Chapter 23 - 1

ISSUES TO ADDRESS...• Price and availability of materials.

• How do we select materials based on optimalperformance?

• Applications:-- shafts under torsion-- bars under tension-- plates under bending-- materials for a magnetic coil.

CHAPTER 22: MATERIALS SELECTION AND DESIGN CONSIDERATIONS

Chapter 23 - 2

• Current Prices on the web: e.g., http://www.metalprices.com-- Short term trends: fluctuations due to supply/demand.-- Long term trend: prices will increase as rich deposits

are depleted.• Materials require energy to process them:-- Energy to produce

materials (GJ/ton)AlPETCusteelglasspaper

237 (17)103 (13)97 (20)20139

-- Cost of energy used inprocessing materials ($/MBtu)

elect resistancepropaneoilnatural gas

25171311

Energy using recycledmaterial indicated in green.

PRICE AND AVAILABILITY

Chapter 23 - 3

RELATIVE COST, c, OF MATERIALS

• Reference material:-- Rolled A36 plain

carbon steel.• Relative cost, ,

fluctuates lessover time thanactual cost.

Based on data in AppendixC, Callister, 7e.AFRE, GFRE, & CFRE = Aramid,Glass, & Carbon fiber reinforced epoxy composites.

c

material ref)kg/($kg/$

=c

Graphite/ Ceramics/ Semicond

Metals/ Alloys

Composites/ fibersPolymers

Rel

ativ

e C

ost (

c)

pl. carbon

Au

Si wafer

PETEpoxy

Nylon 6,6

0.050.1

5

100000

1000020000

50000

5000

20001000

500

200100

50

2010

21

0.5

Steel

high alloy

Al alloysCu alloys

Mg alloys

Ti alloys

Ag alloys

Pt

Tungsten

Al oxide

Concrete

Diamond

Glass-soda

Si carbide

Si nitride

PC

LDPE,HDPEPPPS

PVC

Aramid fibersCarbon fibers

E-glass fibers

AFRE prepregCFRE prepreg

GFRE prepreg

Wood

Chapter 23 - 4

Integrated Circuits• The microelectronic circuitry, including

the integrated circuits are used in • our modern computers, • calculators, and • other electronic devices

Chapter 23 - 5

The heart of the integrated circuit (IC) :the chip

• a small rectangular substrate of high-purity and single-crystal silicon (or Ga-As)

• onto which millions of circuit elements are imprinted.

• circuit elements (i.e., transistors, resistors, diodes, etc.) are created by selectively addingcontrolled concentrations of specific impurities to extremely minute and localized regions near the surface of the semiconducting material.

• the chips are small in size, with the largest being on the order of 6 mm on each side and approximately 0.4 mm thick

Chapter 23 - 6

WAFER MANUFACTURING• The Silicon Crystal is Sliced by Using a Diamond-Tipped Saw into Thin

Wafers• Sorted by Thickness• Damaged Wafers Removed During Lapping• Etch Wafers in Chemical to Remove any Remaining Crystal Damage• Polishing Smoothes Uneven Surface Left by Sawing Process

Chapter 23 - 7

Silicon Wafer

Photograph of a 100-mmdiameter silicon wafer.Each of the small rectangles shown is an individual IC

chip or die.

Chapter 23 - 8

The performance of ICsThe performance of ICs is limited:1.by the characteristics of the semiconducting materials2.by the metallization process, 3. rather by the quality of the package.

Chips are very fragile: i. Silicon is a relatively brittle material ii. Gallium arsenide is even more brittle. On silicon ICs, aluminum or an aluminum–silicon alloy (99 wt% Al, 1

wt% Si) is used as the metal conductor which is metallized onto the chip surface to form a very thin film

Chapter 23 - 9

LEADFRAME DESIGN AND MATERIALS

• A leadframe is a thin layer of metal that connects the wiring from tiny electrical terminals on the semiconductor surface to the large-scale circuitry on electrical devices and circuit boards. The leadframe consists of

1. a central plate onto which the die is mounted,2. and an array of contact leads to which wire connections

may be made from the contactpads on the chip.

Chapter 23 -10

Photograph of a leadframe on which the central plate and contact leads are labeled.This package design is popular with digital IC manufacturers primarily because its production can be highly automated.

Chapter 23 -11

Some of the functions that an integrated circuit package (leadframe) must perform:

1. To permit electrical contact between the devices on the chip and the macroscopic world. The contact pads on the surface of the IC are so minuscule and numerous that accommodation of macroscopic wiring is simply not possible.

2. To disperse excess heat. While in operation, the many electronic devices generate significant quantities of heat, which must be dissipated away from the chip.

3. To protect fragile electrical connections on the chip from chemical degradation and contamination.

4. To provide mechanical support so that the small and fragile chip may behandled.

5. To provide an adequate electrical interface such that the performance of theIC itself is not significantly degraded by the package design.

Chapter 23 -12

The properties of the material to be used for the leadframe:

(1) The leadframe material must have a high electrical conductivity.(2) The leadframe, the die attach central plate, and die-bonding adhesive

must also be thermally conductive so as to facilitate the dissipation of heat generated by the IC.

