mechanical shock durability studies of sn–ag–cu–ni … · the design of the test vehicle,...

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Transactions of The Japan Institute of Electronics Packaging Vol. 2, No. 1, 2009

Mechanical Shock Durability Studies of Sn–Ag–Cu–Ni BGA Solder

Joints on Electroless Ni–P/Au Surface FinishFumiyoshi Kawashiro*, Hajime Yanase*, Masato Ujiie*, Takaki Etou* and Hiroshi Okada**

*NEC Electronics Corporation, 1120 Shimokuzawa, Sagamihara, Kanagawa 229-1198, Japan

**Senju Metal Industry Co.,Ltd, 1, Matsuyama, Mohka, Tochigi 321-4346, Japan

(Received July 30, 2009; accepted November 24, 2009)

Abstract

In this study, 60 solder compositions were examined in an effort to improve the integrity against impact loads of the

solder joints on an electroless Ni–P/Au surface finish. The Ag, Cu, and Ni contents in the Sn-based solder varied from

0 to 4.5 wt%, 0 to 2.0 wt%, and 0 to 0.05 wt%, respectively. Impact shear tests were performed to investigate solder joint

integrity after solder ball reflowing, after reflow soldering twice more, after storage at room temperature for 168 hours,

and after storage at 150°C for 1,000 hours. According to the results, the Ag content should be as low as possible, the Cu

content should be from 0.5 to 0.7 wt%, and the Ni content should be as high as possible. The Ag and Ni contents should

be determined in consideration of the wettability and the board-level reliability of the solder joints.

Keywords: Impact Shear Test, Electroless Ni–P Au, Sn–Ag–Cu–Ni Solder, Automotive, Flip-chip BGA

1. Introduction1.1 Trends

With the miniaturization and very large scale integration

of circuits in Si devices, the latest semiconductors for auto-

motive products such as car navigation systems require

fine-pitch flip-chip BGA packages (FCBGA) whose BGA

solder joints consist of solder balls and electroless Ni–P/

Au (ENIG) surface finish pads, as shown in Figure 1. Lead-

free solders like SnAg3Cu0.5, which are very brittle and

much weaker than tin-lead ones against impact loads, are

used for the BGA solder joints of the package.[1–10]

These solder joints tend to come off easily during handling

or transportation. To resolve this problem, the improve-

ment of the solder joints against impact forces and board-

level thermal cyclic properties is required.

1.2 Impact loads During TransportationTo determine the direction of the impact loads during

transportation, FCBGA packages were packed into a tray

container on a transporter, and the gravitational accelera-

tions of the packages were monitored. These accelerations

are plotted in Figure 2. The graph shows that the shock

that came from the horizontal direction (the X-axis) was

higher than that from the vertical direction (the Z-axis).

Therefore, impact shear testing rather than pull testing

was used to measure the impact properties of solder joints.

2. Experiments2.1 Test vehicle

The design of the test vehicle, which was 27 mm × 27

mm and had 720 BGA pads with a 0.8 mm pitch, is shown

Fig. 1 Structure of FCBGA and its pad surface finish.

Fig. 2 Impact accelerations on a package in the tray con-tainer during transportation.

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in Figure 3. The BGA pad surface finish consisted of

ENIG, with Ni plating and Au plating from 1.6 to 2.0 μm

and from 0.07 to 0.10 μm, respectively. The content of P in

the Ni plating varied from 9 to 11 wt%. After feeding water-

soluble flux to the substrate pads, 0.500 mm diameter

solder balls were attached to the black solder pads shown

in Figure 3, which had a pad opening diameter of 0.400

mm. Next, the solder balls were reflowed at 250°C as

shown in Figure 4 and the flux residue was removed using

de-ionized water.

2.2 Compositions of solder ballIn this study, 57 solder alloys were examined in an effort

to improve the integrity of the solder joints against impact

loads. The concentration of Ag, Cu, and Ni in the Sn based

solder varied from 0 to 4.5 wt%, 0 to 2.0 wt%, and 0 to 0.05

wt%, respectively, as shown in Table 1. In addition, three

solder alloys consisting of SnPb37, SnCu0.7Ni0.05, and

SnAg2.3Ni0.08 were examined.

