CABLES AND CONNECTORS PRINTED CIRCUIT BOARDS (PCBs) PACKAGING OF COMPONENTS & ICs ELECTRONIC INTERCONNECTIONS

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2009-2010

An indigenous work of, Aravind Suresh (09) Nirupama Sreedharan () Sarika R () Sandeep ajayan (52)

Dated: September 19, Sunday

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Coaxial cable Fibre optic cable Twisted pair cable BNC connector PS/2 connector SCSI RJ-45 Parallel and Serial ports USB Firewire

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The following are discussed in detail:-

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Consist of an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield.

Designed to carry radio frequency current The wire braid (shield) reduce EM interference Outer conductor (shield) grounded to avoid radio frequency radiation Characteristic impedance usually 50Ohm or 75 Ohm For high power radio transmission upto 1GHz a solid Cu outer conductor is used Connectors- BNC, RF, UHF etc. are used Thicknet and Thinnet are two variants:-

Thicknet (Thick coaxial cable, an ethernet std. often called yellow cable) Thicknet runs at 10Mbps for max. segment length 500mtrs Thicknet used for 10Base5 ethernet networks Thinnet (Thin coaxial cable) runs at 10 Mbps for 128mtrs Thinnet used for 10Base2 ethernet networks

RG-59 is a flexible coaxial cable Impedance matching is of utmost importance. Ghosting in analog TV transmission

can occur due to impedance mismatch

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Optical fiber typically consists of a transparent core surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by total internal reflection

Multiple optic fibres are contained in a cable Elementary fibres are individually coated with plastic layers within a protective tube An additional jacket of application specific material (PVC, PA etc.) may be used Silica (glass), plastic and special materials like Aramid forms waveguide core Transmission speed in Gbps and used for backbone network for internet, large LAN etc Source- Laser, LED; Modulators- Electro optic, PIN diode; Detectors- PIN diode,

Avalanche photodiode, Wavelength Division Mutiplexers- AWG, optical add-drop etc. are used with optic fibres in communication systems.

Single mode and Multi mode are two types of fibres used:- Single mode has core diameter as low as 9 um and stable LASER sources are used Multi mode with multiple angles of incidence at the core – cladding interface for

which transmission is possible. Core diameter> 50um and even LEDs can be used Connectors like FC, LC, ST are used Different types of cables and jackets are used with color coding indication of the type

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Forward and return conductors are twisted together for canceling out external Eletcromagnetic Interference(EMI) or cross talk between neighbouring pairs.

They may be Unshielded Twister Pair(UTP) or Shielded Twisted Pair(STP) STP with shielding from external EMI and UDP with no shielding. Both are Cu cables Available as stranded or solid core cables The TIA/EIA 568 Commercial Building Wiring Standards and their uses:-

Category Speed Uses

1 1 Mbps For voice, not data (UTP only)

2 4 Mbps Token ring networks (UTP only)

3 10 Mbps Ethernet

4 16 Mbps Token ring networks

5 100 Mbps Ethernet and Fast Ethernet

6 155 Mbps Fast Ethernet

7 1000 Mbps Gigabit Ethernet

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British Naval Connector or Bayonet Neill-Concelman. Named after its bayonet mount locking mechanism and its inventors, Paul Neill and Carl Concelman.

A type of RF connector used for coaxial cables that connects radio, TV and other radio-frequency electronic equipments

Connector is crimped on the cable using a bayonet mount BNC connectors exist in 50 and 75 ohm versions, matched for use with cables of the

same characteristic impedance The 50 and 75 ohm connectors are typically specified for use at frequencies up to 4

and 2 GHz respectively 75 ohm BNC Connectors are primarily used for video and DS3 Telco central office

applications and 50 ohm are used for data and RF At frequencies below 10 MHz the impedance mismatch between a 50 ohm connector

or cable and a 75 ohm one has negligible effects SR-50 and SR-75, TNC (Threaded Neill–Concelman), Twin BNC or twinax, Triaxial, N and C connectors are some variants.

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A computer mini-socket and plug used to connect a keyboard and a mouse to a computer.

