ICEPT2007 Proceedings 410

Interfacial Reactions of Ni-doped SAC105 and

SAC405 Solders on Ni-Au Finish during Multiple Reflows

Toh C.H.1, Liu Hao

1, Tu C.T

2., Chen T.D.

2, and Jessica Yeo

1

1United Test and Assembly Center Ltd, 5 Serangoon North Ave 5, SINGAPORE 554916

2Accurus Scientific Co., Ltd, 508-51, Wen Sien Road, Section 1 Jen-Der, Tainan, TAIWAN

ch_toh@sg.utacgroup.com

Abstract

Solder-joint performances of SAC405 and SAC105 with 200ppm and 500ppm Ni addition were investigated for electrolytic Ni-Au

BGA pad finish. For each alloy system, ball shear tests, cross-sectional analysis and 3-D etching were performed to study the

interfacial reactions after repeated reflows. Also, the effect of solder-mask pad design on solder joint integrity was investigated. In

this study, all three systems gave rise to only (Cu,Ni)6Sn5 IMC layer at the soldering interface after multiple reflows at 245oC. The

(Cu,Ni)6Sn5 IMC had a needle-shaped structure after 1 time of reflow and the morphology remained the same after multiple reflows.

SACN0.05 appeared to be inappropriate for a BGA pad finish. It exhibited a drastic increase in the IMC layer thickness after 11

times of reflow and this corresponded to a significant increase in IMC fracture mode percentage. NSMD pad design led to ball pad

lifting because the bonding between the pad and substrate was weaker than the bulk solder strength and the IMC-pad interface

bonding.

1. Introduction

Solder composition and ball-grid array (BGA) pad finish are

some of the many factors that can affect intermetallic

compound (IMC) formation at the interface. The IMC growth

controls the strength of the solder joint. Excessive brittle

intermetallics and weak interfaces can result in solder joint

reliability concern leading to the BGA package failure.

The adoption of SAC solders doped with nickel has steadily

increased in the recent years, particularly for OSP finish. This

is developed to replace the conventional lead-free alloys such

as SAC405 for drop reliability improvement by means of

retarding the growth of Cu3Sn IMC [1, 2]. In high volume

manufacturing lines, it is always desirable to standardize

solder balls composition for BGA packages with different pad

finishes. However, there is limited report on the use of SAC

solders doped with nickel for the Ni-Au finish, a popular

industrial choice for pad finish.

In the manufacturing assembly processes, multiple reflows

are often required. During these processes, the solder joints

are subjected to the repeated melting and solidification.

Consequently, the microstructure of the BGA solder joints

can be affected by the interfacial reactions such as dissolution

of surface finish layers, compositional changes and IMC

growth. The purpose of this study is to understand how

SAC405 and two nickel doped SAC solders react with the Ni-

Au finish during multiple reflows.

2. Materials and Methods Test vehicles used in this study were BGA packages with

Au/electrolytic Ni/Cu pad. The Au thickness is around 1um,

while the Ni thickness is around 5um. Typical cross-section

SEM image for the as-plated substrate are shown in Figure. 1.

Fig. 1. Typical thickness for electrolytic Au plating

Solders balls with 0.4mm diameter from Accurus Scientific

with three types of Pb-free composition were studied as given

in Table 1. Both solder masks defined (SMD) and non-solder

mask defined (NSMD) were studied. SMD pad refers to a ball

pad defined by a solder-mask in which the copper pad is

much larger. An NSMD pad has a solder mask opening that is

larger than the copper pad area where the copper pad defined

the solder ball structure.

Electrolytic Au Electrolytic Ni

Cu Pad

ICEPT2007 Proceedings 411

Table 1: Composition of Solder Balls

Alloy Typical Composition (wt.%)

SAC405 4% Ag, 0.5%Cu, balance Sn

SACN0.02 1% Ag, 0.5%Cu, 200ppm Ni balance Sn

SACN0.05 1% Ag, 0.5%Cu, 500ppm Ni balance Sn

Solder balls were attached on the BGA substrates using a

commercial flux. The layout of the package is shown in

Figure 2. The reflow was accomplished using a hot air

furnace equipped with 6 heating zones. Reflow temperature

profile with 245oC peak temperature and a time above the

liquidus of approximately 46sec is shown in Figure 3. After 1

time of reflow, the packages were kept at a production floor

at room temperature for around one month before underwent

additional 3 times and 10 times of reflow at 245oC.

