have high speed analysis.

6. It provides NCD files for Gerber data files for photo plotter

3.3 Basic design steps in CAD:

1. Entry of schematic diagram.

2. Net list file creation.

3. Placement of components.

4. Routing.

5. DRC check.

6. Art work generation.

3.4 Standards of Design Rule:

IPC 2221 Generic design identifies the generic physical design principles as well as

the material selection. Besides this, the performance requirement of the finished rigid

printed board are specified in IPC-6012.

3.5 MATERIAL CODE:

Material code designation could be example “l21 1500 C1/C1 A1A”.The first three

characters of the code designate the type of material. “L” indicates the laminate

material and “P” stands for prepreg material and the number designates the glass

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transition temperature. The next four numbers designates the normal dielectric

thickness in um. C stands for copper clad. The thirteenth character denotes the pits

and dents allowed in copper foil.

3.6 Component Placement Rule:

1. In a highly sensitive circuit, the critical components are placed first in a

manner to require minimum length for conductors.

2. In less critical circuit components are arranged in order of signal flow.

3. The components having more connections are placed first.

4. The components should be placed in a grid of 2.5mm.

5. Larger components should be placed first and the space is filled with smaller

ones.

6. Components should be placed in arrow or column to get good over view.

3.7 Conductor Routing Rules:

1. Conductor length should be shortest possible.

2. Conductor forming sharp angles should be avoided other wise it will give

problem in etching.

3. Minimum spacing is provided where it can’t be avoided.

4. In double sided PCB, it is normal practice to draw the tracks on component

side in the direction of y-axis & tracks on soldier side in the direction of x-

axis.

3.8 Hole Diameter Rule:

Hole diameter = effective led diameter + hole location tolerance + 0.2mm.

3.9 Solder Pad Dia Rule:

As a rule, solder pad diameter is approximately 3 times the component lead diameter.

3.10 Voltage and Ground Distribution:

1. The analog and digital circuits on a PCB should strictly have independent

ground network to avoid power loss.

2. Voltage and ground signal conductors have to be provided with sufficient

width to keep resistance and inductance low to carry required current.

3. While connecting voltage and ground priority should be given to the

components with the highest power consumption.

4. Spacing between voltage and ground should be as large as possible.

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5. All I/O voltage and ground conductors should have minimum conductor

length to achieve more efficiency of the circuit.

3.11 Design factors:

1. Type of circuit

2. Board size

3. Number of layers

4. Pad stack sizes

5. Hole sizes

6. Layer thickness

7. Board thickness

8. External connections

9. Mounting holes

10. Supply and ground thicknes

3.12 Capacitance:

Capacitance between two signal conductors is given by the relation:

C = .886* r * A/b

A = Total overlapping area

b = thickness of dielectric

r = dielectric constant

3.13 Resistance:

Specific resistance is given by the relation:

R = > *I/A

I = conductor length

A = conductor cross section.[5]

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Chapter 4

4.1 Mechanical design consideration:

1. Board size should be compatible with PCB manufacturing process.

2. Board mounting hole should be provided.

3. Heavy components should be adequately placed.

4. Proper hole dia should be provided.

4.2 Artwork Generation:

The film positive which is finally used for the direct exposure of photo resist coated

PCB or alight sensitive screen is called artwork.

The generation of PCB artwork has to be considered as an important step for the PCB

fabrication process. To maintain a reliable fabrication of PCBs on a high quality

standard one should be familiar with film emulsion, film base material, equipment and

processing.

Any photographic material consists basically of two layers, the emulsion and the base.

The emulsion has an approximate thickness of 3-6um and counts mainly for

photographic characteristic of the film. Under unexposed conditions, it contains the

light sensitive silver halides stabilized in a suspension. The thickness of base layer

should not be typically less then 150um in order to provide dimensional stability.

Polyester offers a good compromise between dimensional stability, easy processing

and handling convenience.[3]

4.3 Film Prossing:

4.3.1 Exposing:

Before the actual exposure, the film must reach the dimensional equilibrium. If the

material is kept in the dark room for past 12 hours, it would be the best preparation.

4.3.2 Developing:

For developing of the film, the developer used has to be a type recommended by the

film manufacturer for that particular film. The best recommended temperature is 20-

22 degree for best result. The developer tray should be of stainless steel in order to

avoid scratches created by plastic-trays.

While developing put sufficient developer in the tray so that the film can get

completely converted with it. Film should be held from the corners only. To start the

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development procedure, dive the exposed film sheet with the emulsion up through the

solution so as to wet the emulsion. Start immediately to lift the tray rotationally on

each side by about 2cm. This lifting should continue for the whole developing period.