Pin 1 +DATA Data

Pin 2 Not connected Not connected*

Pin 3 GND Ground

Pin 4 Vcc +5 V DC at 275 mA

Pin 5 +CLK Clock

Pin 6 Not connected Not connected**

Consists of a 6-pin circular connector Purple colour used for Keyboard and Green for Mouse Hot swapping is not preferrable. Also PS/2 connectors only insert in one direction and

must be rotated correctly before attempting connection. The pin configurations are as under:-

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(Centronics 68M)

(Centronics 50F)

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices.

The SCSI standards define commands, protocols, and electrical and optical interfaces SCSI is most commonly used for hard disks and tape drives, but it can connect a wide

range of other devices, including scanners and CD drives Allows up to 15 devices to be daisy-chained using one SCSI controller The last device in a SCSI daisy-chain must have a SCSI terminator Three types of external SCSI connectors:-

25-pin (SCSI-1) 4 Mbps, DB-25 (8-bit bus) 50-pin (SCSI-2) 4 Mbps, supports multiple chained devices, Centronics 50 (8bit bus) 68-pin (SCSI-3) 40 Mbps, “Ultra Wide SCSI”, Centronics 68 (16-bit bus)

The first, still very common, SCSI interface is the parallel SCSI (now also called SPI), which uses a parallel electrical bus design

Serial attached SCSI uses a modified Serial ATA data and power cable USB Attached SCSI allows SCSI devices to use the Universal serial bus. SCSI uses a protocol method(SCSI command protocol) to transfer data between devices

on the bus. It is a circular process which starts and ends up in the same layer

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91 White-Green2 Green3 White-Orange4 Blue5 White-Blue6 Orange7 White-Brown8 Brown

It is one of the many registered jacks. RJ45 was originally a Telephone-only standard. As a registered jack, telephone RJ45 specifies the physical male and female connectors as well as the pin assignments of the wires in a telephone cable.

Connector holds up to eight wires, and is used with twisted-pair wire Connectors are attached to the wires using a crimper Commonly used on certain types of ethernet and token ring networks EIA/TIA 568B (AT&T) and EIA/TIA 568A Color Schemes are shown above The cable types shown above can be used for cross-over cabling, i.e connect two PCs

without use of hub. Cross-over cabling should be at least 3 feet long to prevent signal interference RJ 11, RJ 14, RJ 25, RJ 48 are some variants of this standard connector Telephone RJ45 connectors are a variant of 8P2C, meaning only the middle 2 positions

have conductors in them, while pins 7 and 8 are shorting a programming resistor Computer RJ45 is 8P8C, with all eight conductors present

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(DB-25 Male) (DB-9 Male)

(DB-25 Female)

Two types of serial connectors or ports for PCs are : DB-9 (9-pin) - Usually COM1 DB-25 (25-pin) - Usually COM2

Traditional recommended serial standard is-232 (RS-232)., with cable limit 50 feet Parallel ports are computer sockets that connect printers or any other parallel device,

such as removable disks and tapes backup devices, to a computer These ports are built into the computer motherboard. Parallel ports now enhanced using IEEE 1284 standard, providing bi-directional

transfers and increased speeds, and it defines a cable type that allows data transfers up to 32 feet on a parallel cable. It uses a DB-25 connector on the computer end, and a Centronics 36-pin connector at the printer

Parallel peripheral buses include: ISA, ATA, SCSI, PCI etc. Serial standards include: serial ATA, RS-422, RS-423, RS-485, I²C, SPI etc.

Serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. Whereas several bits are sent as a whole, on a link with several parallel channels.

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Universal Serial Bus (USB) is a specification to establish communication between devices and a host controller (usually personal computers), developed and invented by Ajay Bhatt while working for Intel.

USB is intended to replace many varieties of serial and parallel ports Allows up to 127 devices to be daisy-chained using one USB port Offers a bandwidth of 1.5MB/sec Devices are hot swappable Uses a type A socket (4-pins) Superspeed USB 3.0 provides full duplex communication USB is the paramount connection standard preferred for network adapters, peripheral

devices (mouse, keyboard, printer etc.), external hard disks and flash drives USB 1.1 maximum cable length is 3 metres (9.8 ft) and

USB 2.0 maximum cable length is 5 metres (16 ft) Maximum permitted hubs connected in series is five

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The IEEE 1394 interface is a serial bus interface standard for high-speed communications and isochronous real-time data transfer. The interface is also known by the brand names of FireWire (Apple), i.LINK (Sony), and Lynx (Texas Instruments).

