Development of Low- Temperature Drop Shock Resistant

an Alent plc Company

Development of Low-

Temperature Drop

Shock Resistant

Solder Alloys for

Handheld Devices

Andy Yuen

Alpha

Technical Services Director

Highly Confidential and Privileged Information of Alpha an Alent plc Company

Overview

Drivers for Low Temp Alloys

Existing Low Temp. Alloy Systems

Objectives

Testing Methodology

Approach & Results

Conclusions

Q&A

1

2

3

4

5

6

7


Low Temp.

Solders enables

elimination of

Wave Soldering

process, reduces

exposure to

thermal excursion,

thereby increasing

Long Term

Reliability

Low Temp Alloy &

Solder Paste

enable:

-Use of Low Tg

PCBs,

-Low Temp.

compatible

Components

- Reduced waste

and Scrap (dross)

Elimination of

Wave Soldering

Operation helps

in reduced

Labor and

Equipment

Maintenance

Cost

Significant

reduction in

Reflow Cycle

time.

Elimination of

one full set-up

enables higher

throughput

Reduction in

Energy Costs

Drivers for Low Temp. Alloy


Existing Low Temp. Alloys

Applications for Low Temp. Alloys

Melting Point (oC)

In 157

Sn 232

Bi 271

Pb 327.5

Ag 961

Cu 1084

Sn

37

Pb

Sn

58

Bi

SA

C A

lloys

183oC

138oC

~215oC

Fatigue

Properties

How

Good?Mechanical

Shock

Resistance


Existing Low Temp. Alloys

• Sn-Bi and Sn-In are the most common

lead-free low temperature alloys typically

used in electronic assemblies.

• Sn-Bi systems are preferred as

compared to Indium containing alloys

which tend to be more costly.

• Sn42Bi58 and Sn42Bi57.6Ag0.4 are the

most commonly used alloys in PCB

assembly and other electronic

applications.

Low Temp.

Eutectic Alloys

Bi26In17Sn (79oC)

Bi33In (109oC)

Sn52In (118oC)

Sn58Bi (138oC)


Objectives

• In this paper, we present details of a very systematic

study to further improve properties of Sn42Bi58 based

alloys.

• New Low Temp. Alloys were developed, through

elemental additions to improve mechanical strength and

drop shock resistance.

• Methodology, Test Methods, Results and Discussions

are covered for the new alloys that were developed.


Testing Methodology

Properties Test Method/Equipment Standard

MECHANICAL PROPERTIES

Tensile Test

(Ultimate Strength, Yield Strength

& Elongation)

Instron Universal Testing Machine ASTM E8

Elastic PropertiesOlympus 38DL PLUS ultrasonic

thickness gageInternal SOP

SOLDER JOINT

Solder Joint Microstructure Cross-section Internal SOP

Chip Resistor Shear Force DAGE 4000 JIS Z 3198-7

IMPACT & VIBRATION

Drop Shock Test Lansmont JESD22-B111


Yield

Point

Fracture

Point

Ultimate Tensile

Strength

Yield

Point

Fracture

Point

Ultimate Tensile

Strength

Diameter

(D)

Gage

Length (L)

Shoulder

radius (R)

4mm 16mm 3mmAlpha R&D

Alpha R&D

Instron Universal Testing Machine

Specimen Tested

Tensile Test


Poisson’s Ratio, Young’s Modulus and Shear Modulus are determined through

computations based on measured sound velocities and material density.

Olympus: Ultrasonic pulse-echo technique

Young's Modulus of Elasticity: Stress-

strain ratio under tension or

compression.

Shear Modulus of Elasticity: Stress-

strain ratio under shear stress.

Poisson's Ratio: Ratio of transverse

strain to corresponding axial strain.

Elastic Properties


Test Condition

JESD22-B111

standard.

Condition B (1500

Gs, 0.5 millisec

duration, half-sine

pulse).

Test Vehicles

CTBGA 84

Daisy chain

Drop Shock Test

M23 shock tester

from Lansmont

1500g, 0.5 msec

Dro

p h

eig

ht

Alpha Drop Test Vehicle

Shock Pulse


Mechanical Properties - Results

0

50

100

150

200

Sn-Bi58 Sn-Bi55 Sn-Bi50 Sn-Bi45 Sn-Bi40

Tensile Properties of Sn-Bi Alloys

Elongation (%) UTS (MPa)1000100101

99

90

70

50

3020

10

5

32

1

Number of Drops

Failu

res,

%

0.8992 296.5 27 0.632 0.092

1.131 167.2 30 0.158 >0.250

1.277 117.7 26 0.293 >0.250

Shape Scale N AD P

Sn-Bi40

Sn-Bi45

Sn-Bi58

Variable

Weibull

Drop Shock of Sn-Bi Alloys

Alloy 1st Failure

Sn-Bi40 10

Sn-Bi45 9

Sn-Bi58 4

Compared to Sn-Bi45 and SnBi40, Sn-Bi58 has slightly higher

mechanical strength and much lower elongation at rupture.

