Intermetallic Reactions in Reflowed and Aged Sn-58Bi
BGA Packages with Au/Ni/Cu Pads
C.C. Chi, L.C. Tsao, C.W. Tsao, and T.H. Chuang (Submitted February 14, 2006; in revised form March 7, 2007)
The reflow of Sn-58Bi solder joints in a BGA package with Au/Ni/Cu pads has been performed by employing various temperature profiles, which results in the formation of (Au0.66Ni0.34)(Sn0.82Bi0.18)4 intermetallic flakes in the solder matrix. The reflow operation performed at a peak temperature of 180 C for a melting time of 80 s gives a ball shear strength of 9.1 N, which decreases drastically to lower values between 6.4 and 4.6 N after further aging at temperatures from 75 to 125 C. Double layers of intermetallic compounds with the compositions of (Au0.30Ni0.70)(Sn0.90Bi0.10)4/Ni3Sn4 can be found at the solder/pad interfaces of the aged Sn-58Bi solder joints. Ball shear testing of the reflowed specimens shows ductile fracture through the solder matrix, which changes to brittle cleavage fracture mainly along the (Au 0.30-Ni0.70)(Sn0.90Bi0.10)4intermetallic layer after aging at various temperatures. The measurement of ball shear strengths (S) reveals a linear relation with the thicknesses (X) of (Au0.30Ni0.70)(Sn0.90Bi0.10)4intermetallic layers: S = 7.13 - 0.33X.
Keywords aging, ball shear strength, intermetallics, reflow, Sn-58Bi solder BGA
1. Introduction
Eutectic Sn-58wt.%Bi alloy has been considered a promis-ing Pb-free solder for low-temperature applications. This alloy possesses the merits of low melting point (138C), high tensile strength (Ref 1), and good creep resistance (Ref 2, 3). On the other hand, the Au/Ni metallization has been one of the most commonly used surface finishes for printed circuit boards (PCB) in electronic packages. The Au thin film provides the beneficial effects of oxidation protection and wetting improve-ment for Cu pads, while the Ni layer acts as a diffusion barrier between Au and Cu. During the reflow process, the Au surface finish dissolves quickly into the solder matrix and a liquid/solid reaction occurs at the interface of Sn-58Bi solder and Ni layer. The resultant intermetallic formation affects the bonding efficiency of the solder joints. In addition, the aging effect caused by heat generated from operating the electronic devices leads to intermetallic growth, and this has become quite an important issue for the reliability of the packages (Ref 4).
The intermetallic reactions occurred at the interfaces between Sn-58Bi solders and Ni substrates have been inten-sively investigated (Ref 5-8). After aging the Sn-58Bi/Ni solder joints at 85, 100, and 120C, Chen et al. reported the formation of Ni3Sn4intermetallic compounds at the interfaces,
and the growth kinetics of these Ni3Sn4 intermetallics was
analyzed and an activation energy of 90 kJ/mol obtained. Chiu et al. (Ref 6) further studied the interfacial reactions between
Sn-58Bi solders and Ni substrates at temperatures ranging from 200 to 400C. They found a Ni3(Sn0.99Bi0.01)4 intermetallic
phase with the composition of Ni41.5Sn57.9Bi0.6 formed at the
interface, and the intermetallic growth was diffusion-controlled with an activation energy of 28.3 kJ/mol. Kang et al. (Ref 7) also investigated the intermetallic reaction of a liquid Sn-58Bi solder with Ni(0.5-2.0 lm)/Cu(4 lm)/Si substrate and reported the appearance of Ni3Sn4 intermetallic compounds at the
Sn-58Bi/Ni interface. The growth kinetics of such an interfacial intermetallic was also diffusion-controlled. Young et al. (Ref 8) used electro- and electroless-plated Ni on Cu plates to react with Sn-58Bi solder at 145 and 185C. They found that by increasing the phosphorus content, the electroless-plated Ni could act as a good reaction barrier between Sn-58Bi solder and Cu substrate. As the soldering temperature rose, the growth of Ni3Sn4intermetallic compound was accelerated, which allowed
the Cu atoms to join in the reaction. In this case, the morphology of the Sn-58Bi/Ni interface would change from faceting to a saw-tooth type.