Frequently used by personal computers, as well as in digital audio, digital video, automotive, and aeronautics applications

FireWire is also available in wireless, fiber optic, and coaxial versions using the isochronous protocols

It allows up to 63 devices to be daisy-chained It supports hot swapping Guarantees bandwidth for multimedia, supporting data transfers up to 400 MB/sec Sometimes used for direct (system-to-system) connections for high-speed data transfer FireWire devices are organized at the bus in a tree topology. Each device has a unique

self-id. One of the nodes is elected root node and always has the highest id. The self-ids are assigned during the self-id process, which happens after each bus resets

FireWire 400 (IEEE 1394-1995), FireWire 800 (IEEE 1394b-2002), FireWire S1600 and S3200, FireWire S800T (IEEE 1394c-2006) are some firewire standards

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The following are discussed in detail:- The PCB technology PCB design process steps PCB manufacturing Fabrication PCB assembly

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A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate

A PCB contains a fiber glass epoxy board with thin copper layers attached to it A photographic process is used to etch off the copper where it is not needed, to connect

ICs Prior to etching, all holes are drilled in the PCB and it is electroplated so that the feed

through holes connect to the different copper layers Electronic parts are soldered to copper mounting pads left after etching, and thin copper

traces are used to connect the parts The designers now uses a PCB layout CAD tool to design the pads and interconnect lines The layout tool produces the artwork for the photographic mask that is used to chemically

etch the PCB PCBs may be single side, double side or multi layer Small simple boards have two copper layers Boards like PC motherboards are very complex and have 8-12 stacked layers of copper

and is therefore expensive

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Create new Design Data Base (.ddb) file and add new Schematic diagrams(s) (.Sch) Draw schematic diagram of circuit

Enter components from selected part libraries Create new components in component library

Draw schematic symbol Enter part description Select or create PCB footprint for part.

Connect electric components/parts with wire symbols Run Electric Rule Check (ERC)

Prepare PCB-file: Create new .PCB file and set Layer Stack to desired configuration. Draw the board (Mechanical layer) Draw Keepout layer

Export Schematic file(s) to PCB file Resolve footprint issues

Set PCB Design Rules Place components on “board” Autoroute board Resolve DRC Violations (Modify design rules, route some traces manually or move

components.) and Return to step 8 Generate Gerber and Drill files.

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Insulated substrate

Copper connectionsProtective covering

Conducting layers are typically made of thin copper foil. Insulating layers dielectric are typically laminated together with epoxy resin prepreg. The board is typically coated with a solder mask that is green (red, blue) in color

Dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3 Well known prepreg materials used in the PCB industry are FR-2 (Phenolic cotton paper),

G-10 (Woven glass and epoxy), CEM-1 (Cotton paper and epoxy) Manufacturing glass reinforced epoxy resin copper-clad boards Boards are produced in "clean-rooms” by hot-pressing manufacturing process. It includes:-

Place copper sheet on the lower plate Place few layers of glass cloth impregnated with epoxy on top [IF two-sided PCB's]: Place copper sheet on above Press between steel plates in a steam-heated hydraulic press (~7,000,000 Pa) Water cooled to 25º C & trim to clean out extruded epoxy Punch/Drill holes for alignment Make circuit on PCB (lithography) Drill through holes (for component leads)

Similar process is for multi-layer PCBs (figure)

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Patterning (etching):- Bonding a layer of copper over the entire substrate, sometimes on both sides Removing unwanted copper after applying a temporary mask

Silk Screen Printing- uses etch-resistant inks to protect the copper foil Photographic Methods:-

Uses a photomask is usually prepared with a photoplotter from data produced by a technician using CAM and is used when fine linewidths are required

Etching- (Chemical) done with ferric chloride, ammonium persulfate, or sometimes HCL Lamination:-

For multi-layer PCBs have trace layers inside the PCB These are formed by bonding together separately etched thin boards

Drilling:- Holes through a PCB are drilled with tiny drill bits made of solid tungsten carbide Automated drilling machines with placement controlled by a drill tape or drill file

