2015 NE Regional IMAPS Technical Symposium presentations/B/B2.pdf · 2015 NE Regional IMAPS...

$: 2015 NE Regional IMAPS Technical Symposium presentations/B/B2.pdf · 2015 NE Regional IMAPS Technical Symposium ... offer excellent protection from bump fracture ... Standard Electrode$
2015 NE Regional IMAPS Technical

Symposium

Epoxy Underfill Challenges for Narrow Pitch Copper

(Cu) Pillar Solder Bump Packages

Ken Araujo – Regional Manager NAMICS

Year 2012 2013 2014 2015

Process generation &

LowK generation (nm)

Maximum Die size (mm)20x20 to 26x26 – FC-BGA

10x10 to 15x15 – FC-CSP

Interconnection

Interconnection pitch (um)

130 120 <120 / Sub 40

Eu

Lead freeCu-Pillar

32/28 22/20 16/14

•Using fragile ultra low-k layers Low-k crack

•Using lead-free solder with high rigidity Bump crack

•Using Cu pillar Filler separation

ConcernTrend

Technical Transition in FC-BGA CSP Package

Cu Pillar Package Forecasting

• Fine Pitch Capable • 50 μm in-line• 40 μm / 80 μm staggered

• Cost Reduction

• Superior Electromigration Performance

• Compatible with Existing Wirebond Designs

• Green Solutions

• Available with and without Re-Passivation

• Qualified for advanced silicon node Low-k devices

• Extreme Fine Pitch for Thru Silicon Via (TSV) and Chip-On-Chip (CoC)

• Large Installed Capacity for Assembly and Test

Cu Pilaar, Why?

Underfill for FC-BGA/FC-CSP for Cu Pillar

# An approach optimizing Underfill property by modeling

# Avoiding Filler separation

# Void elimination for narrow gap

Underfill Challenges and Solutions

In Low Modulus state

In High Modulus state

Temperature(deg.C)

Wa

rpa

ge

0

RT 50 100 150 200 220

Process

Reflow

(F/C

Bonding)

Underfill

Curing

Tg HighTg Low

0.01

0.10

1.00

10.00

DMA CHART

High Modulus state

Low Modulus state

Tg High

Tg Low

Underfill

Reflow

(F/C

Bonding)

Underfill

Curing

Temperature

Low warpage

Model 1: UF Modulus=Low Low Tg

Model 2: UF Modulus=High High Tg

Si Die (CTE: 4ppm/C)

Substrate (CTE: 17-18ppm/C)

UnderfillBumpHigh

Low

Low stress

on bump

Low stress on Low-K

High warpage

High stress

on bump

High stress on Low-K

Package

Deformation

by

Temperature

Concept:

Objective:

Comparison between lead-free solder and Cu pillar models.

Die=10000

Fr4=20000

Overall specification of model:

Die: 20mm x 20mm

Substrate: 40mm x 40mm

Bump pitch: 150um

Solver: ANSYS Mechanical 12.1

Mode: 3D (1/8 cut)

Range: -55 to 125C

1. Lead-free solder 2. Cu Pillar

Cu pad

Solder

Cu

1890

60

15

15

45um85

95

UF

SR

63

60

Solder 85

100

95

60um

UF

SRCu pad

63

18

15

90

An Approach Optimizing Underfill by FEM


X-sectional schematics

High strain points shown for Lead Free & Cu Pillar. Tg is strongly correlated to creep strain of solder in both structure. A high Tg underfill is required for bump protection.

0

0.1

0.2

0.3

0.4

7000M

Pa

10000M

Pa

13000M

pa

30M

Pa

180M

Pa

330M

Pa

80C

120C

160C

20ppm

/C28ppm

/C36ppm

/C

70ppm

/C100ppm

/C130ppm

/CE1 E2 Tg CTE1 CTE2

Str

ain

(-)

Lead-free solder

Cu pillar

Solder Creep Strain


X-sectional schematics

Note the significant difference of low-K stress between lead-free solder to Cu pillar. In case of Cu pillar, high Tg is slightly better. Overall there is limited impact on Low-K when looking at Tg.

0

50

100

150

200

250

300

7000M

Pa

10000M

Pa

13000M

pa

30M

Pa

180M

Pa

330M

Pa

80C

120C

160C

20ppm

/C28ppm

/C36ppm

/C

70ppm

/C100ppm

/C130ppm


Str

ess(M

Pa)

Lead-free solder

Cu pillar

Low-K Dielectric Stress

In both cases, lower Tg will provide lower warpage.