2.3 Impact shear testingAn impact shear tester (Dage 4000-HS) was used to

investigate the impact properties of the solder joints, as

shown in Figure 5.[11] The speed of impact testing ranged

from 10 mm/s to 4,000 mm/s. Both the shear load and the

fracture energy of solder joint were plotted. Figure 6

shows the relationship between the shear load and the

fracture energy of the solder joints. According to our pre-

vious studies, the fracture energy of solder joints can

describe their integrity more clearly than the shear

load.[12–13] Therefore, the fracture energy of the solder

joints was also used in this study to assess the solder joint

integrity. Impact shear tests were performed immediately

after solder ball reflowing, after reflow soldering twice

more, after storage at room temperature for 168 hours, and

after storage at 150°C for 1,000 hours.

Fig. 3 27 mm × 27 mm/ 0.8 mm Pitch/ FCBGA.

Fig. 4 Reflow profile.

Table 1 Compositions of Sn based solder balls.

Ag 0

Cu 0 0.1 0.5 0.7

Ni 0 0.0 0.0 0.0

Ag 0.5

Cu 0 0.1 0.5 0.7 1

Ni 0 0.015 0.05 0 0.015 0.05 0 0 0.015 0.05 0

Ag 1

Cu 0 0.1 0.5 0.7 1 2

Ni 0 0.015 0.05 0 0.015 0.05 0 0 0.015 0.05 0 0

Ag 2

Cu 0 0.1 0.5 0.7

Ni 0 0.015 0.05 0 0.015 0.05 0 0 0.015 0.05

Ag 3

Cu 0 0.1 0.5 0.7 2.0

Ni 0 0.015 0.05 0 0.015 0.05 0 0 0.015 0.05 0.0

Ag 3.5 4 4.5

Cu 0 0 0.1 0.5 0.7 0 0.1 0.5 0.7

Ni 0 0 0 0 0 0 0 0 0

Fig. 5 Impact shear tester.

Fig. 6 Relationship between shear load and fracture energyof solder joint.

Kawashiro et al.: Mechanical Shock Durability Studies of Sn–Ag–Cu–Ni (2/7)

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3. Results and Discussion3.1 Shear speed

The minimums of the fracture energies of the Sn–Ag–Cu

solder joints are plotted as functions of their Cu content in

Figure 7. Tests were conducted on 30 solder joint points

under each condition. The speeds of the impact shear tests

were 10, 100, 1,000 and 4,000 mm/s. Figure 8 represents

the rates of the interfacial fracture modes at each shear

speed. The results in Figure 7 show that the sensitive

shear speeds were 10 mm/s and 100 mm/s. Furthermore,

Figure 8 shows that the fractures occurred mainly in the

solder bulk at the speed of 10 mm/s, and almost all the

fractures occurred in the intermetallic compound layer at

the speeds of 1,000 mm/s and 4,000 mm/s. Therefore, the

most sensitive shear speed to distinguish the impact prop-

erty seems to be 100 mm/s.

3.2 Cu content effect on impact shear propertyFrom Figure 7, the fracture energies of SnAgxCu0.5 and

SnAgxCu0.7 solder joints were higher than those of

SnAgxCu0 , SnAgxCu0.1, where x varied from 0 to 4.5 wt%.

It seems that these differences were caused by changes in

the intermetallic compound (IMC) composition. For exa-

mple, the SEM images in Figure 13 show cross-sections of

the solder joints of SnAg0Cuy alloys on ENIG pads and

fractures due to the impact shear tests, where y varied

from 0 to 0.7 wt%. The compositions of the IMC layers

were identified using an EPMA (electron probe micro-

analyzer) and EDX (energy dispersive X-ray fluorescence

spectroscopy). Table 2 shows the typical composition of

each observed IMC layer. The SnAg0Cu0 alloy on ENIG

pads formed a Ni3Sn4 IMC as the first IMC layer. There

were Ni3(Sn,P)2 IMC and P-rich Ni layers between the first

IMC layer and Ni–P surface finish. A fracture due to an

impact shear test occurred between Ni3(Sn,P)2 and

Ni3Sn4, where there were some voids. The SnAg0Cu0.1

alloy formed a (Ni,Cu)3Sn4 IMC as the first IMC layer.