Exposed conductor plating and coating:- PCBs are plated with solder, tin, or gold over nickel as a resist for etching away the

unneeded underlying copper The places to which components will be mounted are typically plated, because

untreated bare copper oxidizes quickly, and therefore is not readily solderable Exposed copper was coated with solder (tin-lead) by hot air solder levelling (HASL)

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Solder resist:- It prevents solder from bridging between conductors and creating short circuits Areas that should not be soldered may be covered with a polymer solder resist (solder

mask- typically 20-30 microns thick) coating Screen printing:-

Used to print Line art and text onto the outer surfaces of a PCB Screen print is also known as the silk screen, or, in one sided PCBs, the red print

Testing:- Unpopulated boards may be subjected to a bare-board test where each circuit

connection (as defined in a netlist) is verified as correct on the finished board For high-volume production, a Bed of nails tester, a fixture or a Rigid needle adapter is

used to make contact with copper lands or holes on one or both sides of the board to facilitate testing

A computer will instruct the electrical test unit to apply a small voltage to each contact point on the bed-of-nails as required, and verify that such voltage appears at other appropriate contact points

Printed circuit assembly- electronic components must be attached to form a functional unit Protection and packaging:-

PCBs intended for extreme environments often have a conformal coating that prevents corrosion and leakage currents or shorting due to condensation

PCBs are static sensitive, and therefore placed in antistatic bags during transport

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In through-hole construction, component leads are inserted in holes. In surface-mount construction, the components are placed on pads or lands on the outer surfaces of the PCB

Assembly can be manual or by machine as shown above The assembly process includes:-

Insert leaded component into holes on PCB Solder- component leads are electrically and mechanically

fixed to the board with a molten metal solder Protective coating

Component input- Leaded components and those with no wire leads are assembled in separate manner (shown alongside)

Surface mount chip assembly:- Silk-screening to apply solder paste on the board Automated assembly of components (>30,000 components/hr) IR or Wave soldering

Cover PCB with solder resist except Lands Flux application: Foam fluxing, Spray fluxing, Ultrasonic Spray

fluxing are used

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20Dip solderingDip soldering

air-jet

Hot air jet knifeSOLDER

WAVE

air-jet

Hot air jet knifeSOLDER

WAVE

Figures shown above:- There are a variety of soldering

techniques used to attach components to a PCB

Automatic removal of solder bridges by Hot air-jet knives

Figure shown aongside:- The basic types of circuit boards

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The fundamental and very basic electronic components and their packages. It includes those of:-

Passive solid state devices resistors, capacitors, inductors, diodes Active solid state devices transistors (BJTs, FETs, MOSFETs etc.)

A chip carrier, also known as a chip container or chip package, is a container for a transistor or an integrated circuit. The carrier usually provides metal leads, or "pins", which are sturdy enough to electrically and mechanically connect the fragile chip to a circuit board. This connection may be made by soldering or by mechanical force applied by springs or a ZIF socket. Most circuit boards today mount the chips on the surface of the board, although previously it was common to place the pins in through-holes punched into the board. As smaller packages are cheaper and ecologically safer, most modern chip carriers are too small for practical installation by humans. Modern microprocessors may have over 1000 pins, so the IC packaging technology to manufacture and install the carrier must be very reliable.

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The following are discussed in detail:-

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Practical resistors are made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome).

Carbon Composite ResistorSlug leads mold bake seal- cheap, poor tolerance, rugged

Wire-wound resistorwind wire leads mold seal- high power, low resistance, high frequency apps

Film resistorFilm is cut by laser high precision- low frequency applications

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The type of internal dielectric, the structure of the plates and the device packaging all strongly affect the characteristics of the capacitor, and its applications.

Disk-ceramic capacitor

Tubular ceramic & Tabbed tubular paper capacitorsPaper was used extensively in older devices and offers relatively high voltage performance.