(3) A coefficient of thermal expansion comparable to that of Si is highly desirable; a thermal expansion mismatch could destroy the integrity of the bond between the IC and the central plate as a result of thermal cycling during normal operation.

(4) The leadframe material must also stick on to the die-bonding adhesive, and the adhesive must also be electrically conductive.

(5) A secure and electrically conductive joint between the leadframe and the connecting wires must be possible.

(6) The leadframe must be resistant to oxidation and retain its mechanical strength during any thermal cycling that may accompany the die-bonding and encapsulation procedures.

(7) The leadframe must also withstand corrosive environments at high temperatures and high humidities.

(8) It must be possible to mass produce the leadframes economically.

Chapter 23 -13

Chapter 23 -14

DIE BONDING

The die-bonding operation consists of attaching the IC chip to the central supporting leadframe plate.

1. For the copper alloys attachment may be made using a gold–silicon eutectic solder; however, melting of the solder requires heating to 500 oC.

2.Another adhesive possibility is an epoxy bonding agent, which is normally filled with metal particles (frequently Ag) so as to provide both a thermally and electrically conductive path between the chip and the leadframe. Curing of the epoxy is carried out at temperatures between 60 oC and 350 oC.

Since the amounts of thermal expansion are different for the Cu alloy leadframe plate and Si chip, the epoxy adhesive must be capable of absorbing any thermal strains produced during temperature changes such that the mechanical integrity of the junction is maintained.

Chapter 23 -15

WIRE BONDING

• The next step in the packaging process involves making electrical connections between the metallized chip pads and the leadframe; this is accomplished using connecting wires .

• The most commonly used wire material is gold(a gold alloy containing a small amount of beryllium–copper that is added to inhibit grain growth).

• Gold wires are round and have diameters that are typically 18 µm , 25µm, or 50 µm. Less costly Cu and Al have also been employed for contact wires.

• A wire-bonding procedure is normally carried out using a microjoining operation

Chapter 23 -16

WIRE BONDING

Chapter 23 -17

Two different types of microjoints are possible: ball and wedge.

• Ball joints are possible for gold wires since the melted wire end forms into a small ball because

• of the high surface tension of gold. Bonding of this molten ball with the contact

• pad or leadframe is accomplished by making mechanical contact with the bonding

• surface while both wire and surface are subjected to ultrasonic vibrations.

Chapter 23 -18

A ball microjoint

Chapter 23 -19

Chapter 23 -20

A scanning electron micrograph of a ball microjoint and a wedge microjoint

Chapter 23 -21

Materials of encapsulation

Both ceramic and polymeric materials are used to encapsulate IC packages;

1)Ceramics are extremely resistant to moisture penetration and are chemically stable and chemically inert.

• Glasses are the most commonly utilized ceramic materials.

2)Polymeric materials are used in the largest volume for packaging encapsulation because they are cheaper, may be produced at lower temperatures.

• Epoxies and polyurethanes are commonly used.

Chapter 23 -22

Package Encapsulation:( hermetic sealing)• The microelectronic package must be provided

some type of protection from corrosion, contamination, and damage during handling and while in service.

Chapter 23 -23

Another packaging design, tape automated bonding (or TAB),

Chapter 23 -24

Chapter 23 -25

Chapter 23 -26

Bibliography

http://www.casetechnology.com/implanter/implanter.html

http://www.micron.com/content.jsp?catID=-8178&edID=16482

http://www.casetechnology.com/links.html

http://www.msil.ab.psiweb.com/english/msilhist5-e.html

http://www-3.ibm.com/chips/bluelogic/manufacturing/tour/

http://www.sematech.org/public/news/mfgproc/mfgproc.htm

http://www.hongik.edu/~photonic/pe2k1/semi/index.html

http://my.netian.com/~jinimp/semi/_lappingpolishing.html

http://jas2.eng.buffalo.edu/papers.html

http://www.photronics.com/internet/corpcomm/publications/basics101/basics.htm#section3

Chapter 23 -2718

• Material costs fluctuate but rise over the longterm as:--rich deposits are depleted,--energy costs increase.

• Recycled materials reduce energy use significantly.• Materials are selected based on:

--performance or cost indices.•An IC chip is bonded to the leadframe plate using either

--a eutectic solder or an epoxy resin. •The leadframe material must be both

--electrically and thermally conductive, and, ideally,

--have a coefficient of thermal expansion that matches the IC chip material (i.e., silicon or gallium arsenide)

SUMMARY

Chapter 23 -28

SUMMARY• Copper alloys are commonly used leadframe materials. • Very thin wires (preferably of gold, but often of copper or

aluminum) are used to make electrical connections fromthe microscopic IC chip contact pads to the leadframe. Ultrasonic microjoining welding techniques are used where each connection joint may be in the form of either a ball or wedge.

• The final step is package encapsulation, wherein this leadframe–wire–chip assembly is encased in a protective enclosure.

• Ceramic glasses and polymeric resins are the most common encapsulation materials.

• Resins are less expensive than glasses and require lower encapsulation temperatures; however, glasses normally offer a higher level of protection.

Chapter 23 -29

Reading:

Core Problems:

Self-help Problems:

ANNOUNCEMENTSReading: Failure of an Automobile Rear Axle