There were Ni3(Sn,P)2 IMC and P-rich Ni layers between

the first IMC layer and the Ni–P surface finish. A fracture

Fig. 7 Fracture energies of SnAgCu solder joint on ENIG.

Fig. 8 Interfacial fracture rate of SnAgCu solder joint onENIG.

Table 2 Typical composition of IMC layers.

Layer Observed composition [at%]

Ni-P layer Cu0.0 Ni90.0 Sn0.0 P10.0

P-rich Ni layer Cu0.0 Ni73.8 Sn1.9 P24.3

Ni3(Sn,P)2 Cu0.0 Ni58.2 Sn25.6 P16.2

Ni3 Sn4 Cu0.0 Ni45.9 Sn54.1 P0.0

(Ni,Cu)3 Sn4 Cu6.5 Ni34.5 Sn59.3 P0.0

(Cu,Ni)6 Sn5 Cu37.8 Ni18.6 Sn43.6 P0.0

Fig. 9 Fracture energies of SnAgCu solder joints as a func-tion of Ag content.

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also occurred between the Ni3(Sn,P)2 and Ni3Sn4. Mean-

while, the SnAg0Cu0.5 and SnAg0Cu0.7 alloys formed a

(Cu,Ni)6Sn5 IMC as the first IMC layer. The Ni3(Sn,P)2

IMC layer was too thin to observe. Fractures occurred in

the solder bulk. According to these results, the Cu content

should be from 0.5 to 0.7 wt%, and a (Cu,Ni)6Sn5 IMC pro-

vides good durability against impact loads.

3.3 Ag content effect on impact shear propertyFigure 9 shows the averages of the fracture energy of

the Sn–Ag–Cu solder joints as a function of Ag content.

The averages of the fracture energy of the Sn–Ag–Cu solder

joints tended to decrease as the Ag content increased.

Also, the Cu content seemed to have little effect on the

fracture energies. Therefore, the microstructures of the

SnAgxCu0 alloys were investigated, where x varied from 0

to 3 wt%. Figure 10 shows SEM images of the microstruc-

tures of the SnAgxCu0 alloys, where the Ag content affects

the Ag3Sn intermetallic compound dispersion and Sn grain

size. The relationship between the fracture energy of the

SnAgxCu0 solder joints and Vickers hardness is plotted in

Figure 11. The fracture energy of the solder joints

decreased as the Vickers hardness of the solder alloy

increased. For the high Ag content solder alloy SnAg3, the

microstructure of the solder alloy has a finely dispersed

Ag3Sn IMC and a minute Sn grain size, which make the

solder hard and the solder joint brittle. Therefore, the Ag

content should be as low as possible to obtain good dura-

bility against impact loads.

3.4 Ni content effect on impact shear propertyThe effect of the Ni content on the fracture energy of the

Sn–Ag–Cu–Ni alloys is shown in Figure 12. It can be seen

that the fracture energies of the SnAgxCu0.7Ni0.05 solder

joints were higher than those of the SnAgxCuyNiz solder

joints, where x, y and z varied from 0 to 4.5, 0 to 0.7 and 0

to 0.05 wt%, respectively. It seems that these differences

were caused by changes in the intermetallic compound

(IMC) composition. For example, the SEM images in

Figure 13 show cross sections of the solder joints of the

SnAg3Cu0.7NiZ alloys on ENIG pads and the fractures due

to the impact shear tests. The compositions of the IMC lay-

ers were identified using an EPMA and EDX and are shown

in Table 2. The fracture energies of the SnAg3Cu0.7NiZalloys increased as the Ni content increased. The

SnAg3Cu0.7Ni0 alloy on ENIG pads formed a (Cu,Ni)6Sn5

IMC as the first IMC layer. There was a P-rich Ni layer

between the first IMC layer and the Ni–P surface finish. A

fracture due to an impact shear test occurred at the first

IMC layer. The SnAg3Cu0.7Ni0.05 alloy also formed a

(Cu,Ni)6Sn5 IMC as the first IMC layer. The P-rich Ni

layer was too thin to observe, which implies that the

consumption rate of Ni from the Ni–P surface finish

decreased. A fracture occurred between the first IMC

layer and the Ni–P surface. Based on these results, the Ni

content should be as high as possible, and a (Cu,Ni)6Sn5

IMC provides good durability against impact loads.

