3D Interconnection and Packaging.pdf

7/26/2019 3D Interconnection and Packaging.pdf

1/57

Ingrid De Wolf

With input from REMO group

Packaging Reliability


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OUTLINE

Introduction Package levels

Function of a package

What can go wrong

Reliability Definition

Early failures

Standard tests

FMEA

What can go wrong

Crack growth (Si, ) Delamination

Corrosion

Diffusion processes (thermal

diffusion, electromigration,

thermo-migration)

Solder issues: whisker

growth, material degradation

(creep, fatigue, )

Conclusions


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INTRODUCTION: Package levels

Source: B. C. Johnson, Overview of c hip-level packaging, in ASM International Handbook Committee: Electronic

materials handbook, volume 1 Packaging. ASM INTERNATIONAL, Materials Park, Ohio, USA, 1989, pp. 398-407.

Si

PCB

chip

Level 0

MEMS-cap


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Humidity, gasses, pressure, light,chemicals, particles,

Input:electrical,pressure,acceleration, drugs

ThermalPower

ICMEMS

keep bad things out:

particles, humidity,

keep good things in:pressure, getters,

throw excess things out: heath,

allow easy in-output:

electrical,optical signals

give mechanical support,

without adding stress

gives the IC a standarized footprint

be reliable

It functions as Gate keeper

Output:electrical,optical,

A package should provide an electrical connection to the outside world,

give mechanical support and protect the device from mechanical,chemical and physical loads

INTRODUCTION: Package function


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MEMS substrate

capping chip

resonator

-level package

The IC can fail: not scope of this lecture

The package can cause the IC to fail

The package can fail: loss of contact to board, shortsbetween feet, cracks, delamination,

e-

EXAMPLE:

Electromigration failure in Cu BEOL

EXAMPLE:

Si stress resonator, measuring stress induced from packaging

INTRODUCTION: What can go wrong?


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RELIABILITY: Definition

Classical definition of Reliability:

Reliability = theprobabilitythat an item will perform a requiredfunction understated conditions for a stated period of time

Alternative definition of Reliability Testing:Predict the effect of design, processing,packaging and use indifferent environments and conditions on the functioning and the

lifetime of devices and define corrective actions

Specified lifetime


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RELIABILITY: When?

During its life the IC and the package are subjected to

various loads due to:

Manufacturing

temperature (0-level package T, cooling down from solderingreflow), vibrations (ex. ultrasonic cleaning), bending (on assemblymachines), mechanical shock

Distributionvibration and shock during transport, handling, storage

Customer use (in the field)environmental loads: cyclic temperature, thermal shock,mechanical shock, vibration (ex. mobile phone), humidity, dust,chemical, operational loads,

Early failures

Normal life and wear-out


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RELIABILITY: Early failures

Any product can have failures due to small variations in manufacturing :

- Can be high for new technologies- To be removed before making the final product

time

Failure

rate

The batht ub curve

Early failures =Infant mortality


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Why to be removed?Innovative package designs and multi-chip-modules (MCM) are expensive.When testing after packaging youll have to throw away good packages andchips (MCM).

How?Wafer-level probing or

Burn-in tests

- Place the chip in aburn-in socket- Place the socket in a burn-in chamber- Stress the chip at certain T and V for a certain time (product dependent)- Throw away the failing ones, package the known-good die

Demands for socket:should keep contact, should not damage the device

(chip, solder bumps,), it should not stick to thecontact.