Electrolytic capacitors

Generally small, cheap and useful for high frequency applications, although their capacitance varies strongly with voltage and they age poorly

They use an aluminum or tantalum plate with an oxide dielectric layer. The second electrode is a liquid electrolyte, connected to the circuit by another foil plate. Electrolytic capacitors offer very high capacitance but suffer from poor tolerances, high instability, gradual loss of capacitance especially when subjected to heat, and high leakage current

Usually constructed as a coil of conducting material, typically copper wire, wrapped around a core either of air or of ferromagnetic or ferrimagnetic material

Core materials with a higher permeability than air increase the magnetic field and confine it closely to the inductor, thereby increasing the inductance

Low frequency inductors are constructed like transformers, with cores of electrical steel laminated to prevent eddy currents

'Soft' ferrites are widely used for cores above audio frequencies, since they don't cause the large energy losses at high frequencies that ordinary iron alloys do

Most are constructed as enamel coated wire wrapped around a ferrite bobbin with wire exposed on the outside, while some enclose the wire completely in ferrite and are called "shielded“. Some inductors have an adjustable core

High frequencies are sometimes made by stringing a ferrite cylinder or bead on a wire Al interconnects typically laid out in a spiral coil pattern on PCB also form inductors

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Typically is a conducting wire shaped as a coil, the loops help create strong magnetic field inside the coil due to Ampere's Law. Due to the time-varying magnetic field inside the coil, a voltage is induced, according to Faraday's law, which by Lenz's Law opposes the change in current that created it.

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Diodes are made of a crystal of SC like Si that has impurities added to it to create a region on one side that has electrons as carriers, called n-region and a region on the other side that contains positive charge carriers (holes), called p- region.

Different types of junction diodes either emphasize a different physical aspect of a diode often by geometric scaling, doping level, choosing the right electrodes, are just an application of a diode in a special circuit, or are really different devices like the Gunn and laser diode and the MOSFET

Normal (p-n) diodes are usually made of doped silicon or, more rarely, germanium Before invent of modern Si-diodes, earlier cuprous oxide and later selenium was used The vast majority of all diodes are the p-n diodes found in CMOS integrated circuits Some varieties of diodes include:-

Cat’s whisker - thin or sharpened metal wire (anode) pressed against a SC crystal (cathode), typically galena or a piece of coal

As a dopant, gold (or platinum) acts as recombination centers, which help a fast recombination of minority carriers. They operate at heigher signal frequencies, at the expense of a higher forward voltage drop

Zener, Varactor, Point contact, PIN and Schottky (a metal to SC contact) diodes

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The first BJTs were made from germanium. Silicon types currently predominate but certain advanced microwave and high performance versions now employ the compound semiconductor gallium arsenide and the semiconductor alloy SiGe.

The two main categories are through-hole (or leaded), and surface-mount, also known as surface mount device (SMD)

Packages are made of glass, metal, ceramic, or plastic Package often dictates the power rating and frequency

characteristics Power transistors have larger packages that can be

clamped to heat sinks for enhanced cooling Power transistors have the collector or drain physically

connected to the metal can/metal plate SM u-wave transistors are as small as grains of sand Recent FET development, the high electron mobility

transistor (HEMT), has a heterostructure i.e, ( AlGaAs) -(GaAs) junction. They work at 12 GHz in satellites.

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There are more than about 100 different type of packages. The two main categories are: through-hole (or leaded), and surface-mount, also known as surface mount devices (SMD).

IC Packages include inexpensive plastic packages (<200 pins) packages with pins>1000 (e.g: Xilinx FF1704, 1704-ball flip-chip

BGA) Packing materials include, plastic, ceramic, laminates (fiberglass,

epoxy resin), metal etc. Chip-to-Package Bonding

Traditionally, chip is surrounded by pad frame Metal pads on 100 – 200 mm pitch Gold bond wires attach pads to package Lead frame distributes signals in package Metal heat spreader helps with cooling

Advanced Packages Bond wires contribute parasitic inductance Fancy packages have many signal, power layers (tiny PCBs) Flip-chip places connections across surface of die rather than

around periphery

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Thermal considerations & heat dissipation in IC packages Chips have enormous power densities due to small area Cooling is a serious challenge Thermal resistances combine like resistors (series & parallel) T = jaP

T : temperature rise on chip ja: thermal resistance of chip junction to ambient P: power dissipation on chip

Package spreads heat to larger surface area. Heat flows from junction to mounting base which is in close contact with case, which by covection or radiation release heat

Heat sinks may increase surface area further Fans increase airflow rate over surface area Liquid cooling used in extreme cases