Drop resistance is higher for lower Bi content, for example, Sn-Bi40 has

150% higher average number of drops than Sn-Bi58.


1000100101

99

90

70

50

30

20

10

5

32

1

Number of Drops

Failu

res,

%

2.17529 502.068 0.991 15 0

2.00233 999.322 0.952 18 12

2.15339 908.766 0.932 19 10

Shape Scale Corr F C

Table of Statistics

SAC305

SAC305 + Underfil l

SACX Plus 0307

Variable

Drop Shock of SAC Alloys

Type 1 (Time) Censored at 1250 - LSXY EstimatesWeibull

Drop shock

characteristic life of

Sn-Ag3-Cu0.5 and

ALPHA SACX Plus

0307 is much higher

than of Sn-Bi alloys.

Illustrates complexity

involved in

improving drop

shock performance

of low temperature

alloys.

Drop Shock of Sn-Ag-Cu alloys


Mechanical Properties –

Approach for Tensile Properties

In general, an alloy with higher modulus will be

stiffer, i.e., less flexible under tension and will

have lower elongation

Develop an ideal alloy composition which will

have high enough modulus with balanced

ductility and elongation

Approach: Develop an alloy with an ideal composition that

enables desired Tensile Properties


AlloysUTS

(MPa)

Elongation

(%)

E

(GPa)

Sn42Bi58 63.6 48.2 39.0

Sn42Bi57.6Ag0.4 67.4 52.6 39.3

Alloy Composition-A 73.0 69.8 38.8

Alloy Composition-B 70.2 66.1 39.1

Small Ag addition to Sn-Bi58 results only in minor UTS and %

Elongation improvement.

Only minor variations were observed in the Modulus (~39GPa).

Mechanical Prop of Sn-Bi58 Plus alloys


Drop Shock - Approach

Target higher strength, % elongation and impact energy,

and lower modulus.

Microstructure modification:

Break continuity of large brittle Bi phase with minor

additions of alloying elements.

Further, with minor additions contribute to precipitate

strengthening of the Sn-Bi matrix.

Continuous and thickness controlled intermetallics

formation.

Approach: Engineer the alloy microstructure with inhibitors and

additives to improve the joint strength


1000100101

99

90

70

50

3020

10

5

32

1

Number of Drops

Failu

res,

%

1.358 191.4 29 0.744 0.047

1.088 306.2 45 0.716 0.058

1.277 117.7 26 0.293 >0.250

1.040 147.2 30 1.241 <0.010

Shape Scale N AD P

Alloy A

Alloy B

Sn-Bi58

Sn-Bi57.6-Ag0.4

Variable

Weibull

Drop Shock of Sn-Bi58 Plus Alloys Characteristic life

higher than SnBi58

• SnBiAg0.4: 25%

• Alloy A: 60%

• Alloy B: 160%.

First failure later than

Sn-Bi58

• Sn-Bi57.6-Ag0.4

and alloy A: 5x

• Alloy B: 9x.

Drop Shock of Sn-Bi58 Plus alloys


Sn-Bi58

Sn-Ag3-Cu0.5

Sn-Bi57.6-

Ag0.4

Sn-Ag3-Cu0.5

Sn-Ag3-Cu0.5

Alloy A

Sn-Ag3-Cu0.5

Alloy B

Cross sectional

analysis of the BGAs

after the drop shock

test.

BGAs with Sn-Ag3-

Cu0.5 spheres were

used.

Cracks develop and

propagate mostly

through the Bi-rich

areas, near the

intermetallics region.

BGAs After Drop Shock Test


1. Alloys with higher Bi addition have lower ductility, which directly

impacts their drop shock performance. The average number of

drops of Sn-Bi40 is 150% higher than of Sn-Bi58.

2. Lowering Bi content does not improve the excessive number of

early failures or solderability issues that may be caused by their

wide melting range. Instead, small alloying additions to the

eutectic Sn-Bi58 were used.

3. Characteristic life of Sn-Bi57.6-Ag0.4 is modestly 25% higher

than Sn-Bi58, whereas of alloy A is 60% higher and of alloy B is

160% higher.

4. The results presented here show that Sn42-Bi58 properties can be

improved further by addition of micro-additives. The degree of

improvement depends on the nature of the alloying addition and

how it interacts with the basic alloy.

Summary / Conclusions


• Alloying additions used in Alloy B seem to be the most

promising to improve drop shock performance of Sn42-

Bi58 alloy.

• Additional data being compiled include thermal cycling

(-40C ↔ +125C thermal profile) and its effect on shear

strength and intermetallics thickness.

• Upon completion, these results will be compiled in a

final report and published.

Next Steps


©2014 Alpha

Thank you !

Documents

Development of Low- Temperature Drop Shock Resistant