pb-free issues for the semiconductor industry cost531, vienna, 18 th of may 2007 pascal oberndorff...

Pb-free issues for the Semiconductor Industry

COST531, Vienna, 18th of May 2007

Pascal Oberndorff

NXP Semiconductors

IC Manufacturing Operations Back End Innovation

Nijmegen, The Netherlands

[email protected]

PaOb0709 Pb-free Issues for the Semiconductor Industry, P. Oberndorff, 18-05-2007

CONFIDENTIAL

Introduction

Pb could be found on the leads in electrolytic finish of semiconductor components

Pb also present in some die – attach, but exemption: alternatives for high Pb solder??

In case of Ball Grid Arrays (BGA) Pb was present in the solder balls

Semiconductor Industry is also impacted by higher temperature of the soldering process in Pb-free (Moisture Sensitivity)


CONFIDENTIAL

Introduction

Leaded Devices:– Change Electroplating

• NiPdAu Preplated Leadframes (costly)

• Pure Sn (Whiskers)

BGA– Change Solder balls to Pb-free balls– Sometimes balls go missing?!

Leaded Components ElectroplatingPre Plated Leadframes NiPdAu


CONFIDENTIAL

NiPdAu Structure

Au or Au Alloy

Pd

Ni

Base Material

•Prevent Pd Oxidation•Prevent Outgas absorbed by Pd

•Prevent Ni Surface Oxidation

•Prevent Cu Defusing into Pd Layer

Source : Sumitomo

Thicknesses:

Ni 0.5- 1.5 m; Pd 5- 60 nm; Au 3-10 nm


CONFIDENTIAL

Note that when come into contact with molten solder, the Pd and Au/Au Alloy layer dissolves into the matrix structure of the solder, leaving only the Ni Layer.

Molten Solder

Au or Au Alloy

Pd

Ni

Base Material

Molten Solder

PdAu

AuPd

Au or Au Alloy

Pd

Ni

Base Material

Soldering NiPdAu

Source : Sumitomo


CONFIDENTIAL

Conclusion for PPF

Only applicable for Small Components due to moisture absorption


CONFIDENTIAL

Moisture Sensitivity

During storage the component willl absorb moisture from the ambient humidity

During board assembly the absorbed moisture will vaporize and escape from the package

More critical for Pb-free reflow, due to higher temperatures of Pb-free solder

Source Infineon

Adhesion of moulding compound on PPF is worse than on Cu alloys


CONFIDENTIAL

IC damage risk due to:

Popcorn effect

Moisture Sensitivity


CONFIDENTIAL

Remarks on PPF

Good solution for small packages but costly!

How to improve adhesion to plastic;– Different plastic– Rougher surface of leadframe– Chemicals?

Leaded Components

Electroplating Sn based Finishes


CONFIDENTIAL

Comparison of Various Plating Options

+/-

!

Matte Sn was selected, mass production within NXP since 2003


CONFIDENTIAL

Problem: Whisker Growth

Most literature agrees whiskers grow because of internal compressive stress in plating layer

Important: whiskers not only grow on pure Sn but also on Sn alloys; even SnPb!


CONFIDENTIAL

Whisker growth not only with pure Sn!

Whisker growth on SnCu1 after one year storage at ambient temperature

Whisker growth on SnBi after 8 months storage at ambient storage


CONFIDENTIAL

Reasons for Stress in Plating

Irregular intermetallic formation

Mismatch of CTE with leadframe

Incorporation of foreign particles in plating

Mismatch of crystal orientation

Mechanical operations

The growth of whiskers is relieving the present stress


CONFIDENTIAL

Cu Substrate

Sn Deposit

Tin Whiskeris forced out

Cu6Sn5

Whiskers grow because of compressive stress in the plating which is caused by irregular growth of intermetallics

Whisker

Cu L/F

Cu6Sn5

Whisker Growth @ Ambient


CONFIDENTIAL

• Because of higher temperature diffusion will shift from grain boundary to bulk diffusion and thus regular intermetallics

• Grain Growth of Sn

• Diffusion barrier for further intermetallic growth• Possible annealing of stress• Postbake does NOT change CTE mismatch!

Countermeasure: Postbake (1h, 150 oC)(Within 24 hours of plating)

No whisker!

Sn deposit

Cu based LF

Cu6Sn5/Cu3Sn


CONFIDENTIAL

Sn Deposit

Cu6Sn5

Cu based LF

Experimental: Selective Etching

Useful for looking at morphologies of Cu6Sn5 and by differential measuring able to calculate the amount of intermetallic.


CONFIDENTIAL

1 hour after plating 1 week after plating

Intermetallic (Cu6Sn5) grows after plating by grain boundary diffusion of Cu into Sn. After one week the height is more than 3 m.

