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Front Line Processes
Engr. Kath Virtusio
Page 1
The Semiconductor Manufacturing Industry typically consists of the
following steps:
1) Production of silicon wafers from very pure silicon;
2) Fabrication of integrated circuits onto these wafers;
3) Assembly of every integrated circuit on the wafer into
a substrate to have an electrical connection. (front line
processes)
4) Back end processing (e.g. encapsulation)
5) Testing of the finished products.
Assembly Flow
Die preparation
Wire Bonding
Encapsulation
Die attach
Wafer cut into individual chip by means of
sawing.
Attaching chip unto the support structure
using conductive epoxy or through Eutectic
process.
Connecting the Silicon chip unto its external
leads to form a complete circuit using Au
wire.
Serves as the body of the package which
serves as protection from Chemical and
Physical damages.
The process of putting the semicon chip inside a package to
make it reliable and convenient to use is known as
semiconductor package assembly, or simply 'assembly'. In
general, an assembly process would consist of the following
steps:
Page 4
Die preparation
is the process by which the wafer is
singulated into individual dice in
preparation for assembly.
Major Steps in Die Preparation
1.Wafer mounting
2.Wafer sawing
3.Cleaning
Wafer mounting
• is the process of providing support to the wafer to facilitate
the processing of the wafer from Wafer Saw through Die
Attach.
• During wafer mounting, the wafer and a wafer frame are
simultaneously attached on a wafer or dicing tape.
Wafer Mounting Steps
StartFrame
loadingWafer
loading
Application of tape to the wafer and wafer frame
Cutting of excess tape
Unloading of the mounted
wafer
Wafer Mounting Materials
1. Wafer frame
• may be made of plastic or metal,
but it should be resistant to
warping, bending, corrosion,
and heat.
2. Dicing tape (also referred to as a wafer film)
• is just a PVC sheet with synthetic
adhesive on one side to hold both
the wafer frame and the wafer.
• typically measuring 3 mils thick
• flexible yet tough and strong,
and with low impurity levels as well.
Common Wafer Mounting Problems
1. Wafer cracking or breakage
2. Bubble trapping on the adhesive side of the tape
3. Scratches on the active side of the wafer
4. Non-uniform tape tension which can result in tape wrinkles.
Wafer sawing
• is the process of cutting the wafer into individual dice for
assembly.
Wafer Sawing Requirements/Materials
1. Blades
• Hubless
• Hubtype
2. De-ionized Water (DI-H2O)
3. Surfactants
4. CO2 Bubbles
Hubtype Blade Hubless Blade Type
Common Wafer Sawing Defects
1.Chipout
2.Washout
3.Die crack
4.Saw-through cutting
5.Saw dust accumulation
Page 12
Die Attach
Page 13
Also known as Die Mount or Die Bond, is the process of attaching the silicon chip to the
die pad or die cavity of the support structure (e.g., the leadframe, pcb) of the
semiconductor package.
There are two common die attach processes, i.e., adhesive die attach and eutectic die
attach. Both of these processes use special die attach equipment and die attach tools to
mount the die.
Eutectic D/A-a process in which the chip backside itself has
lead (Pb) which serves to bind chip unto the
support structure.
Adhesive D/A-a process in which conductive epoxy (e.g.,
Silver) used to attach chip unto the support
structure.
Page 26
Die Attach DetailsDie Attach cross section
Fillet – the mass of epoxy climbing the edges of the die.
FILLET
BLT ( Bond Line Thickness ) – thickness of the epoxy material between
the die and die pad. Serves as contact mostly cathode side of the chip to
the die pad & cushion to prevent die stress.
BLT DIE PAD
DIE
Die – Basic building block of the circuit or semicon package.
Silicon chip is the most commonly used semiconductor.
Die Pad – an area on the support structure where die is placed.
DIE PAD
Basic Material Needed During Die Attach
Page 27
Leadframe – Support
structure of the package.
Epoxy – Silver filled,
conductive.
Chips – sawn silicon
wafer.
Support Structure
Is the “skeleton” of the package, providing mechanical support to the die or chip during itsassembly into a finished product. It consists of a die paddle, to which the die is attached, andleads which serve as the means for external connections to the outside world.