Hig

h

L

ow

-360

-340

-320

-3007000M

Pa

10000M

Pa

13000M

pa

30M

Pa

180M

Pa

330M

Pa

80C

120C

160C

20ppm

/C28ppm

/C36ppm

/C

70ppm

/C100ppm

/C130ppm


Warp

age(u

m)

Lead-free solder

Cu pillar

Package Warpage

- Cu Pillar & Lead free bumps require a higher Tg to offer excellent protection from bump fracture during TCT.

- However low Tg may can assist with warpage, delamination and failure.

Modeling Summary

Bump Protection

Warpage

‘Filler separation’ is NOT a phenomenon of ‘more

settled down’, should be occurred by another factor

Cause: Gravitation forceCause: Unknown

Has no filler Fine particles

are remaining

Filler separation Filler settling

Confirmation of Filler Separation vs Filler Setting

Eutectic

solder (Sn/Pb)

Cu

Delamination by sample preparation

Separation is specifically on Cu-Sn interface.

V

2 different metal

2 different metal

electrolysis solution

Voltaic Battery

Element E0(V)

Li -3.045

K -2.925

Ca -2.840

Na -2.714

Mg -2.356

Al -1.676

This phenomenon means

Cu becomes positive(+) and Sn becomes

negative(-) electrode by the difference between

their Standard Electrode Potential (table right)

We think that Cu and Eutectic

solder works as a battery

And the voltage separates filler

and resin

Cu

0.340V

Sn

-0.138V

+

-

Standard Electrode Potential

Element E0(V)

Zn -0.763

Fe -0.440

Ni -0.257

Sn -0.138

Pb -0.126

H 0.000

Element E0(V)

Cu 0.340

Hg 0.796

Ag 0.799

Pt 1.188

Au 1.520

0.478V

1. Electrostatic charge of particle (Filler) 2. Electrophoretic effect

Filler

+

-- - -

----

-

+ + + ++

+

+

+

+

+

+

++

+ + +

---

- - -

SiO2 filler is Electrostatic charged and it

can be kept dispersed by the repulsive

force of charged ion

++

-+

-

-+

-+

+

+

+

+

+

+

+

Resin +

+

+

+

+

+

+

+

+

+

-

-

-

-

-

+

+

+++

++

+

+

+++

++

+

+

+++

++

+

+

+++

++

+

+

+++

++

+

+

+++

++

+

+

+++

++

+

+

+++

++

+

+

+++

++

+

+

+++

++

-

-

-

-

-

-

-

-

-

-

-

-

-

+

+

+++

++

-

-

-

-

-

Filler is migrated with the charged ion

together while voltage occurred

Ion charge

Electrostatic

charge

Our hypothesis

Why the voltage makes separation?

Test specimen

InitialCu wire (plated Sn)

L/S=25/25um

Electrode

Polyimide base film

Diagram

Procedure

After curedUnderfill

1. Put underfill onto the Cu wire

2. Put the specimen into oven (165C)

3. Turn on the switch(SW) to apply voltage

between 2 electrodes

4. Cure the specimen (2h)

5. Turn off the switch(SW)

6. Cut the specimen and the cross section

view is observed

SW

Underfill

Cross-section line

Electrode

Confirmation Test

(+)(-)

Filler separation like occurred in Cu-pillar PKG is confirmed by applying voltage.

(+)(-)

- No

applied

voltage

- Applied

+5VDC

Testing Results

The phenomenon

is influenced to

voltage.

- Not applied voltage

- Applied +5VDC

- Applied +1VDC

(+)(-)

(+)(-)

Step 1; Influence of Voltage

Step 2; Filler Size

Std Filler

Fine Filler

(+)(-)

The phenomenon is

confirmed with underfill

using fine filler.

The phenomenon is

confirmed.

The phenomenon seen with variable filler size.

Step 3; Curing System

Different curing system

to previous material.

Faster Gel time!

The phenomenon is

improved with underfill has

different curing system

Faster gel time does offer some assistance, but phenomena is still seen.

The phenomenon is

confirmed and

almost same to

165C curing

165C/2h

(+)(-)

The phenomenon is

confirmed.

175C/2h

(+)(-)

The phenomenon is not changed by underfill curing condition

Step 4; Curing Conditions

Condition Filler separation

1. Voltage Accelerated by voltage increasing

2. Underfill with fine filler Not improved

3. Underfill with different curing system Improved

4. Underfill curing condition Not improved

Filler separation is occurred by voltage and it is changed

by underfill curing system.

Summary

(+)(-)(+)(-)

Not applied voltage Applied +5VDCInfluence of voltage

1. Our opinion is the filler separating phenomenon has been occurred by the migration of filler by voltage

Battery effect of 2 different metals

2. And the voltage is related to the bump material

Eutectic solder

(Sn/Pb)

Cu

There are a possibility that it

happens on other bump structures.