RELIABILITY: Early failures


10/5710Old reliability engineer

One cannot test during 5 or 10 years with these real-life loads

and see whether it still works therefore:speed-up testing time

Normal life and Wear-out Accelerated tests

StandardTests

Failuredriventesting

RELIABILITY: Normal life and wear-out

time

Failure

rate

The batht ub curve


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STANDARD TESTS

Test at higher stress (thermal, electrical, mechanical, environmental)than in normal life

The test procedure and conditions are described in the standards

Examples of committeesMIL (Military) standards

JEDEC (Joined Electron Devices Engineering Council)

IPC (originally Institute for Printed Circuits but has broader scope)

IEC (International Electro-technical Commission) standards

Telcordia



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+ Easy, well defined and used all over the world

- Not all tests are useful for all kinds of packages

- The tests are time consuming, expensive

- Wrong failure modes might be tested and others are not tested:

The product should exhibit the same failure mechanism and mode in the testunder high stress conditions during a short time as it would exhibit undernormal life stress conditions during a longer time

Lifetime

Stress

Measurements done at high stress

Projection

A

O

H

P

P = the predicted lifetime

is only valid if:- The algorithm is correct

- The field stress indicated asH is the true field stress

- There is no other

(competing) degradationmechanism in the systemwhich will make the device in

field fail much earlier


STANDARD TESTS


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EXAMPLE: Temperature CyclingMILSTD 1010.8

Exposure of an assembly

to cyclic T changes with

parameters:

Tmin and Tmax

ramp rate

dwell time

Accelerated testing: STANDARD TESTS

TESTING and CHARACTERIZATION QUALIFICATION

Test type: THERMAL CYCLINGExample: the low air pressure test: 20h at15kPacorresponds to an altitude of about14 km which simulates (worst case) for anairplane. No use to test this on applicationsfor car or GSM or devices that are inside theplane...

Specific test conditions:Temperature range will be different forautomotive and consumer applications

Test results:Pass/no-pass criteria should be linked

with a required lifetime for specificproduct

e TR

Q

TMTTF

TMTTFAF

1exp

Acceleration factor


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Recognize the expected failure mechanisms that can occur in asystem for a certain application in a certain environment

Define tests that accelerate these failure mechanisms

Questions to answer:

Q1: what is the application?

Ex. for a mobile phone, for a car, for an airplane,Q2: where?Ex. a mobile phone for Singapore, or Siberia or Belgium, for a carunder the hoot or on the mirror or in the wheel or inside the cabin,for an airplane inside the cabin or in the wings,

Q3: what does the system see (environment)?

low pressure, vibrations, heath, cold, dirt,Q4: what can go wrong due to this environment?

Failure Mode Effect Analysis


FAILURE DRIVEN TESTING

Different concept of testing is valuable for new products and applications


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RPN

Accelerated testing: FMEA TESTS


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RPN = risk priority factor= severity x occurrence x detect-ability


What is first measured indicating a failure: failure mode

What is observed, the signature of the failure mode: failure defect What is the physics, chemistry causing the failure: failure mechanism

What is the cause of the failure mechanism: failure cause


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Find and explain one example of a FMEA of a specific application


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WHAT CAN GO WRONG?

Ingrid De Wolf

And how to test and inspect them?

C it t k DT


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Cracking: Si die fracture

Cause: the thermal-mechanical induced stress is higher than

the strength of silicon Influenced by the quality of the die: roughness, way of cutting,IC design and lay-out (3D-Cu-plugs through thin die can act as crackinitiators)

Wider (thicker) or asymmetrical fillets result in larger stresses

at the chip edges, which may induce die cracking

Sipackage

Composite stack DT

Source: Takahashi et al., ASET;ECTC Proceedings, 2004, p 601


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Cracking: Si die cratering

Is the fracture of silicon under the bond/bump during the bonding

process, flip chip assembly or field service More critical for advanced low-K materials

Source: C. Wang and A. S. HolmesIEEE TRANSACTIONS ON ELECTRONICSPACKAGING MANUFACTURING, VOL. 24,NO. 2, APRIL 2001

Wire bond damage Shear force on solder bump


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Delamination: underfill/die-attach/resin ...

