Download - MSE111-0 Lecture 5 [Semiconductor Assembly]

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Page 1: MSE111-0 Lecture 5 [Semiconductor Assembly]

Front Line Processes

Engr. Kath Virtusio

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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.

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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:

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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

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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

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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.

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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.

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Wafer sawing

• is the process of cutting the wafer into individual dice for

assembly.

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Wafer Sawing Requirements/Materials

1. Blades

• Hubless

• Hubtype

2. De-ionized Water (DI-H2O)

3. Surfactants

4. CO2 Bubbles

Hubtype Blade Hubless Blade Type

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Common Wafer Sawing Defects

1.Chipout

2.Washout

3.Die crack

4.Saw-through cutting

5.Saw dust accumulation

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Die Attach

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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.

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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

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Basic Material Needed During Die Attach

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Leadframe – Support

structure of the package.

Epoxy – Silver filled,

conductive.

Chips – sawn silicon

wafer.

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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

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Die pad PRS / Mechanical Alignment

Chip Ejection

Chip suction / Pick-up

Epoxy Application

Chip Attach / Bond

Chip PRS

Epoxy Cure

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Auto Die Attach Process

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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.

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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

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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

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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

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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.

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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

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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.

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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

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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.

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Bonding Theory

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Intermetallics

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What Is Inter-Atomic Diffusion?

Gold atom Aluminum atom

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What Is Inter-Atomic Diffusion?

Gold atom Aluminum atom

FORCE

FORCE

ULTRASONIC SCRUB

ULTRASONIC SCRUB

HEAT

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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.

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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

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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

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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.

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• 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

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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.

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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

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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

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Grain Size Distribution

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STEP 1: PRS FOR FIRST BOND

BALL BONDING PROCESS

HEAT

WIRE CLAMP

GOLD WIRE

CAPILLARY

BONDING PAD

FREE AIR BALL

CHIP

LEAD/POST

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STEP 2: FIRST BOND FORMATION

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STEP 2: FIRST BOND FORMATION

Ultrasonic Scrub

Force

HEAT

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STEP 2: FIRST BOND FORMATION

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STEP 3: VERTICAL HEIGHT EXIT FORMATION

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STEP 3: VERTICAL HEIGHT EXIT FORMATION

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STEP 3: VERTICAL HEIGHT EXIT FORMATION

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STEP 3: VERTICAL HEIGHT EXIT FORMATION

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STEP 4: WIRE LOOP FORMATION

REVERSE HEIGHT

REVERSE DISTANCE

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STEP 4: WIRE LOOP FORMATION

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STEP 5: 2ND BOND FORMATION

Ultrasonic Scrub

Force

HEAT

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STEP 5: WIRETAIL FORMATION

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STEP 5: WIRETAIL FORMATION

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STEP 5: WIRETAIL FORMATION

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STEP 5: WIRETAIL FORMATION

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STEP 5: WIRETAIL FORMATION

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STEP 6: FREE AIR BALL FORMATION

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STEP 1: PRS FOR FIRST BOND

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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

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• 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

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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

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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

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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

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Smashed ball Lifted wedge Metal-off

Wire tailing

Foreign material on

pad

Lifted ball

Common WB specifications and failure mechanism

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Example of Wirebond Machine

Bond head assembly

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Bond Head Assembly

Pre-heater

Clamp

Clamp

Fingers

Main heater

EFO Torch

Capillary

Transducer

horn

Clamp

Wire

tensioner

Transducer

screw

Camera

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• 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

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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