In some cases of the differential of

alignment of die and substrate will

make metals working as a battery.

Conclusion

Bump pitch=150

SR=33

UF3=42

UF2=21

130 92.206

137.206

85UF1

Bump pitch=150

SR=33

UF1=42

UF2=21

130

85UF3

Bump pitch=150

SR=33

UF3=42

UF2=21

130 92.206

137.206

85UF1

Bump pitch=150

SR=33

UF1=42

UF2=21

130

85UF3

Underfill property UF1 UF2 UF3

TgDMA

C140

TMA 120

C.T.E. <Tg (0-20)

ppm/C20 58 34

>Tg 75 185 120

Storage modulus

<TgGPa

13.2 3.7 8.2

>Tg 0.85 0.09 0.3

UF2 = 0wt% filler(filler separated)

UF1 = 70wt% filler(filler concentrated)

UF3 = 50wt% filler (based material)

Objective:

To confirm how filler separation impacts PKG reliability by FEM

Design and condition

Design and material property has been defined from filler separation occurred in actual PKG

Filler Separation Impact on Stress

Low-K stress

0

50

100

150

200

250

300

Without filler separation With filler separation

Low

-K s

tres

s (M

Pa)

+7.7%

Stress on Low-K was increased by filler separation

FEM Low-K Dielectric Stress w/ Filler Separation

Underfill stress

0

50

100

150

200

250

300

350

400

Without filler separation With filler separation

Un

der

fill

stre

ss (

MP

a)

A: Fillet top B: Die corner C: Around the bump

A: no changeB: no changeC: +22%

A: Fillet top

B: Die corner

C: Around the bump

A,B C A,B C

Underfill stress around the bump increased due to filler separation

FEM Underfill Stress w/ Filler Separation

Filler cohesion during underfill flow reduces flow velocity in locations, this phenomena will cause flow marks and capture voids.

Flo

w d

irection

Flowmark

FastSlow

Captured void

Cross-sectional schematics

Uniform flow= Flows fast

Filler cohesion / stuck= Flows slow

Flow direction Flow direction

Mechanism of Flowmarks

Applying proprietary treatment to filler prevented the filler from cohesion.

Conventional UnderfillMany flow marks and voids

Improved w/ filler treatmentNo flow mark

Flow mark

Test specimen for flowability

Gap:10um

Underfill

Glass

Glass

10mm

20mm

Hot plate

Captured void

Improvement of Voids caused by Flowmarks

Vacuum assisted process Curing process with Pressure oven

Equipment Vacuum system Pressure oven

Void reduction process At underfilling At curing

A sort of void to eliminate Capture Capture, Moisture

Size of void to eliminate Large ~ Small Small

Underfill VCUF CUF

Dispensing Underfilling Curing Completion

Vacuum assisted process

Void elimination

Curing process with Pressure oven

Void elimination

Void Elimination Process

Under a vacuum condition

Round shape dispensing

DispenseComplete

Underfilling

Flow by Capillary action

and differential pressureNo void

Gap: 20um

Demonstration 1

Tem

pera

ture

Pre

ssu

re

Time20sec 20sec

About About

Set onto heated stage in

vacuum chamber

(130Pa, 70 – 110C)Penetration process

Accelerates with positive pressure

negative

pressure

positive

pressure

C-SAM View

IBM test vehicle

19 x 19mm die

150um pitch, Pb free C4Ref. Horibe, ECTC2011

Microscope View

Vent

Under atmospheric pressure

Vacuum Assisted Processing

Before cure

After cure

UFPressure-oven

TypePressure value

NAMICS VFS-60A-JP 7 kg/cm2

Capture void investigation

Voids disappear.

Capture void

Voids remain.

Pressure oven profile

2030405060708090

100110120130140150160170180

0 15 30 45 60 75 90 105 120 135 150 165 180

Time (min.)

Tem

pera

ture

(d

eg

.C)

1

2

3

4

5

6

7

8

9

10

Pre

ssu

re (

Kg

/cm

2)

Temperature Pressure

Pressure-ovenOven

TEST vehicleDie size : 20mmx20mmx0.73mm(t)Passivation : PIBump material : Sn/3Ag/0.5CuBump pitch : 175umFlux : non-cleanSubstrate size : 52.5mmx30.0mmx0.73mm(t)

Void Investigation w/Pressure-oven

FEM helps us to tailor underfill materials to the package needs.

Needs vary from package to package design.

Avoid filler separation as it can lead to increases in stress concentration

CUF provides narrow gap penetration and high jet dispense stability, also

new function with high thermal conductivity, vacuum assist process.

Summary

2015 NE Regional IMAPS Technical Symposium presentations/B/B2.pdf · 2015 NE Regional IMAPS...

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