MSE111-0 Lecture 5 [Semiconductor Assembly]

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Transcript of MSE111-0 Lecture 5 [Semiconductor Assembly]

Front Line ProcessesEngr. Kath VirtusioPage 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 FlowThe 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: Die preparation sawing.Wafer cut into individual chip by means of

Die attach

Attaching chip unto the support structure using conductive epoxy or through Eutectic process.

Wire Bonding

Connecting the Silicon chip unto its external leads to form a complete circuit using Au wire.

Encapsulation

Serves as the body of the package which serves as protection from Chemical and Physical damages.

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 sawing3. 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 StepsStart Frame loading Wafer loading Application of tape to the wafer and wafer frame Cutting of excess tape

Unloading of the mounted wafer

Wafer Mounting Materials1. 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 Problems1. 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.

Hubtype Blade

Hubless Blade Type

Wafer Sawing Requirements/Materials1. Blades Hubless Hubtype

2. De-ionized Water (DI-H2O)3. Surfactants 4. CO2 Bubbles

Common Wafer Sawing Defects1.Chipout2.Washout 3.Die crack

4.Saw-through cutting5.Saw dust accumulation

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Die AttachAlso 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 13

Die Attach DetailsDie Attach cross section

DIE

FILLET

BLTDie Basic building block of the circuit or semicon package.Silicon chip is the most commonly used semiconductor.

DIE PAD

Fillet the mass of epoxy climbing the edges of the die. BLT ( Bond Line Thickness ) thickness of the epoxy material betweenthe die and die pad. Serves as contact mostly cathode side of the chip to the die pad & cushion to prevent die stress.

Die Pad an area on the support structure where die is placed.Page 26

Basic Material Needed During Die AttachChips sawn silicon wafer. Epoxy Silver filled, conductive. Leadframe Support structure of the package.

Page 27

Support Structure Is the skeleton of the package, providing mechanical support to the die or chip during its assembly into a finished product. It consists of a die paddle, to which the die is attached, and leads which serve as the means for external connections to the outside world. Types of support structure

Lead frame

PCB

Metal Header

Ceramic Header

Auto Die Attach ProcessDie pad PRS / Mechanical Alignment

Epoxy Application

Chip PRS

Chip suction / Pick-up

Chip Ejection

Chip Attach / Bond

Epoxy CurePage 29

Auto Die Attach Process and basic Machine partsDie pad PRS / Mechanical AlignmentPRS (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 Air controlled

Epoxy Application

Air motor controlled

Epoxy dispensing

Upon epoxy alignment, epoxy is adjusted per machine X/Y/Z location. Volume however is by adjusting dispensing delays or air pressure.

Epoxy StampingPage 30

Auto Die Attach Process and basic Machine partsChip PRSChip 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 suction / Pick-up

Actual BF indicator Spring tensioner Bond arm Rubber tip

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.Page 31

Auto Die Attach Process and basic Machine partsChip EjectionBond 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. Pepper pot Wafer holder Theta adjustment Multiple ejector 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. Single Ejector Multiple Ejector

Page 32

Auto Die Attach Process and basic Machine partsChip Attach BondBond arm it is a part of machine that picks, and bonds chip unto support structure

Epoxy CureChip Attach Oven curing

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.Page 33

Common Die Attach-related Failure MechanismsDie 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 34

Common Die Attach-related Failure MechanismsDie 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, wornout 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. Lifted bond/ball N material P material Bond padPage 35

Bond Post Epoxy Fillet

Common Die Attach-related Failure MechanismsDie 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 36

Die Shear And Flick TestProcess QualificationDST Tool Die Shear / Flick test a type of test to determine the lateral force of epoxy after curing process. Typical Epoxy coverage And Alignment

IRED Die Shear test

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 Maximum Epoxy Fillet

Page 40

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.

Bonding Theory

Page 45

Intermetallics

Page 46

What Is Inter-Atomic Diffusion?

Gold atomPage 47Page 47

Aluminum atom

HEAT

What Is Inter-Atomic Diffusion?FORCE

ULTRASONIC SCRUB

ULTRASONIC SCRUB

FORCE Gold atom Aluminum atom

The Wire bond

The parts of Wirebond are:Ball bond The bond formed after the FAB was squashed onto the bonding pad by the capillary. 2) Ball Neck The termination point between 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. 1)

Wire bonding material and toolsBonding Wires

Are very thin wires of high purity (99.99% or 4N) used as interconnect between the chip and lead frame leads. Typical wires used as interconnects 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 3.5 mils

Wire bonding material and toolsBonding Capillary Are axial-symmetric ceramic tools with vertical feed holes. The tools tip is shaped

to 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

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.

Basic Wire bond Terminologies

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.

Page 53

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.

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 leads of leadframe. Power 2 Force 2 Time 2 Temperature

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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 ofReverse Angle Reverse Angle

Clamp or InsertHorizontal Distance Reverse Distance

Reverse Height (Vertical Height Exit)

Grain Size Distribution

BALL BONDING PROCESS

STEP 1: PRS FOR FIRST BOND

WIRE CLAMP

GOLD WIRE

CAPILLARY

LEAD/POST

FREE AIR BALL BONDING PAD

CHIP HEAT

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 FORMATIONREVERSE DISTANCE

REVERSE HEIGHT

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

Wire Pull Test Break Mode

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 breakWedge heel Wedge Ball bond Loop VHE / Neck

Page 77

Ball Shear TestBall 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. 2. 3. 4. 5. 6. Ball bond must be at 2 to 5 times wire diameter Ball height must be greater than half of wire diameter 75% of ball bond must be inside bond pad if pad size is bigger than ball bond 50% of ball bond is inside bond pad if and only if pad is equal to ball bond size Shear arm/tool must be of good condition Shear height must be at 10 microns

h

Common WB specifications and failure mechanismMinimum 2mils of wire vertical exit from the ball bond

Tight loop is called if the wire exit is less than 2mils long vertically before starting to arc for the wedge bond.

Any evidence of non-sticking or lifted ball due to poor bonding.

Any evidence of wires overlapping or crossing each other.Wire exit outside of the periphery of ball bond

Minimum 1 wire diameter

Club bond or golf bond if wire exit is outside ball periphery.

Sagged wire is a defect when wire touches the chip edge or near chip edge by less than one wirebond diameter.

Common WB specifications and failure mechanismAny evidence of cratering, the chips underlying silicon material is already exposed.X (wire )

1.5 to 5X wire

1.2 to 5X wire

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. When a wirebond is misplaced on top of another wirebond or bond over bond, it is called stacked bond.

D = wire diameterMinimum height is 1/2X wire diameter

Wrong 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.

Bonding pad is equal or smaller than the ball

Smashed ball is reject if the ball bond height falls below one-half of wire diameter.

Common WB specifications and failure mechanism

Smashed ball

Lifted wedge

Metal-off Lifted ball

Wire tailing Foreign material on pad

Example of Wirebond Machine

Bond head assembly

Bond Head Assembly

Camera Clamp Fingers Clamp Pre-heater Main heater Wire tensioner Clamp Transducer horn Transducer screw Capillary EFO Torch

Machine Parts Terminologies 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

Auto Wire bond Process Flow

Leads PRS

Chip PRS

FAB Formation

Reverse Distance

Reverse Heig5ht

1st Bond applica5tion

Pre-bond position 1

Reverse Angle

Pre-bond position 2

2nd Bond application

Tail formation