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Interconnection in IC Assembly
Level of Interconnection, Wire Bonding, Tape
Automated Bonding (TAB), Flip Chip
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Flip Chip
First invented in 1962 at IBM; called Solid Logic Technology
Converted in 1970 to Controlled Collapse Chip Connection (C4)
C4 is one type of mounting used for semiconductor devices, such as
Chips, MEMS or components, which is using solder bumps instead of
wire bonds.
The solder bumps are deposited on the chip pads, located on the top
side of the wafer, during the final wafer processing step.
In order to mount the chip to external circuitry (on a circuit board or
another wafer or a chip), it is flipped around – the top-side facing
down the mounting area.
The solder bumps are used to connect directly to the associated
external circuitry.
DMT 243 – Chapter 4
M.Nuzaihan
Interconnection in IC Assembly
Flip Chip
Basic structure of flip chip consists of IC or chip, interconnection system
and substrate.
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IC or chip –Si, GaAs, IP, Si-Ge and etc
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Interconnection system (IC bond pad interface) –under bump
metallization (UBM), chip bumps, bond material between the bump and
substrate metallization, encapsulation and substrate metallization
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Substrate –Ceramic (C4), epoxy glass laminate, polymer thin-film buildup, resin coated copper (RCC), glass, silicon, dieletric-coated metal,
liquid crystal polymer, dielectric metal matrix composite, low-temperature
co-fired ceramic (LTCC), ceramic thick film, multilayer high temperature
co-fired ceramic and etc.
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Flip Chip
DMT 243 – Chapter 4
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Flip Chip Technology
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Flip Chip
Advantages
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Lower cost than wirebonding since the bumping done at wafer level; high I/O;
all connections are made simultanenously.
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Higher reliability than either wirebonding or beam lead bonding (TAB). Book
claims flip chip was succesfully made from 1960-1990 without any failure
related to FC  implemented for ceramic substrates
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Better electrical performance due to lower resistance, capacitance and
inductance. Provide shorthest path between IC and substrate.
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Repairability—If an IC was defective, either during assembly or during usage,
it can be removed and a new IC if flip chip bonded on the same ceramic again.
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Improved chip designs may be substituted on multichip modules
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Flip Chip
Disadvantages
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More cost on infrastructures.
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Involved of a few process before interconnection can be made.
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Unprotected flip chips are prone to thermally induced cracking solder
connections and even damage the device itself. The effects of this thermal
mismatch can be lessened considerably with the use of an underfill.
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Not a good choice (low cost solution) for less I/O
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Flip Chip - Process steps
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Flip Chip - Process steps
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DMT 243 – Chapter 4
Integrated circuits are created on the wafer
Pads are metalized on the surface of the chips
Solder dots are deposed on each of the pads
Chips are cut
Chips are flipped and positioned so that the
solder balls are facing the connectors on the
external circuitry
Solder balls are then remelted (typically using
ultrasound)
Mounted chip is “underfilled” using an
electrically-insulating adhesive
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DMT 243 – Chapter 4
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DMT 243 – Chapter 4
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Interconnection in IC Assembly
Under Bump Metallization (UBM)
UBM is a multilayer thin films between IC
metallization, passivation layer and solder bump.
The structure consists of
1. an adhesion layer covering up the chip metallization
(Cr, Ti, Ni, W TiW )
1. A barrier layer (Cr, W, Ti, TiW, Ni, Cr-Cu)
2. A wetting layer (Cu, Ni, Pd, Pt)
3. An anti-oxidation barrier layer (Au)
DMT 243 – Chapter 4
M.Nuzaihan
Interconnection in IC Assembly
Under Bump Metallization (UBM)
UBM is necessary.
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To protect the Al metallized chip from moisture and corrosion.
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To remove the Al2O3 and provide a good adhesion to Al and reduce
the interfacial resistance.
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Barrier layer to prevents the solder (mainly Sn) from corroding
the Al or dewetting Cr wetting layer.
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Wetting layer because solder will not wet the metal deposited for
the barrier or bare Al.
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Under Bump Metallization (UBM) - Process
Evaporation
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Under Bump Metallization (UBM) - Process
Electroplated
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Under Bump Metallization (UBM) - Process
Sputtering
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Under Bump Metallization (UBM) - Process
Electroless
DMT 243 – Chapter 4
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Interconnection in IC Assembly
Flip Chip Bumps
The bumps provide four functions
1. Electrical connection between the chip and the substrate
2. A heat dissipation path from the chip
3. Environmental protection to the final metal layer (Al bond pad)
4. A structural link between the chip and substrate
DMT 243 – Chapter 4
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Various Flip Chip Bumps
Gold Stud Bumps
Indium Bumps
Nickel Bumps
Stacked Stud Bumps
Gold Bumps
DMT 243 – Chapter 4
Solder Bumps
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Interconnection in IC Assembly
Solder Bumps
1.
2.
3.
4.
5.
6.
Solder bumps are deposited onto the UBM –
Several method:
Evaporation
Electroplated
Screen printing
Solder jet technology
Ball drop
Micro Ball Placement and etc.
DMT 243 – Chapter 4
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Solder Bumps - Printing
DMT 243 – Chapter 4
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Solder jet technology
DMT 243 – Chapter 4
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Solder Ball Bumping (SBB) – Ball Drop
DMT 243 – Chapter 4
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Micro Ball Placement
DMT 243 – Chapter 4
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Solder Bumps
Solder primary material system
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High temperature with melting points in excess of 250°C (examples
include 95%Pb - 5%Sn and 97% Pb – 3%Sn)
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Moderate temperature with melting points between 200 and 250°C
(examples include 95.5%Sn-3.5%Ag-1%Cu; CASTIN® Cu-Ag-Sb-Sn
and 85.9%Sn-3.1%Ag-10%In-1%Cu and 96.5% Sn-3.5% Ag)
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Low temperature with melting points less than 200°C (examples
include 37%Pb-63%Sn eutectic, 88% In – 12%Pb, 100% In and 48%
Sn-52% In)
DMT 243 – Chapter 4
M.Nuzaihan
Flip Chip (Bump Interconnection)
Processing
There are several interconnection processing technologies:
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Solder interconnect processing
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Isotropic conductive adhesive
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Anisotropic conductive adhesive
DMT 243 – Chapter 4
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Solder interconnect processing
DMT 243 – Chapter 4
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Conductive Adhesive Interconnection
System
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The basic structures of two major adhesive interconnection
systems:
Isotropic conductive adhesive
Anisotropic conductive adhesive
DMT 243 – Chapter 4
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Solder Bumped Flip Chip
on Organic Substrate
Underfill
TCE of Silicon Chip = 2.5x10-6/oC
oC) o
Pick
&
Place
part
on
PCB
Room
Temperature
(25
o(75
Underfill
melting
& reflowing
Forming
solder
joint
(183
C) C)
Solder-Bumped
Flip
Chip
Stencil Printing Solder Paste on PCB
Solder
Joints
Organic Substrate
TCE of Organic Substrate = 18x10-6/oC
DMT 243 – Chapter 4
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Underfill Encapsulation and Processing
DMT 243 – Chapter 4
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Flip Chip – Product Examples
Flip Chip on Organic Circuit Board Application
DMT 243 – Chapter 4
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Flip Chip – Product Examples
Flip Chip for a Consumer Product
DMT 243 – Chapter 4
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