Cause: the shear force at the interface is higher than the

adhesion forces Depends on many factors: materials, surface chemistry

It is an indirect failure mode (the device may still work) butit will lead to device failure at the end (redistribution ofmechanical and thermal stress)

DIE

Molding Resin

leadframe

leadframe

Si-pass

Organic solder mask


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Popcorn effect

One of the main causes of mold/resin delamination and

cracking Cause: moisture absorption

Test: T shock (popcorn test)

by diffusionthrough voids,delamination Steam: increase in

pressure: delamination,cracks, shear on balland wire bonds

moisture vaporizes resulting in steam

Moisture absorption

Solder reflow: high T (~ 230 oC)

1

2

3


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Corrosion

Destructive interaction between material and environment

An electrochemical process which may occur if there are:

- a conductive anode and cathode- an electrolyte bridging anode to cathode (moisture)

- an electrical potential between them

Corrosion of metal pads at the anode occurs by dissolution ofthe metal until an electrical open terminates the process.Dendrite growth (the precipitation of the dissolved metal ion atthe cathode) causes shorts.

Au Cu

Preferential attack of

Less noble CuPreferential attack inside Cu structure: PITTING

due to micro-structural differencesDendritic growth


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Solder joint reliability

The package reliability is mainly determined by the

robustness of the solder joints

Solder failure

dominateswear-out region


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Solder joint reliability

The package reliability is mainly determined by the

robustness of the solder joints

The robustness of the solder joints is defined by

- Intrinsic material properties: Creep and Fatigue behavior

- Metal finish interactions: Intermetallic compound formation

Solder joints are connecting

two different material worlds:Si versus laminate technology,

with highly differing CTE values

(Coefficient of Thermal Expansion)

Difference in deformation is mainly

taken up by the solder joints


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Fatigue occurs when a material is subjected to cyclic loading and the

material goes from the elastic region to the plastic region (at the pointswith highest stress) and back

The plastic deformation initiates micro-cracks, which propagate duringsubsequent cycles and can cause sudden failures

Fatigue is the dominant failure mode for flip chip devices

The crack growth is a function of the applied stress (s) andtemperature T, the material properties, the load rate, the history of thematerial

Fatigue life is the number of cycles required to initiate a micro crackand to propagate it to a critical length

Solder issues: Fatigue


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Solder issues: Fatigue

High-cycle fatigue: stresses remain in the elastic region.

Expected lifetime > 10000 cycles.

Low-cycle fatigue: the yield point is exceeded in each cycle

Expected lifetime < 10000 cyclesIs the most typical failure mode for solder joints (solders

have a low yield stress)

% failures

Number of T cycles

Weibull/Lognormal plot

0.1

100

500 1000

N50%

Determine N50% to

characterize thereliability of a

package assemble.


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Solder joint reliability: Fatigue

(Au,Ni)3Sn4

Ni3Sn4

(Au,Ni)Sn4

Sn

Pb

Brittle fracture

Brittle fracture


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Define acceleration factor for solder joint fatigue

Law Coffin-Manson

Solder joint reliability: Fatigue


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Solder issues: Creep

Creep is a time dependent visco-plastic deformation: change of

strain in time due to an applied load (mechanical stress) s Creep is a function of the applied load (s) and temperature (T)

Creep produces dislocation migration, grain-boundary sliding, reductionof residual stress, void formation,...

Creep in metals can occur at stress levels below the yield point

and at temperatures > 0.5 TM (TM = melting T in K)

Example: solder SnPb (60/40) melts at ~458K (= 183 oC)0.5 TM = 229 K (RT=298K) and has potential for creep even at room T

Example: ceramic substrates melt above 2000oC, no problem expected


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y = 5E-18x10.16

R2= 0.9133

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1 10 100Stress (MPa)

StrainRate(1/s)


Testing procedure:

Constant strain (displacement) rate (measure load) Constant load (measure displacement)

Aim:

Determine steady state strain rate as a function of stress andtemperature, to be implemented in FE models