Power Distribution Network functions Carry current from pads to transistors on chip Maintain stable voltage with low noise Provide average and peak power demands Provide current return paths for signals Avoid electromigration & self-heating wearout

Itanium 2 die dissipates 130 W over 4 cm2

Thermal evaluation includes:- Power dissipation limit Continous dissipation

Junction temperature is analysed for following waveforms:-

Periodic waveform Single-shot waveform Composite waveform A burst pulse Non-rectangular pulse

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Various package functions are as follows:- Electrical connection of signals and power from chip to board Little delay or distortion Mechanical connection of chip to board Removes heat produced on chip Protects chip from mechanical damage Compatible with thermal expansion Inexpensive to manufacture and test

Two main IC package categories are:- PTH (pin-through-hole)

Pins are inserted into through-holes in the circuit board and soldered in place from the opposite side of the board

Sockets available Manual pick and place possible

SMT (surface-mount-technology) SMT packages have their leads that are soldered directly to corresponding

exposed metal lands on the surface of the circuit board SMT offers several advantages and is generally preferred in modern electronics

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The SMT process-

It offers:- Elimination of holes Ease of manufacturing (high-speed P&P) Components on both sides of the PCB Smaller dimensions Improved package parasitic components Increased circuit-board wiring density

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IC packing materials:- Plastic-

Die-bonding and wire-bonding the chip to a metal lead frame Encapsulation in injection-molded plastic Inexpensive but high thermal resistance Hygroscopic (Absorb moisture)

Stored moisture can vaporize during rapid heating and lead to hydrostatic pressure during reflow process (a part of IC manufacturing)

Results in delamination within the package, and package cracking. Early device failure

Ceramic Consists of several layers of conductors separated by layers of ceramic

(Al2O3 “Alumina”) Chip placed in a cavity and bonded to the conductors (with no lead-frame) Metal lid soldered on to the package Sealed against the environment Ground layers and direct bypass capacitors possible within a ceramic package High permittivity of alumina (εr=10), leads to higher propagation delay Expensive

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32 Popular IC packages:- The pins are all parallel, point downward, and extend past the

bottom plane of the package at least enough to be through-hole mounted to a PCB

Available from SO8..SO28, with Gull-wing leads Dimensions: 8.6mm x 3.9mm x 1.75mm Pin-to-pin: 1.27mm (50mil) IC

Available up to TSSOP64 for low-profile applications Dimensions: 5.0mm x 4.4mm x 1.2mm Pin-to-pin: 0.65mm (25mil)

Available pin count >1700 with advanced Chip-scale package (CSP) package for high-density & low-profile applications

Dimensions: 8.0mm x 5.5mm x 1.4mm. Pin-to-pin: 0.8mm Low lead inductance

An SMP with body thickness 1.0 mm , 2.0 mm lead footprint Lead counts range from 32 to 176 with pitch-0.4mm to 1mm Body sizes from: 5 mm x 5 mm to 20  x 20 mm

Plastic Dual-In-Line(PDIP)Figure: PDIP14

Small OutlineIC (SOIC)Figure: SO14

Thin Shrink Small Outline (TSSOP)

Ball Grid Arrays (BGA).Figure: BGA54

Thin Quad Flat Package (TQFP)Figure: TQFP32

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Metal lines (within one or several layers) routed for sending signals between devices, distributing clocks, power and ground

Metal via plugs for connecting two metal layers on different planes Inter Metal Dielectric (IMD) separating the metal lines Pads for input/output of signals Ohmic contact to silicon There are six layers of Cu metallization:-

Lower layers are finer and are used for “local” interconnection between cells Middle layers are wider and are used for global interconnection between blocks Upper layers are wider and are used for clocks, ground and power distribution Oxide is the Inter Metal Dielectric (etched)

Al used to be the pre-dominant metal for VLSI because it meets all the requirements for interconnects or has a way around any problem.

Cu is now the metal for IC interconnects

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Desirable interconnect wiring properties include:- Electrical

Low resistance Low ohmic contact resistance to Si

Physical Adhesion to all surfaces Good step coverage Stable and reliable (corrosion and

electromigration resistant) Process Compatibility

Can easily be anisotropically etched Survive subsequent processing steps

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