Intermetallic Growth Directly after Plating


CONFIDENTIAL

Growth of Cu6Sn5

Growth of intermetallic Cu6Sn5 compared

@ room temperature and @ 55 oC

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 1000 2000 3000 4000 5000

t (hours)

mas

s in

crea

se (

g)

55 degrees Room temperature


CONFIDENTIAL

Growth Curves at Elevated Temperature

Growth rate gives idea about amount of intermetallics

However, morphology of IMC is different

0

0.05

0.1

0.15

0.2

0 20 40 60 80 100

t (hours)

mas

s in

crea

se (

g) 150 degrees C 125 degrees


CONFIDENTIAL

Morphology of the Intermetallics

Bulk Diffusion

Recrystallization

Grain Boundary Diffusion

Less Recrystallization

1 h 150 oC 4 h 125 oC

42 days 55 oC 1 year RT

Same amount of intermetallics!


CONFIDENTIAL

6 Cu + 5 Sn => Cu6Sn5

Calculation of the volume change with help of X-ray data of the unit cell results in:

V = 128.08/124.06 = 1.0418 = 3.2%

Not taking into account the contribution of the reacting Cu:

V = 128.08/81.4= 1.588 = 57.3 %

Thus: Cu6Sn5 formation can result in compressive stress.

Calculation of Volume Change

avuc

m Nn

VV *


CONFIDENTIAL

Grain Growth

Before postbake: 3-10 m After postbake: 5-20 m

Sn

C19400

Cu6Sn5


CONFIDENTIAL

Diffusion barrier

• This graph shows that @ ambient there is no additional intermetallic growth up to 2 years after postbake

• No whisker observed after postbake @ ambient for > 4 years, irrespective of Sn thickness

0

0.01

0.02

0.03

0.04

0 200 400 600 800

time (days)

mas

s in

crea

se (

g)

Postbake @ ambient

w/o bake @ ambient

Intermetallic Growth

12

2

1


CONFIDENTIAL 26

CTE Mismatch

Difference in CTE

Sn ~23 ppm/o

Cu ~17 ppm/o

FeNi42 ~4 ppm/o

Sn= ~ 23 ppm/o

FeNi42= ~4 ppm/o

This situation is only relevant for assembled components

Whiskers will grow during temperature cycling


CONFIDENTIAL

CTE Mismatches

5 m Sn on NiFe after 500 cycles -55/85oC

5 m Sn on CuFe after 500 cycles

-55/85oC

No whisker growth on NiFe leadframes in isothermal storage,but after temperature cycling whiskers grow, contrary to Cu-based leadframes.


CONFIDENTIAL

Temperature Cycling on Assembly

Source: Infineon

Whisker length after –40 oC/ 85 oC, 5K/min, 30 minutes dwell time on unsoldered components and modules

(SnPbAg and SnAgCu soldered)

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500cycles

wh

iske

r le

ng

th i

n µ

m

Components

SnPbAg soldered

SnAgCu soldered

0/+125, air to air shock, 20 mindwell SnAgCu soldered


CONFIDENTIAL

Corrosion may happen due to water condensation in association with exposed base material forming a galvanic couple (exposed base material)

Also local defects within the tin may cause localised different nobility forming such a galvanic couple

Presence of water w/o condensation may support the corrosive effect

Whisker in high humidity

CathodeCu2+ / Cu = 0.34 V

AnodeSn / Sn2+ = -0.14 V

O2 in H2O

Sn Sn2+ + 2e-Cu2+ + 2e- Cu

Sn + 2OH- Sn(OH)2 + 2e-

O2 + 2H2O + 4e- 4OH-

2Sn + 2H2O + O2 2Sn(OH)2


CONFIDENTIAL

Sn Deposit

Tin Whiskeris forced out

Cu6Sn5

Whiskers grow because of compressive stress in the plating which is caused by irregular growth of intermetallics or oxidation/corrosion products in the Sn layer.

Formation of oxide results in volume increase (29/47% SnO and 32% SnO2)

Whisker Growth Associated with Corrosion

Cu based LF

SnOx


CONFIDENTIAL

Relation to Field Life

• After 4000 hrs even SnPb shows whisker growth (max length ~ 100 m)

• Board assembled units do not show this corrosion or extreme whisker growth during the test time


CONFIDENTIAL

Three mechanisms for whisker growth related to field conditions of SMT components have been idenified:

– Irregular intermetallic growth at ambient temperatures– CTE mismatch during temperature cycling– Corrosion induced at elevated temperatures & high humidities

Open Questions:– Were does Sn from a whisker come from?– What is mechanism on micro scale?– Why SnPb helps mitigation of whisker growth?– What does board soldering/reflow do to whisker growth? – What can be used as accelerated test with a relation to field life?