0. 00

0. 10

0. 20

0. 30

0. 40

0. 50

0. 60

0. 70

0. 80

0. 90

1. 00

0.E +00 1.E +05 2.E +05 3 .E +05 4 .E +05 5 .E +0

Time (sec)

Strain

(absolute)

PrimaryCreep

Secondary /

Steady StateCreep

TertiaryCreep

0. 00

0. 10

0. 20

0. 30

0. 40

0. 50

0. 60

0. 70

0. 80

0. 90

1. 00

0.E +00 1.E +05 2.E +05 3 .E +05 4 .E +05 5 .E +0

Time (sec)

Strain

(absolute)

PrimaryCreep

Secondary /

Steady StateCreep

TertiaryCreep

Strain rate is defined by slope


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Is there a difference in creep behavior between eutectic Sn-Pb

versus Pb-free solder alloys?


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Diffusion processes

emassjq*Z

kT

DCCDJ

Driving forces for diffusion

1. Chemical gradient

2. Electric field (ions move in opposite direction of electric field,along the direction of the electrons, by momentum exchange)

3. Stress gradient (atom movement occurs from compressed to

tensile stressed regions)More. Thermal gradient

Not yet considered for solder bumps,

known in conductor lines (Cu, Al) asBlechs length

s

q*ZjL cecc

321

Electromigration can enhance or reduce the

intermetallic and void formation

ELECTROMIGRATION

BACK STRESS

Riet Labie imec restricted 2010

Reliabilit cha acte isation


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Intermetallic growth occurs by interdiffusion M S

Based on diffusion model of ideal solid solutions

Based on following assumptions:

- flux is identical in both directions (M in S and vice versa)

- one IMC is formed (constant concentration gradient)

Reliability characterisationSolid state ageing

tDx .~2

).

(exp.0~~

TR

QDD

with x = intermetallic thickness~

Interdiffusion coefficient is defined by Maxwell-Boltzmann equation

Reliability characterisation


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Based on diffusion model of ideal solid solutions

Based on following assumptions:

- flux is identical in both directions (M in S and vice versa)

- one IMC is formed (constant concentration gradient)

Ficks first law: concentration gradient is driving force

Ficks second law: conservation of mass

jM S

jS M

x

CDJ

.

t

C

x

J

x

CD

xt

C

.


jx

jx+Dx

Reliability characterisation


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x

CD

xt

C

.

2

2

.x

CD

t

C

tDx .~2

).

(exp.0~~

TR

QDD


~

Dt

xtxC exp~),(

with x = intermetallic thickness~

~

Interdiffusion coefficient is defined by Maxwell-Boltzmann equation

Intermetallic growth in solid state


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Cu Sn experimental measurements

Cu6Sn5

Cu3Sn initial 100h

1000h

Ageing temperature of 175 oC

Dominant h-phase,

non-continous e

Sn

Cu3Sn

Cu6Sn5

Pronounced scalloping after reflowseems to decrease

500h

Kirkendall voids trapped at Ti barrier

Cu

Transformation from e to h

Reduction of Kirkendall voids



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Ni

Ni3Sn4

Sn

initial

500h 1000h

100h

Ageing temperature of 175 oC

Ni3Sn4

Rather uniform IMC thickness,

needle-shaped or dendritic interface

Ni3Sn4

Ni3Sn4

More scalloping effect of interface

Crack formation inside IMC layer


Ni Sn experimental measurements



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39-16.0

-15.0

-14.0

-13.0

-12.0

-11.0

-10.0

0.0002 0.00025 0.0003 0.00035 0.0004

1/RT

ln

D~

Ni - Sn

Cu - Sn

150 100 oC

1/RT


experimental measurements

Cu-Sn

Ni-Sn

Ageing at 150oC

0.0

2.0

4.0

6.0

8.0

10.0

0 500 1000 1500 2000

time [sec]1/2

IMCthickness[um]