Fig. 2. Substrate layout: FBGA 4x4mm-16B, 0.8mm pitch with

0.4mm solder ball diameter.

Fig. 3. Reflow temperature profile

Fig. 3. Reflow temperature profile

A Dage 4000 tester was used for the high-speed ball zone

shear test. A constant shear speed of 50mm/sec was applied.

The gap between the substrate surfaces to the shear tool was

kept at 30um. Fracture mode distribution was studied using

optimal microscopy. Four levels of failure modes were

defined as schematically shown in Figure 4. For each alloy

composition & reflow conditions, about 80 solders balls from

various BGA substrates were sheared.

Fig. 4. Failure mode definition (a) bulk solders fracture with zero

IMC fracture; (b) IMC fracture surface > 1% and <50%; (c)IMC

fracture surface > 50%; (d) pad lifting

Specimens were cold mounted and cross-sectioned through a

row of solder balls. The specimens were then ground with

2000-grit SiC paper, and mechanical polished using 0.3 &

0.05µm Al2O3 powder. Micro-hardness tests were performed

using 50gf load on the cross-section in the middle of a solder

ball to examine the micro-hardness value after 1 time, and 11

times of reflows.

A HITACHI S-3000N scanning electron microscope (SEM)

operating at 15KeV was used to study the interfaces and IMC

microstructure. The SEM is equipped with an energy

dispersive spectrometer (EDS) to analyze the IMC and phase

composition. IMC thickness (in um) is defined as the ratio of

IMC layer area to IMC layer length as illustrates in Figure 5.

Fig. 5. Definition for IMC layer’s thickness

3. Results and Discussion

3.1 Zone Balls Shears and Failure Modes Figure 6 plots the total number of IMC fracture for the

various solders before and after multiple reflows. Only

fracture mode for the first balls are reported and compared to

avoid the neighboring balls sheared effect as schematically

shown in Figure 7. Dark shaded bar represents more than

50% of the fracture surface (individual ball) exhibiting IMC

interface fracture, whereas light shaded bar corresponds to

less than 50% of the fracture surface is IMC fracture.

80

100

120

140

160

180

200

220

240

260

0 50 100 150 200

Time (sec.)

Temperature (°C) peak temperature=245℃dw ell time above

217℃=46 sec.

IMC area

(brittle failure mode) Bulks solder area

(ductile failure mode)

(a) (b)

(c) (d)

Length of IMC layer = L (µm)

IMC layer

Ni layer

Area of IMC layer = A (µm2)

ICEPT2007 Proceedings 412

0%

20%

40%

60%

80%

100%

SAC405

(as-reflowed)

SAC405

(11x reflows)

SACN0.02

(as-reflowed)

SACN0.02

(11x reflows)

SACN0.05

(as-reflowed)

SACN0.05

(11x reflows)

0%<IMC<50% 50%<IMC<100%

Accumulation of IM

C (%) failures

Fig. 6. Accumulative IMC fracture percentage comparison after 1

time and 11times of reflow at 245oC. SAC405 with NSMD pad

designs showed 100% pad lifting after 1 time of reflow.

Solder bump Shear tool

Pad

1st 2

nd 3

rd

Fig 7. Zone balls shear failure mechanism. As the tool piece

touches the first solder ball, the solder undergoes plastic

deformation followed by fracture. The neighboring ball is

then sheared by the first ball. The process is repeated until the

whole row of solder balls is sheared.