Remarks on Pure Sn finish

Leadfree solder ball adhesion improvement in BGA-packages


CONFIDENTIAL

BGA Soldering


CONFIDENTIAL

Root Cause Description of Missing Ball Issue

Present used Pb-free solder (SAC405) is very sensitive for impact loading because of:

– Very high strength of the bulk solder– Formation of brittle intermetallic layer between solder ball and package

substrate.– Formation of large Ag3Sn needles

Image of 0hr SAC-ball overview

Ni

(CuNi)6Sn5(CuNi)3Sn

4


CONFIDENTIAL

Cross sections of products showing missing SAC405 solder balls on NiAu pad

SEM/BSE images of PSK sample (missing bump), showing almost no intermetallic residues

Ni

NiNi

Ni


CONFIDENTIAL

Solution

1. Controlling growth and structure of the IMC layer

2. Reducing the strength, while improving the ductility of the Pb-free solder used

￫ modification of the SAC solder ball composition from SAC405 to SAC101

Composition of SAC101 SAC405– Ag : 1% 4%– Cu : 0.1% 0.5%– Ni : dopant no– X : dopant no– Sn : balance balance


CONFIDENTIAL

Effects of Ag and Cu concentration

Effect of Cu in SAC alloys on brittle failure

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2

Cu level (%)

Bri

ttle

fai

lure

s (%

)

Effect of Cu-content (Supplier Presentation)

Effect of Ag-content (Supplier presentation)


CONFIDENTIAL

Effect of Ag on liquidus temperature


CONFIDENTIAL

High speed shear test

•Test equipment : Instron micro tester•Test location : Instron Singapore

•Instruction and test set up by IME-Instron-Apptech Singapore

•Test speed : 0.45m/s•Shear height: 50m•Ball diameter : 600m

High speed ball shear tester


CONFIDENTIAL

High speed shear test-Failure modes

Three typical failure modes were found:

• Fracture in IMC• Fracture in Bulk solder• Pad peel

Fracture in IMC Fracture in Bulk solder

Fracture of Pad peel


CONFIDENTIAL

Alloy Composition Supplier

SnPb (ref1) eutectic A

SAC405 (ref2) Sn-4Ag-0.5Cu A

SAC305 + NiGe Sn-3Ag-0.5Cu-xNiGe B

Castin 258 Sn-2.5Ag-0.8Cu-0.5Sb C

(SAC101 + doping) Sn-1Ag-0.1Cu-0.02Ni-x A

SACX Sn-0.3Ag-0.7Cu-0.1Bi – X D

High Speed Shear testing


CONFIDENTIAL

High speed shear testFailure modes on Supplier A finish

• Bulk failure happens in SnPb and in SAC101 solder joints• The SAC0535 (SAC-X) alloy, SAC305 and SAC405 mainly fail in the intermetallic compounds• In general more intermetallic failure than for Supplier B finish

Failure modes of SAC 0535 on supplier A substrate

48%50%

2%

Pad peel

IMC

Bulk

Failure modes of SAC101 on supplier A substrate

62%

0%

38%

Pad peel

IMC

Bulk

Failure modes of SAC305 + NiGe on supplier A substrate

3%

97%

0%

Pad peel

IMC

Bulk

Failure modes of Castin 258 on supplier A substrate

16%

84%

0%

Pad peel

IMC

Bulk

Failure modes of SAC 405 on supplier A substrate

38%

62%

0%

Pad peel

IMC

Bulk

Failure modes of SnPb on supplier A substrate

9%

0%

91%

Pad peel

IMC

Bulk


CONFIDENTIAL

High speed shear test – Failure modes on Supplier B finish

• Bulk failure only happen in SnPb solder joints• SAC101 only failed with Pad peel• Assume the IMC strength > the strength in Pad interface if

failure in pad peel, SAC101 is better than other solder alloys to resist the high speed impact.

Failure modes of SAC 0535 on Supplier B substrate

85%

15%

Pad peel

IMC

Failure modes of SAC101 on Supplier B substrate

100%

0%

Pad peel

IMC

Failure modes of SAC305 + NiGe on Supplier B substrate

71%

29%

Pad peel

IMC

Failure modes of Castin 258 on Supplier B substrate

44%

56%

Pad peel

IMC

Failure modes of SAC 405 on Supplier B substrate

83%

17%

Pad peel

IMC

Failure modes of SnPb on Supplier B substrate

14%

0%

86%

Pad peel

IMC

Bulk


CONFIDENTIAL

Test board : standard JEDEC test board according to JESD22-B111

BLR tests: JEDEC Drop testing


CONFIDENTIAL

Test packages:

