CHAPTER 22: MATERIALS SELECTION AND DESIGN CONSIDERA CONSIDERATIONS TIONS ISSUES TO ADDRESS... • P rice rice and av availabilit ailability y of of mat mater erials ials. • How do we we select select materials aterials based based on opt optimal imal performance? • Ap App plica licattion ions: -- shaft shafts unde underr torsion orsion -- bars ars unde underr tension ension -- plat plates unde underr bend bending ing -- mater aterials ials for a magn magnet etic ic coil.
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PRICE AND AVAILABILITY • Curr urrent ent P rices ices on th the web web:: e.g., http://www.metalprices.com -- Short term trends rends: fluctuat fluctuations ions due to supply/dem upply/demand. and. -- Long term trend: rend: prices prices will increase as rich depos deposit its s are depleted.
• Materials Materials require require energ energy y to to process them: hem: -- E nerg nergy y to pro produ duce ce mater aterials ials (GJ (GJ /ton /ton)) Al P ET Cu steel glass paper
237 (17) 103 (13) 97 (20) 20 13 9
-- Cost of ener energ gy used in in processing materials ($/MBtu) elect resist resistance propane oil natural gas
25 17 13 11
Energy using recycled material indicated in green. Chapter 23 - 2
RELATIVE COST, c , OF MATERIALS Metals/ Alloys 100000 50000 20000 10000 5000
) 2000 c ( 1000 t 500 s o 200 C 100 e 50 v 20 i t 10 a l e 5 R 2 1 0.5
0.1 0.05
Pt Au
Graphite/ Ceramics/ Semicond
Polymers
Composites/ fibers
c
Diamond
Si wafer Si nitride Ag alloys Tungsten Ti alloys
Cu alloys Al alloys Mg alloys high alloy
CFRE prepreg AFRE prepreg Carbon fibers Aramid fibers GFRE prepreg
Si carbide
Al oxide Glass-soda
Steel pl. carbon
Concrete
Nylon 6,6 PC Epoxy PVC PET LDPE,HDPE PP PS
E-glass fibers Wood
=
$ / kg ($ / kg)ref material
•Reference material: -- Rolled A36 plain carbon steel. •Relative cost, c , fluctuates less over time than actual cost. Based on data in Appendix C, Callister, 7e. AFRE, GFRE, & CFRE = Aramid, Glass, & Carbon fiber reinforced epoxy composites.
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Integrated Circuits • The microelectronic circuitry, including the integrated circuits are used in • our modern computers, • calculators, and • other electronic devices
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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 adding controlled 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
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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
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Silicon Wafer
Photograph of a 100-mmdiameter silicon wafer. Each of the small rectangles shown is an individual IC chip or die.
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The performance of ICs The performance of ICs is limited: 1.by the characteristics of the semiconductingmaterials 2.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
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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 contact pads on the chip.
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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 - 10
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 andnumerous that accommodation of macroscopic wiring is simply not possible. 2. To disperse excess heat. While in operation, the many electronic devices generatesignificant 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 be handled. 5. To provide an adequate electrical interface such that the performance of the IC itself is not significantly degraded by the package design.
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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) I t must be possible to mass produce the leadframes economically.
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DIE BONDING The die-bonding operation consists of attaching the IC chip to the central supportingleadframeplate. 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 theepoxy 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 capableof absorbing any thermal strains produced during temperature changes such that the mechanical integrity of the junction is maintained.
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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. L ess costly Cu and Al have also been employed for contact wires. • A wire-bonding procedure is normally carried out using a microjoining operation
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WIRE BONDING
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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.
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A ball microjoint
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A scanning electron micrograph of a ball microjoint and a wedge microjoint
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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 stableand 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.
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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.
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Another packaging design, tape automated bonding(or TAB),
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Bibliography
http://www.casetechnology.com/implanter/implanter.html http://www.micron.com/content.jsp?catI D=-8178&edI D=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 - 26
SUMMARY • Material costs fluctuate but rise over the long term as: --rich deposits are depleted, --energy costs increase. • Recycled materials reduce energy use significantly. • Materials are selected based on: --performanceor cost indices. •An IC chip is bonded to the leadframeplate using either --a eutectic solder or an epoxy resin. • The leadframematerial 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) Chapter 23 - 18 27
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 from the 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 expensivethan glasses and require lower encapsulation temperatures; however, glasses normally offer a higher level of protection. Chapter 23 - 28
ANNOUNCEMENTS Reading: Failure of an Automobile Rear Axle
Core Problems:
Self-help Problems:
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