Types of support structure
Lead framePCB
Metal Header Ceramic
Header
Die pad PRS / Mechanical Alignment
Chip Ejection
Chip suction / Pick-up
Epoxy Application
Chip Attach / Bond
Chip PRS
Epoxy Cure
Page 29
Auto Die Attach Process
Page 30
PRS (pattern recognition search) - a machine feature
wherein a specific pattern on work area is thought for
placement of epoxy or chip
Epoxy application – there are 2 types of epoxy
application, stamping using epoxy disk/showerhead, and
epoxy writing/dispensing
Auto Die Attach Process and basic Machine parts
Die pad PRS / Mechanical Alignment
Epoxy Application
Epoxy Stamping
Epoxy dispensing
Air controlled
Air motor
controlled
•Upon epoxy alignment, epoxy is adjusted per machine
X/Y/Z location. Volume however is by adjusting
dispensing delays or air pressure.
Page 31
Chip pick-up – There are 2 types of pick-up tool, Rubber
tip and Collet
Rubber tip are usually made of Nitrille rubber usually
colored black.
Collets are usually made of Delrin plastic, these are
designed for smaller chips.
Chip PRS
Chip suction / Pick-up
Rubber tips and Collet
•Machine will perform Chip PRS then bond arm
will go to pre-pick position and will apply its
actual force as per set-up done. BF can be
obtained by adjusting spring tensioner located
on bond arm.
Actual BF indicator
Bond arm
Rubber tip
Spring tensioner
Auto Die Attach Process and basic Machine parts
Page 32
Bond arm – it is a part of machine that picks, and bonds
chip unto support structure
Ejection – a part of machine process wherein chip is
ejected from WAFER tape using ejector pin or Push-up
needle.
Chip Ejection
Multiple ejector
Pepper pot
Wafer holder
Theta
adjustment
•Ejector pins or Push-up needles are designed
to remove chip from wafer tape by piercing.
•Upon chip PRS, Pepper pot will automatically
apply vacuum sucking the wafer tape, then
Bond head will go to its Pre-pick position
applying force on chip. Once Rubber tip
touches the chip, ejector/s will rise piercing
wafer tape and ejecting chip to be attached.
Ejector height should be at 2 die thickness
maximum.
Auto Die Attach Process and basic Machine parts
Multiple EjectorSingle Ejector
Page 33
Bond arm – it is a part of machine that picks, and bonds
chip unto support structureChip Attach Bond
Epoxy Cure
Oven curingChip Attach
Chip sensor
•Bond arm will pass thru a light source to detect if chip is present on Rubber tip, once machine detects chip
presence, it will directly go to die pad to attach. Force is applied on the chip depending on spring tensioner actual
BF.
•Curing will take place after completing die attach.
Auto Die Attach Process and basic Machine parts
Page 34
Common Die Attach-related Failure Mechanisms
Die Lifting - detachment of the die from the die pad or cavity.
Common Causes: contamination on the die pad or cavity, die
backside contamination, excessive die attach voids, incomplete die
attach coverage, inadequate die attach curing.
Die Cracking - occurrence of fracture anywhere in the die. Common
Causes in the context of Die Attach: excessive die attach voids, die
overhang or insufficient die attach coverage, insufficient bond line
thickness, excessive die ejection force on the wafer tape, absence of
die attach voids.
Page 35
Common Die Attach-related Failure Mechanisms
Die Scratching - inducement of any mechanical damage on the die,
as when an operator scratches a die with tweezers due to
mishandling. Common Causes: insufficient operator training, worn-
out or contaminated pick-and-place tool, disorderly workplace, use
of improper tools.
Bond Lifting - lifting of the first or second bond from the die or
leadfinger, respectively. From the DA process point of view, this is
often due to resin bleeding of the die attach material into the bond
pads or leadfingers, inhibiting good intermetallic formation.
Bond Post
Epoxy
Fillet
N material
P material
Bond pad
Lifted bond/ball
Page 36
Common Die Attach-related Failure Mechanisms
Die Metallization Smearing - depression or deformation of any metal
line on the die surface. Common Causes: dirty or worn-out die attach
pick-and-place tool, wafer mishandling, wafer related issue.
Page 40
Die Shear / Flick test – a type of test to determine the
lateral force of epoxy after curing process.
Die Shear And Flick Test
Process Qualification
•DST/DFT tool should be equal or greater than chip size
•There should be 50% of Silicon left on die pad after DFT
•Epoxy on chip perimeter should cover at least 75%
•Chip alignment should be follow device requirement
DST Tool Typical Epoxy coverage
And Alignment
Maximum Epoxy Fillet
IRED Die Shear test
Page 43
WIRE BONDING is a process of providing electrical connection between
the chip and the external leads of the device using very fine bonding wires and a
combination of heat, pressure and/or ultrasonic energy at specified time. Wire
bonding is a solid phase welding process, where the two metallic materials (wire
and pad surface) are brought into intimate contact. Once the surfaces are in
intimate contact, electron sharing or inter-atomic diffusion takes place, resulting
in the formation of wire bond.