Component UBM finish Solder ball Supplier

LFBGA240 NiAu SAC 101 (0.3mm) A

LFBGA240 NiAu SAC 101 (0.3mm) B

LFBGA240 OSP SAC101 (0.3mm) A

LFBGA240 NiAu SAC405 (0.3 mm) A

LFBGA240 NiAu PbSn A

BLR tests: JEDEC Drop testing


CONFIDENTIAL

Drop test set-up

BLR tests: JEDEC Drop testing Method

To event detector (AnaTech: Model 128-256 STD)


CONFIDENTIAL

-200

0

200

400

600

800

1000

1200

1400

1600

1800

398.0 398.5 399.0 399.5 400.0 400.5 401.0

t (ms)

a (g

) exp

calc

Test conditions– Acceleration: 1500g, 0.5ms half-sine pulse– Test duration: 30 drops– Failure criteria: daisy chain resistance larger than 500 and the first event of intermittent

discontinuity followed by 3 additional such events during 5 subsequent drops

Sample size: 6 boards per type of package

BLR tests: JEDEC Drop testing Method

Acceleration measured from base plate


CONFIDENTIAL

A-NiAu B-NiAuUpper Limit 84 68Drops 56 45Lower Limit 37 30

BLR tests: JEDEC Drop testing results.SAC 101 versus SAC405

• SAC 101 clearly shows improved drop test performance compared to SAC405.

• Failure distribution of supplier B is more narrow than that of supplier A

• Following table shows the characteristic lifetime at 63.2% failure with 95% 2-sided confidence level, for SAC 101.

Supplier A-NiAu SAC101

Supplier B-NiAu SAC101

Supplier A-NiAu SAC405

Supplier A-OSP SAC101


CONFIDENTIAL

BLR tests: JEDEC Drop testing results. PbSn versus SAC 101

• Drop test performance of SAC 101 on NiAu is comparable to that of PbSn on NiAu!

1. 100.10.1.

5.

10.

50.

90.

99.

ReliaSoft's Weibull++ 6.0 - www.Weibull.com

Probability

N-dropsC

um

ula

tive

fa

ilure

s (%

)

2005-11-02 13:17Philips CFTPhilips

WeibullNiAuW2 RRX - SRM MED

F=9 / S=36CB[FM]@95.00%2-Sided-B [T1]

PbSn SAC 101


CONFIDENTIAL

(c) U13, down corner. Complete fracture through Ni/Ni3Sn interface at the component side.

1st2nd3rd

(d) Details of (c).

(a) U5 right corner 2nd bump, fail @ 7 drops. Crack through Ni/Ni3Sn interface at the component side.

(b) U5, right corner 1st bump. Crack in PCB.

Failure analysis – NiAu-SAC 405


CONFIDENTIAL

Failure analysis (1) – NiAu-SAC 101

(a) U3 up corner, fail @ 16 drops. (b) Details of (a). Fracture through first Cu6Sn5 IMC layer then metal line at the substrate side.

(c) U3 down corner, fail @ 16 drops. (d) Details of (c).


CONFIDENTIAL

Failure analysis – summary

SAC 101 solder ball, supplier A and B– From the cross sectioning analysis, it becomes clear that all failures are caused by

the Cu track fracture in the daisy chain on PCB– Drop test performance of SAC 101 /NiAu is comparable to that of PbSn / NiAu.

Drop test performance of SAC 101/OSP is a factor 2 better than PbSn / NiAu

SAC405 solder ball, supplier A– Complete fracture through Ni/Ni3Sn interface at the component side is observed in

many cases. Fracture of the Cu traces in the PCB, and of the PCB does occur, but is not the main failure mode.


CONFIDENTIAL

– High Temperature Storage Tests

• No missing balls after 1500 hrs testing

• Single uniform IMC layer with decreased thickness compared to SAC 405 (see picture)

Reliability Test Results of new solder ball

Image of 0hr SAC405-ball overviewImage of 0hr SAC101-ball overview


CONFIDENTIAL

Summary BGAJEDEC drop test improvement– All SAC101and SAC105 products showed failures at the PCB side.– SAC101on NiAu shows equivalent performance as PbSn on NiAu– SAC101 on OSP shows a factor 2 improvement in JEDEC drop testing

compared to PbSn on NiAu

Temperature Cycling Data– SAC101 outperforms SAC405– SAC101 ‘outperforms’ SnPb

Results from high speed shear test:– SAC101 closest to SnPb with respect to failure mode; no intermetallic

failure observed for this alloy

Open Questions:

What is influence of dopants?????

What does the Ag and Cu concentration do exactly??

But remember:It’s not easybeing green

pb-free issues for the semiconductor industry cost531, vienna, 18 th of may 2007 pascal oberndorff...

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