Three failure modes were observed in this study, namely IMC

fracture, bulk solder fracture and pad lift. The IMC fracture

was with a shinny fracture surface, a typical character of

brittle fracture. Fracture in the bulk solder was with a dull

fracture surface depicting a ductile fracture. SACN0.02

solders showed the least total number of IMC fracture after 1

time and 11 times of reflow at 245oC. Although SACN0.05

solders exhibited less total number of IMC fracture than

SAC405 after 1 time of reflow, the total number of IMC

fracture for SACN0.05 increased dramatically from 15% to

65% after 11 times of reflows at 245oC.

3.2 IMC and Interfacial Microstructures The average thickness of IMC layer that formed at the

interface of a solder ball and Ni-Au overplated Cu pad with

reflow time is given in Figure 8. The IMC layer in SAC405,

SACN0.02 and SACN0.05 systems was about 0.9, 0.7 and

1.2um, respectively after 1 time of reflow. SACN0.05

exhibited a significant increase in IMC layer thickness up to

3um after 11 times of reflow while an average of 1.2um was

maintained for SAC405and SACN0.02 solders system. This

corresponded to a significant increase in IMC fracture mode

percentage as shown in Figure 6. The thick and brittle IMC

layer may provide a low energy path for crack propagation.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

as-reflowed

4x reflows

11x reflows

as-reflowed

4x reflows

11x reflows

as-reflowed

4x reflows

11x reflows

Average(µm) Standard deviation(µm)

IMC layer thickness (µm)

SAC405 SACN0.02 SACN0.05

Fig. 8. IMC layer thickness comparison at as-reflowed, 4 times and

11times of reflow.

Figure 9 shows that the bulk solder hardness for each solder

system remained after multiple reflows. Indentations were

carried in the middle of the solder ball, at least 50um away

from the IMC layer.

0

2

4

6

8

10

12

14

16

18

20

SAC405

(as-reflowed)

SAC405

(11x reflows)

SACN0.02

(as-reflowed)

SACN0.02

(11x reflows)

SACN0.05

(as-reflowed)

SACN0.05

(11x reflows)

Max Microhardness (Hv50) Average Microhardness (Hv50)

Min Microhardness (Hv50) Stdev(Hv50)

Microhardness (Hv50)

Fig. 9. Bulk solder micro-hardness comparison after 1 time and 11

times of reflows at 245oC

Typical cross-sectional SEM images for the SAC405,

SACN0.02 and SACN0.05 system after 1, 4 and 11 times of

reflows are shown in Figure 10 and Figure 11. EDS analysis

indicated the IMC layer compositions at the soldering

interface are consistent with the stoichiometry of the

compound (Cux,Niy)6Sn5. No other IMC was detected after

multiple reflows at 245oC. The (Cux,Niy)6Sn5 is a uniform

layer conforming to the nickel surface finish.

ICEPT2007 Proceedings 413

Ni layer

Sn

Ag3Sn

(Cux,Ni1-x)6Sn5

SAC405

Ag3Sn Sn

(Cux,Ni1-x)6Sn5

Ni layer SAC405

Ni layer

Sn

Ag3Sn

(Cux,Ni1-x)6Sn5

SACN0.02

Ag3Sn

Sn

(Cux,Ni1-x)6Sn5

Ni layer

(Cux,Ni1-x)6Sn5

SACN0.02

Sn

(Cux,Ni1-x)6Sn5

Ni layer

Ag3Sn

SACN0.05

(Cux,Ni1-x)6Sn5 Ni layer

Sn

AuxCuyNizSn1-x-y-z

(Cux,Ni1-x)6Sn5

SACN0.05

Fig. 10. Solder joint microstructures comparison after 1 time of

reflow@245oC (left) and 4 times of reflows@245oC (right)

Ni layer

(Cux,Ni1-x)6Sn5

Ag3Sn Sn

SAC405

Ni layer

(Cux,Ni1-x)6Sn5

Ag3Sn

Sn

SACN0.02

Ni layer

(Cux,Ni1-x)6Sn5

Ag3Sn

Sn AuSn4

SACN0.05

Fig. 11. Solder joint microstructures comparison after 11 times of

reflow @245oC

Figure 12 shows the 3-D IMC images for each solder system.

Deep etching revealed that the morphology of (Cux,Niy)6Sn5

were needled shaped compound protruding into the solder,

making the interface very rough as seen in Figures 10-11.