Page 45
Bonding Theory
Page 46
Intermetallics
Page 47Page 47
What Is Inter-Atomic Diffusion?
Gold atom Aluminum atom
What Is Inter-Atomic Diffusion?
Gold atom Aluminum atom
FORCE
FORCE
ULTRASONIC SCRUB
ULTRASONIC SCRUB
HEAT
The Wire bond
The parts of Wirebond are:
1) Ball bond – The bond formed after the FAB was squashed onto
the bonding pad by the capillary.
2) Ball Neck – The termination pointbetween the Ball bond and the wire
exit.3) Span – The gold wire from ball neck
to stitch heel.4) Stitch Heel – The termination point
between the Span and the Wedge.
5) Stitch/ Wedge – Second Bond or Wire Tail.
Bonding Wires
Are very thin wires of high purity (99.99% or4N) used as interconnect between the chip andlead frame leads. Typical wires used asinterconnects are Gold (Au), Silver (Ag), Copper(Cu) and Aluminum (Al).
Consideration in choosing bonding wires
Package type
e.g. plastic package, hermetic
Diameter
e.g. 0.8 mil, 1.0 mil
Tensile strength
e.g. 15 – 17 grams, 31-34 grams
Elongation property
e.g. 2 – 8%, 9 – 13%
Heat Affected Zone
e.g. 1.5 – 2.0 mils, 3.0 – 3.5mils
Wire bonding material and tools
Bonding Capillary
Are axial-symmetric ceramic tools with vertical feed holes. The tool’s tip is shapedto give clearance needed in fine-pitch bonding.
Considerations in the design of capillary tools.
Wire Diameter
Bond pad opening
Bond pad pitch
Capillary Dimensions
Tip Diameter
Hole Diameter or Size
Chamfer Diameter
Inside Chamfer
Inside Chamfer Angle
Face Angle
Outside Radius
Wire bonding material and tools
Basic Wire bond Terminologies
• Bond Pad - A metal area in a die used for electrical contact with a package.
• Capillary - A tubular wire guide use in wire bonding.
• Free Air Ball (FAB) - A resultant formation on tip of the wire which is achieved after EFO firing.
• EFO (Electronic Flame Off) - A spark coming from a EFO Torch/Wand which is a capacitance discharge to
form FAB.
Page 53
• Ball Bond (1st Bond)- The bond formed after the Free Air Ball is squashed onto the bonding
pad by the capillary.
• Wedge Bond (2nd Bond)- The wire tail or Stitch.
Basic Wire bond Terminologies
Basic Wire bond Parameters
• Bond force - The amount of force exerted on the wire while the Ultrasonic Energy is being applied.
• Ultrasonic Power - The amount of vibrating energy/force applied to the bond.
• Time - The duration that the ultrasonic energy and force are applied to the bond.
• Heater Block Temperature (HBT) - The temperature of the work holder that serves as ‘surface softener' to
match hardness of the bonding pad in relation to the bonding/gold wire.
Page 55
First Bond
First bond or ball bond is the bonding on the bond pad of the device.
Ultrasonic Power 1
Force 1
Time 1
Temperature
Second Bond
Second bond or wedge bond is the bonding done most of the time on leadsof leadframe.
Power 2
Force 2
Time 2
Temperature
Reverse Height
(Vertical Height Exit)
Reverse Distance
Reverse Angle
Horizontal Distance
Basic Wire bond Terminologies
• HAZ (Heat Affected Zone) - Is the length of Au wire which has had its microstructure and properties
altered during Free Air Ball formation.
• Reverse height - Sets the amount of wire fed out above first bond before reverse motion starts.