Cu-Sn

D~max = 6,3.10-6mm2/secD~average = 2,9.10

-6mm2/se

D~min = 2,3.10-6mm2/se

Ni-Sn

D~max = 2,3.10-6mm2/se

D

~

average = 1,7.10

-6

mm

2

/seD~min = 8,1.10-7mm2/se



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Cu-Sn: 100-150oC: Q=64kJ/mol, Do=226mm2/sec

150-175o

C: Q=138kJ/mol, Do=4.105

mm2

/sec 2-phase formation Cu3Sn (e) and Cu6Sn5 (h),

sum e+h follows interdiffusion laws

D > literature values

Validation experiment: blind experiment with unknown T

based on measured IMC thickness, estimated temperature of 166oCcompared to 163oC actual, compared to estimated value of 148oC

for literature data


experimental measurements

~

~

Diffusion processes


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y = 0.126x + 1.5125

y = 1.0013x + 6.3231

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10

t1/2, days1/2

IMCthickness,m

Ni/AuHASLLinear (Ni/Au)Linear (HASL)

2 days 60 days

0 days 60 days

HASL

Ni/Au

SOLDER: SAC (SnAgCu) on HASL vs. Ni/Au finish

Diffusion processesChemical gradient Thermal diffusion

Diffusion processes


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Ernest Kirkendall

WIRE BOND: Au wire on Al bond pad

- Formation of IMC: AuAl2 (purple plague has purple colour)

- When diffusion flux in one direction is larger than diffusion fluxin opposite direction this results in material shortage (voids)and excess material (hillocks)

Voids are created at the side ofthe fastest diffusing species:

KIRKENDALL VOIDS

Au

Al

Diffusion processesChemical gradient Thermal diffusion

Diffusion processes


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Diffusion can completely absorb one metal into the other. Example:

The complete dissolution of the UBM may result in solder/UBMdelamination

Intermetallics often cause weak bonds because embrittlement.Example: excessive Sn-Au, Sn-Cu and Sn-Ni intermetallics may causesolder joint embrittlement

SnSn

Cu

Cu3Sn

Cu6Sn5 Cu3Sn

Cu6Sn5

SiO2SiO2

Diffusion processesSolder intermetallic related failures

Diffusion processes


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Fracture of brittle intermetallics when high stresses and/or

deformations are appliedExamples:

-Bending of assemblies during

shipping and handling with insufficient mechanical support

in-circuit test, rework

insertion and removal of boards in chassis,

attachment or removal of press-fit connectors and fasteners

- Fast temperature changes

- Mechanical shock, vibration

- Volume change (VIMC


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Solder issues: Brittle fracture in solder

Metals lose ductility below a certain temperature :

Ductile to Brittle Transition Temperature (DBTT) Shock loads can cause premature failure due to brittle

fracture normally not associated with ductile failures

Increasing %Ag -> increase T at which brittlefracture occurs

Mini-Charpy system

Cooling block

0

10

20

30

40

50

60

70

-200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100

Temperature, oC

Fracturetoughness,

J/cm

2

Sn-5%Ag

Sn-4%Ag-0.5%Cu

Sn-3%Ag-0.5%Cu

Sn-37%Pb

Sn-0.7%Cu(Ni)

99.99%Sn

Sn-0.7%Cu

brittle

ductile

Ag:

0%

3%

4% 5%

0

10

20

30

40

50

60

70

-200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100

Temperature, oC

Fracturetoughness,

J/cm

2

Sn-5%Ag

Sn-4%Ag-0.5%Cu

Sn-3%Ag-0.5%Cu

Sn-37%Pb

Sn-0.7%Cu(Ni)

99.99%Sn

Sn-0.7%Cu

brittle

ductile

Ag:

0%

3%

4% 5%

IMEC: Presented at IMAPS Europe & IPC


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Brittle fracture in solder: Role of Ag

Sn-3%Ag-0.5%Cu Sn-4%Ag-0.5%Cu Sn-5%Ag

The increase of the Ag content leads to increase of theintermetallics volume fraction:Possibly the reason for the transformation shift?