Needle-shaped IMC morphology was reported by Lee et al.

[3] for SAC305 on electroplated Cu/Ni/Au after 20 times of

reflows. The morphology remained the same after repeated

reflows at 245oC up to 11 times. It is interesting to note that a

ring pattern of IMC with a relatively larger IMC was

observed for SACN0.02 and SACN0.05 after 1 time and

multiple reflows. SACN0.05 solders which exhibited a

significant growth in (Cux,Niy)6Sn5 IMC layer thickness after

11 times of reflow showed no morphology changed. The

thick IMC shown in Figure 11 was attributed to the IMC

layer thickening of (Cux,Niy)6Sn5.

acicular (Cux,Ni1-x)6Sn5

acicular (Cux,Ni1-x)6Sn5

Ag3Sn

Ag 3 Sn

acicular (Cux,Ni1-x)6Sn5

acicular (Cux,Ni1-x)6Sn5 Residual solder

Fig. 12. 3D-IMC images comparison for as-reflowed, 4 times and 11

times of reflow at 245oC.

(a) SAC405 (as reflowed)

(b) SAC405 (4x reflows)

(c) SAC405 (11x reflows)

(d) SACN0.02 (as reflowed)

(e) SACN0.02 (4x reflows)

(f) SACN0.02 (11x reflows)

ICEPT2007 Proceedings 414

Fig. 12 (con’t). 3D-IMC images comparison for as-reflowed, 4

times and 11 times of reflow at 245oC.

Figure 10 and 11 showed the Ag3Sn inside the bulk solders

were pebble or plate-like. Figure 13 shows a typical 3-D

image of Ag3Sn for SAC405 after 4 times of reflow. The

Ag3Sn had a prismatic shape and the sizes were in the ranges

of 6-12um. AuSn4 platelet was detected in the bulk solders

for SACN0.05 after 11 times of reflow (see Figure 11). Also,

a compound of AuxCuyNizSn-x-y-z was found inside the bulk

solder for SACN0.05 after 4 times of reflow (see Figure 10).

Ag3Sn

acicular (Cux,Ni1-x)6Sn5

Residual solder

Fig. 13. Ag3Sn morphology for SAC405 with Ni-Au surface finish

after 4 times of reflow.

During soldering, the Ni from BGA surface finish diffused into

the solder and an IMC formed. If the solder did not contain Cu,

the IMC at the interface was typically Ni3Sn4, however, when Cu

existed as little as 0.6wt.% in the solder, the IMC became

(Cu,Ni)6Sn5 [4]. In the Cu-Ni-Sn ternary system as shown in

Figure 14, (Cu,Ni)6Sn5 is more stable than Ni3Sn4, (Cu,Ni)6Sn5

preferentially formed at the interface with the Cu in the solder

[5]. In this study, only Cu6Sn5 with some solution of Ni exited

at the interface up to 11 times of reflow for all alloys system.

Fig. 14. Isothermal Cu-Sn-Ni phase diagram at 235oC. Redrawn

from [5]

As more reflow cycles were applied, the IMC were

precipitated out in the solder matrix and thereby the amounts

of Cu and Ni were reduced. Interestingly, the available of the

Cu inside the solder matrix next to the IMC is seems to

sustain the substantial growth of (Cu,Ni)6Sn5 after multiples

reflows for SACN0.05. However, this is not the case for the

identical SACN0.02 but with 300ppm less Ni concentration.

Further investigation will be required to understand the

mechanism behind the much higher IMC growth for

SACN0.05 than SACN0.02 after 11 times of reflows.

3.3 Effect of solder mask design In this study, SAC405 solder balls with the SMD pad design

exhibited either bulk solder fracture or IMC fracture while all

those with NSMD pads design showed pad lift phenomenon.

Figure 15 provides a side-by-side comparison of such pad

designs at the same length scale. For SMD pad design, the

bonding between solder balls and copper pads was provided

by the top pad area. The solder resist overlapped the pad area

and enhanced the adhesion between the copper pad and the

substance. This helped to enhance adhesion of the copper pad,

resulting in a ball shear failure mode.