4.3.3 Stopper bathing:

After the development is over, the film should be lifted above the developing tray for

about 2 seconds which enables excess developer to drop in tray.

Afterwards the film is immersed in to the stopper-bath keeping emulsion side upwards

to avoid mechanical damage on the soft emulsion.

4.3.4 Fixing bath:

In this bath, the unexposed emulsion will be dissolved in the solution. However the

temperature of fix bathing is less critical then developer bath.

4.3.5 Film washing:

After all the silver halide has been removed, the film is still wet with some chemicals

and dissolved silver compounds of the fixed bath. If they are not removed the will

attack the image, causing stained and fade film.

4.3.6 Drying:

After washing, the film should be dried slowly at room temperature. For good results,

drying should be take place at room temperature. Avoid forced drying which may

effect the dimensional stability of film.[3]

Fig 2:Architecture of PCB designing[3]

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Fig.3: Showing OrCAD Capture[1]

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Fig.4: Showing Layout Of OrCAD Capture[1]

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[1]

14

15

[1]

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GLOSSARY

1: ADHESIVE:- A substance such as glue or cement used to fasten object together. In

surface mounting, an epoxy adhesive is used to adhere SMD to the substrate.

2: ANSI:- “American National Standard Institute”, an organization formed by

industry and the US government to develop trade and communication standard.

3: Assembly drawing:- A drawing depicting the location of components, with their

4: Bare board:- An unassembled PCB.

5: Base material:- An insulating material as well as copper foil bondeor both sides.

6: BGA:- Ball grid array.

7: Bill of material:- An itemized list of all components, hardware, wire, on PCB.

8: Blind via:- A via that reaches only one layer beneath the outer layer on one side

9: Bridging:- Short circuit of the pads to an adjacent track or pad on a board during

soldering.

10: Buried via:- A via that does not reach a surface layer on either side of a multi-

layer board.

11: Card:- Another name of a printed circuit board.

12: Capture:- Graphical representation of the circuit.

13: Clad:- A copper object on the board.

14: Clearance:- Metal to metal gaps on the board.

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15: Datum:- A specific location that serves as a reference

16: Die:- The uncased and normally leadless form of an electronic component that is

either active or passive.

17: DRC:- Design rule check.

18: Electroless copper:- A thin layer of copper deposited on PCB without the use of

electric current.

19: Electrolytic copper:- A thin layer of copper clad on conductive areas of PCB by

means of electric current.

20: Footprint:- The pattern and space on a board taken up by the component.

21: Gerber file:- Data file used to control a photo plotter.

22: Land:- Synonym for pad and terminal area.

23: Legend:- Pattern printed onto the surface of aboard which defines the position of

components to be mounted on board.

24: NC drill:- Numerical control drill machine.

25: NC drill file:- A text file which tell NC drill where to drill is holes.

26: Netlist:- List of parts and their connection points.

27: Photoplotter:- Device used to generate artwork photographically by plotting

objects(opposed to copying an entire image at once as with a camera) on to the film

used in manufacturing PCB.

28: Prepreg:- B-stage sheets of glass cloth pre-impregnated with epoxy resin, used to

bound multi layer boards.

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29: SMD:- Surface mount devices.

30: Silkscreen:- In is used as reference designators on the printed wiring board.

31: Routing:- Placing tracks between components.

32: Track:- Trace.

33: Wire:- Wire on a printed board also means a route on track.

34: Zero reference line:- Also called zero datum line.[3]

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CONCLUSION

During my Training I learnt a lot, due to the result of hard working of my guides &

co-operation of my parents. It was an interesting experience to know the spirit of,

human relationship, punctuality & hard work. The period of 30 days training was

really a great experience in industries field.

I also learnt how to co-ordinate with the colleagues & also increased my personal

skill it will be helpful a lot in my future. It has given a lot of my knowledge & is an

essential part of technical education programme.. we were PCB designing

My training section mainly included knowing about the basics of pcb designing

through the use of orcad software.

It was great pleasure learning and we were given a lot of knowledge & this training

was an essential part of technical education programme. On completion of this 30

days training I am very well aware about substation, its working and importance.

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REFERENCE

[1] Manual provided by the institute.

[2] S.M.Sze,”VLSI TECH.”,second edition,chapter:2,9,7.

[3] WALTER C BOSSHART, “PCB design and techenology”, second edition

chapter no.1,2,9,11

[4] Carrow, R.A., “Soft Logic; A Guide to Using a Printed ckt Board”, McGraw

Hill, 1997.chapter no.3,5.

[6] Batten, G.L., Applications of Printed Ckt Board, Second Edition, McGraw-

Hill, 1994.ch no.10

[7] PCB software library.

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