CHAPTER 07:
MATERIALS SELECTION
FOR INTEGRATED CIRCUIT
PACKAGES
ISSUES TO ADDRESS...
• Price and availability of materials.
• How do we select materials based on optimal
performance?
•Materials selection for integrated circuits
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PRICE AND AVAILABILITY
• Current Prices on the web(a):
--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:
--Cost
Cost of energy used in
processing materials ($/GJ)(g)
--Energy
Energy to produce
materials (GJ/ton)
Al
PET
Cu
steel
glass
paper
237 (17)(b)
103 (13)(c)
97 (20)(b)
20(d)
13(e)
9(f)
Energy using recycled
material indicated in green.
elect resistance
propane
natural gas
oil
a
a
b
c
d
e
f
g
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9
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http://www.statcan.ca/english/pgdb/economy/primary/prim44.htm
http://www.metalprices.com
http://www.automotive.copper.org/recyclability.htm
http://members.aol.com/profchm/escalant.html
http://www.steel.org.facts/power/energy.htm
http://eren.doe.gov/EE/industry_glass.html
http://www.aifq.qc.ca/english/industry/energy.html#1
http://www.wren.doe.gov/consumerinfo/rebriefs/cb5.html
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RELATIVE COST, $, OF MATERIALS
Metals/
Alloys
Relative Cost ($)
100000
50000
20000
10000
5000
Pt
Au
20
10
5
2
1
0.5
0.1
0.05
Composites/
fibers
$=
Diamond
Si wafer
2000
1000
500
200
100
50
Graphite/
Ceramics/ Polymers
Semicond
Si nitride
Ag alloys
Tungsten
Ti alloys
Si carbide
Cu alloys
Al alloys
Mg alloys
Al oxide
high alloy
CFRE prepreg
Glass-soda
Steel
pl. carbon
Concrete
AFRE prepreg
Carbon fibers
Aramid fibers
GFRE prepreg
Nylon 6,6
PC
Epoxy
E-glass fibers
PVC PET
LDPE,HDPE Wood
PP
PS
$ /kg
($ /kg)ref material
• Reference material:
--Rolled A36 plain
carbon steel.
• Relative cost, $,
fluctuates less
over time than
actual cost.
Based on data in Appendix
C, Callister, 6e.
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
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
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 semiconducting materials
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
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.
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 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 diebonding 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.
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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.
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
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WIRE BONDING
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
A scanning electron micrograph of a ball
microjoint and a wedge microjoint
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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.
A schematic diagram of an encapsulated IC
package.
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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 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.
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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
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:
--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)
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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 expensive than glasses and require
lower encapsulation temperatures; however, glasses
normally offer a higher level of protection.
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