Ag3SnAg3Sn and Cu6Sn5Ag3Sn and Cu6Sn5

Find and discuss an example of IMC related solder joint failures

Diffusion processes


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Driving force for diffusion is an electrical current: metal migrates in

the direction of the electron flow A reliability concern for the future high density microelectronic

packaging and power electronic packaging. The interconnecting solderjoints are getting smaller in size and, thus, carry higher currentdensity

Current crowdingat turning point for current

Current distribution inside bump by FE simulations

Diffusion processesElectro-migration

Diffusion processes


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Lifetime of diffusion driven mechanisms can be described by Arrhenius

law:

What will be the impact of scaling flip chip interconnections andincreased user current ?

RT

QjAMTTFv

EMnexp..~

1

Diffusion processesElectro-migration

Diffusion processes


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Electro-migration is metal migration in the direction of the electron flow

It acts as an additional driving force for diffusion It has an impact on IMC formation and UBM consumption

Diffusion processesElectro-migration related failures

Initial state

Cu3SnCu

6Sn

5

Sn

Cu3SnCu6Sn5

Sn

Cu

Cu

Cu6Sn5

Sn

500h at 150oC

electrons

Void propagation

Atom pile-up

No Cu left after 30h at 150oC and 1A

Diffusion processes


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T-gradient induced migration of solder bump material

with time which can result in an open bump

Current: joule heatingMetal lines on chiphave a smaller X-section: becomehotter(can be > Tm solder)

ColderT-gradient from chip side(warm) to substrate side (cold):material transport

Diffusion processesThermo-migration

Discuss an example of failure by thermo-migration.

Which gradients are needed to induce thermo-migration ?

Solder issues: Tin whiskers


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Solder issues: Tin whiskers

Crystalline extrusion structures of tin (mm lengths, electrically

conductive) They grow from surfaces where thin tin (especially

electroplated tin) is used as a final finish

They can bridge closely-spaced circuit elements maintained at

different electrical potentials.

Ban of lead: PbSn plating: trend to use pure Sn instead of SnPb,seems an easy and cheap alternative

http://nepp.nasa.gov/whisker/background/index.htm

the precise mechanism

for whisker formationremains unknown

Avoid the use of PURE TIN plated components

Solder issues: Tin whiskers pictures


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52Courtesy: Eddy Blansaer

Solder issues: Tin whiskers pictures

Solder issues: Voids


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Solder issues: Voids

Voids in solder: in general not a problem, but mightgive problems if they become too big: seen typically inPb containing solder ball on Pb-free solder paste.

X-ray imagesof SnPb BGA

X-sectionimages

Solder issues: BGA voiding problem


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Solder issues: BGA voiding problem

Pb-free paste and

Sn-Pb BGA ballSn-Pb

BGA

PCB

SAC Melting T= 217oC

Melting T= 183oC

Pb-free solder paste:Contains solvents and activators that become active andvolatilize at T > 183 oC (melting point of Sn63):

So the solder paste is still wetting and volatizing withinthe ball when the solder ball joint is in the process of

forming.

Solder issues: Solder extrusion


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Solder issues: Solder extrusion

Wang et al.http://www.advanpack.com/techlib/Reliability%20studies%20flipchip_package%20with%20Reflowable%20Underfill.pdf

Previti et al.http://www.cooksonsemi.com/tech_art/pdfs/NUF%20Reliability%20is%20Here.pdf

Solder flows into a void inside theunderfill or along delaminated parts(can occur after reflow or T-tests)

Conclusions


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Conclusions

Moores law is also affecting the package: front end, backend, package and board cannot be looked at separatelyanymore

Pb-free: causes new reliability problems (Tin whiskers,

brittle fracture,)

Packaging reliability: Standard testing towards failuredriven reliability testing

Accelerated testing required, but be careful whenextrapolating to real life


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3D Interconnection and Packaging.pdf