• Reverse distance/Angle - movement of unit usually up / down in motion caused by improper set-up of
Clamp or Insert Reverse Angle
Grain Size Distribution
STEP 1: PRS FOR FIRST BOND
BALL BONDING PROCESS
HEAT
WIRE CLAMP
GOLD WIRE
CAPILLARY
BONDING PAD
FREE AIR BALL
CHIP
LEAD/POST
STEP 2: FIRST BOND FORMATION
STEP 2: FIRST BOND FORMATION
Ultrasonic Scrub
Force
HEAT
STEP 2: FIRST BOND FORMATION
STEP 3: VERTICAL HEIGHT EXIT FORMATION
STEP 3: VERTICAL HEIGHT EXIT FORMATION
STEP 3: VERTICAL HEIGHT EXIT FORMATION
STEP 3: VERTICAL HEIGHT EXIT FORMATION
STEP 4: WIRE LOOP FORMATION
REVERSE HEIGHT
REVERSE DISTANCE
STEP 4: WIRE LOOP FORMATION
STEP 5: 2ND BOND FORMATION
Ultrasonic Scrub
Force
HEAT
STEP 5: WIRETAIL FORMATION
STEP 5: WIRETAIL FORMATION
STEP 5: WIRETAIL FORMATION
STEP 5: WIRETAIL FORMATION
STEP 5: WIRETAIL FORMATION
STEP 6: FREE AIR BALL FORMATION
STEP 1: PRS FOR FIRST BOND
Wire Pull Test
The purpose of this test is to measure bond strengths, evaluate bond
strength distributions, or determine compliance with specified
bond strength requirements of a semiconductor device
Page 77
• A – Ball lift
• B – Stitch lift
• C – Metal lift at pad
• D – Metal lift at post
• E – Span break
• F – Ball neck break
• G – Stitch heel break
Loop
Wedge heel
Wedge
VHE /
Neck
Ball
bond
Wire Pull Test Break Mode
Ball Shear Test
Ball shear test provides means of determining the
metallurgical bond which has been formed, and the
quality of Gold-Aluminum wire bonds to die.
Rules before performing BST;
1. Ball bond must be at 2 to 5 times wire diameter
2. Ball height must be greater than half of wire diameter
3. 75% of ball bond must be inside bond pad if pad size
is bigger than ball bond
4. 50% of ball bond is inside bond pad if and only if pad
is equal to ball bond size
5. Shear arm/tool must be of good condition
6. Shear height must be at 10 microns
h
Tight loop is called if the wire exit is less than
2mils long vertically before starting to arc for the
wedge bond.
Minimum 2mils of wire vertical exit from the ball
bond
Any evidence of non-sticking or lifted ball due to
poor bonding.
Any evidence of wires overlapping or crossing
each other.
Minimum 1 wire
diameter
Sagged wire is a defect when wire touches the
chip edge or near chip edge by less than one
wirebond diameter.
Wire exit outside of
the periphery of ball
bond
Club bond or golf bond if wire exit is
outside ball periphery.
Common WB specifications and failure mechanism
Any evidence of
cratering, the chip’s
underlying silicon
material is already
exposed.
Small/insufficient wedge is the term when the length
of wedge bond is less than 1.5times the wire
diameter or when the width is less than 1.2times the
wire diameter.
D = wire diameter
Minimum height is 1/2X
wire diameter
Bonding pad is equal or
smaller than the ballWrong bond placement
occurs if the ball bond is
less than 75% inside the
bond pad. But if bond
pad is equal or smaller
than the ball, the criteria
is 50% inside.
When a wirebond is misplaced on top of
another wirebond or bond over bond, it is
called stacked bond.
Smashed ball is reject if the ball bond height falls
below one-half of wire diameter.
1.2 to 5X wire Ø
X (wire Ø)
1.5 to 5X wire Ø
Common WB specifications and failure mechanism
Smashed ball Lifted wedge Metal-off
Wire tailing
Foreign material on
pad
Lifted ball
Common WB specifications and failure mechanism
Example of Wirebond Machine
Bond head assembly
Bond Head Assembly
Pre-heater
Clamp
Clamp
Fingers
Main heater
EFO Torch
Capillary
Transducer
horn
Clamp
Wire
tensioner
Transducer
screw
Camera
• Camera – allows user to view unit to be bonded and is also used during
programming
• Clamp finger – part of Clamp which is to grip leads during wire bonding
• Pre-heater – used to pre-heat LF before wire bonding which primarily serves
as ‘surface softener’
• Main heater – serves as surface tensioner during wire bonding
• Wire tensioner – serves as wire stiffener and optimized loop formation
• Transducer horn – holds capillary in place during wire bonding, it is the main
machine part used for wire bonding
• Transducer screw – holds and attaches capillary on Transducer horn
• Capillary – a tubular wire guide used in wire bonding
Machine Parts Terminologies
Auto Wire bond Process Flow
Pre-bond position 1
Leads PRS FAB Formation
Reverse Heig5htReverse Distance
Tail formation
1st Bond applica5tion
2nd Bond applicationReverse Angle Pre-bond position 2
Chip PRS