(g) SACN0.05 (as reflowed)

(h) SACN0.05 (4x reflows)

(i) SACN0.05 (11x reflows)

ICEPT2007 Proceedings 415

Fig. 15. Comparison of SMD (top) and NSMD pads (bottom) after

SAC405 balls soldering. In this study, the overplated Cu pad

diameter is 0.33mm.

Figure 16 shows a typical force-displacement diagram for a

sheared single ball with SMD and NSMD pad design. The

slope to failure was produced after removal of the

neighboring balls. In this example with a 0.33mm pad

diameter and a 0.4mm SAC405 ball, SMD pad design

exhibited slightly higher shear force than NSMD pad design.

Also, the large area under the force-displacement diagram for

SMD design indicates its ability to absorb more energy up to

a fracture. A study by Lim et al.[6] has shown that the relative

strength of SMD and NSMD ball pads was determined by the

pad size and substrate thickness. It is also interesting to note

that the diagram shows multiple stress peaks before failure

for NSMD pad design.

0

100

200

300

400

500

600

700

0 50 100 150 200 250 300

NSMD_1

NSMD_2

NSMD_3

SMD_1

SMD_2

SMD_3

Displacement (µm)

Force (g)

Fig. 16. Typical force-displacement graph for single shear test.

Comparison of SMD and NSMD pad design for SAC405 solder. A

constant shear speed of 500um/sec was applied.

4. Conclusions The following conclusions could be drawn for the SAC405,

SACN0.02 and SACN0.05 solders when reacted with Ni-Au

surface finish.

• All three systems gave rise to only (Cu,Ni)6Sn5 IMC

layer at the soldering interface after multiple reflows at

245oC.

• The (Cu,Ni)6Sn5 IMC had a needle-shaped structure after

1 time reflow and the morphology remained the same

after multiple reflows.

• SACN0.05 appeared to be inappropriate for a BGA pad

finish. It exhibited a drastic increase in the IMC layer

thickness after 11 times of reflow and this corresponded

to a significant increase in IMC fracture mode

percentage.

• NSMD pad design led to ball pad lifting because the

bonding between the pad and substrate was weaker than

the bulk solder strength and the IMC-pad interface

bonding.

5. Acknowledgement The authors would like to thank the management teams of

United Test and Assembly Test Center Ltd (UTAC) and

Accurus Scientific Co., Ltd for their support on this project.

6. References 1. J.Y. Tsai, Y.C. Hu, C.M. Tsai and C.R. Kao, “A study on

the reaction between Cu and Sn3.5Ag solder doped with

small amount of Ni”, J. Electronic Materials, Vo1. 32, No.11,

2003, pp12-3-1208.

2. I.E. Anderson, J.C. Foley, B.A. Cook, J. Harringa, R.I.

Terpstra and O. Unal, “Alloy effects in near-eutectic Sn-Ag-

Au solder alloys for improved microstructral stability”, J.

Electronic Materials, Vol.30, No.9, 2001, pp.1050-1059.

3. KY. Lee, M Li, D.R. Olsen and W T. Chen,

“Microstructure, Joint Strength and Failure Mechanism of Sn-

Ag, Sn-Ag-Cu versus Sn-Pb-Ag Solders in BGA Packages”,

2001 ECTC.

4. C.E Ho, R.Y. Tsai, Y.L. Lin and C.R. Kao, “Effect of Cu

Concentration on the Reactions between Sn-Ag-Cu Solders

and Ni”, J. Electronic Materials, Vol. 31, No. 6 2002, pp-584-

590.

5. K. Zeng and K. N. Tu, "Six cases of reliability study of Pb-

free solder joints in electron packaging technology,"

Materials Science and Engineering Reports, R38, 55-105

(2002). (A review paper).

6. A.C.P. Lim, T.K Lee, and Airin Alamsjah, “The Effect of

Ball Pad designs and substrate materials on the performance

of second-level interconnects,” 2003 EPTC, pp.563-568.