Fig. 10 Microstructures of SnAgxCu0 alloys (x: 1, 0, 3).

Fig. 11 Microstructures of SnAgxCu0 alloys (x: 1, 0, 3). Fig. 12 Fracture energies of SnAgCuNi solder joints as afunction of Ni content.


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3.5 Effects of pre-conditionsThe effects of pre-conditions on the fracture energy of

Sn–Ag–Cu solder joints are plotted in Figure 14 of the data

for the Sn–Ag–Cu–Ni and common solder alloys like

SnPb37, SnCu0.7Ni0.05, SnAg1Cu0.1Ni0.015 and SnAg2.3Ni0.08

are plotted in Figure 15. The pre-conditions written as As

reflowed and Reflow × 2 indicate that the impact tests were

performed immediately after solder ball reflowing, and

after reflow soldering twice more, respectively. RT 168 h

and HT 1000 h mean that the tests were performed after

storage at room temperature (RT) for 168 hours and after

storage at a high temperature (HT) of 150°C for 1,000 hours,

respectively. The fracture energies of the SnAg3Cu0.5 alloys

were too low compared with those of SnCu0.7Ni0.05,

SnAg1Cu0.5, and SnAg2Cu0.7Ni0.05. Although the fracture

energy of SnAg1Cu0.15Ni0.015 was very high after reflow

soldering, it became worse after the pre-conditions were

applied. The fracture energy of the SnPb37 alloy was also

high after reflow soldering, and it also became worse after

storage at 150°C for 1,000 hours. The fracture energies of

the SnAg2.3Ni0.08 alloys were also very low. These results

indicate that such solder alloys as SnCu0.7Ni0.05,

SnAg1Cu0.7Ni0.05, and SnAg2Cu0.7Ni0.05, with Ag content

not more than 3 wt%, Cu content from 0.5 to 0.7 wt%, and

Ni content not less than 0.05 wt%, seem to form solder

joints that are robust against impact loads under the vari-

ous pre-conditions.

3.6 Melting point of solder and board level reliabilityFCBGA packages for automotive products have a thick

substrate and a Cu-based lid. Therefore, the reflow temper-

ature during board-level assembly is lower than that of

packages for portable products. Solder balls whose melt-

Fig. 13 IMCs of various solder alloys (left side: SnAg0Cuy (y: 0, 0,1, 0.5, 0.7), right side: SnAg3.0Cu0.7Niz (z: 0, 0.05)].

Fig. 14 Effects of pre-conditions on the fracture energies ofSnAgCu solder joints.

Fig. 15 Effects of pre-conditions on the fracture energies ofvarious solder joints.

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ing points are higher than the reflow temperature can

cause solder joint failures such as open joints during

board-level assembly. Therefore, the melting point of the

solder, including the liquidus temperature, should be as

low as possible. Liquidus temperatures of solders are

shown in Figures 16 and 17. Preferred compositions

should be within 2–3 wt% for Ag, 0.5–0.8 wt% for Cu, and

not more than 0.05 wt% for Ni. Moreover, the board-level

thermal cyclic property should be taken into consider-

ation.[14]

4. ConclusionsThe mechanical shock durabilities of Sn-based solder

alloys, with Ag, Cu and Ni contents varying from 0 to 4.5

wt%, 0 to 2.0 wt%, and 0 to 0.05 wt% respectively, on an elec-

troless Ni–P/Au surface finish, were investigated by

impact shear tests at the speed of 100 mm/s under various

pre-conditions. The results show that the following points

are important for solder alloys to perform well:

(1) The Ag content should not be more than 3.0 wt%,

and solder alloys with 0 wt% Ag content will be

stronger than the others against mechanicals

shocks.

(2) The Cu content should be from 0.5 to 0.7 wt%.

(3) The Ni content should not be less than 0.05 wt%.

(4) Such solder alloys as SnCu0.7Ni0.05, SnAg1Cu0.7Ni0.05,

and SnAg2Cu0.7Ni0.05 seem to form solder joints

that are robust against mechanical shocks under

various pre-conditions.

(5) The Ag and Ni contents should be determined

with consideration for the wettability and the

board-level reliability of solder joints.

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