In a reflowed Sn-37Pb solder BGA package with Au/Ni surface finish, the dissolved Au atoms were found to form flake-shaped AuSn4 intermetallics in the solder matrix, and
further aging of the reflowed specimen caused the AuSn4
intermetallic phase to migrate from the solder matrix to the Sn-37Pb solder/Ni layer interface (Ref 9). According to the explanation of Ho et al., it has been the Ni layer that attracts AuSn4 intermetallics and causes their migration to the Ni/Cu
pads (Ref 10). However, the behavior of AuSn4intermetallics
in a Sn-3.5Ag solder BGA package with Au/Ni/Cu pads could be quite different. In this case, the AuSn4intermetallic flakes
after aging would continue to remain in the solder matrix. The discrepancy might be attributed to the peening effect of Ag3Sn
precipitates in the Sn-3.5Ag solder matrix on the AuSn4
intermetallic flakes (Ref 11).
In this present study, the morphology and compositions of various intermetallic phases formed in a Sn-58Bi BGA package with Au/Ni surface finish after reflow and aging processes are
C.C. Chi, L.C. Tsao, C.W. Tsao, and T.H. Chuang, Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan. Contact e-mail: tunghan@ntu.edu.tw.
JMEPEG (2008) 17:134–140 ASM International
identified. The growth kinetics of intermetallics formed at solder/pad interfaces are analyzed. Finally, the bonding strengths of the solder joints are measured via ball shear testing.
2. Experimental
The Sn-58Bi solder BGA package used in this study contained a Si dummy die on a resin substrate, encapsulated with molding compound. Each substrate was fitted with 49 (7· 7) Cu pads which were electroplated with 5 lm thick Ni, and immersion coated with 0.5 lm thick Au. The eutectic point (Te) of Sn-58Bi as measured by a differential scanning
calorimeter (DSC) was 138C. The Sn-58Bi (wt.%) solder balls of 0.4 mm in diameter were dipped in rosin mildly activated (RMA) flux, placed on the Au/Ni surface finished Cu pads, and then reflowed in a hot-air furnace. The reflow furnace was installed with five heating zones. Temperature profiles with various peak temperatures (Tmax) and melting times (Dtm,
T > Te) as shown in Fig.1 were employed for the soldering
process. In addition, a certain number of specimens after reflow at a peak temperature 180C for a melting time of 80 s were
further aged at 75, 100, 115, and 125C for various times ranging from 100 to 1000 h.
The reflowed and aged Sn-58Bi BGA packages were cross-sectioned through a row of solder balls, ground with 2000 grit SiC paper and polished with 0.3 lm Al2O3 powder. The
microstructure of solder joints was observed using a scanning electron microscope (SEM). The chemical compositions of
Fig. 1 Temperature profile for the reflow process of Sn-58Bi solder BGA package in this study (Tmax= peak temperature, Dtm= melting time for T > Te, Te: eutectic point)
Fig. 2 Typical morphology of a solder ball in Sn-58Bi BGA pack-age after reflow
Fig. 3 Microstructure of the Sn-58Bi solder joints reflowed at vari-ous peak temperatures for a melting time of 60 s: (a) 160C, (b) 170C, (c) 180 C, and (d) 190 C
various intermetallic phases were analyzed via an energy dispersive x-ray spectrometer (EDX) installed in the SEM. The bonding strengths of the solder joints were measured by ball shear tests, for which the shear rate and shear height were set at 0.1 mm/s and 80 lm, respectively.
3. Results and Discussion
The typical morphology of Sn-58Bi solder balls reflowed on Au/Ni/Cu pads in BGA packages is shown inFig.2. The solder ball contains a eutectic lamellar microstructure of pure Bi phase (in white) and b-Sn solid solution (in black). In addition, some flake-shaped intermetallics in the color of gray appear in the solder matrix. EDX analysis indicates that the chemical composition (at.%) of these gray intermetallic flakes is Au:Ni:Sn:Bi = 13.05:6.84:65.63:14.48, which corresponds to the (Au0.66Ni0.34)(Sn0.82Bi0.18)4 phase (IM1). Metallographic
observations of a higher magnification inFig.3 reveal that the distribution of these flake-shaped IM1 intermetallic compounds is dependent on the reflow conditions. On the reflow profile having lower peak temperatures (Tmax), the (Au
0.66-Ni0.34)(Sn0.82Bi0.18)4 intermetallic flakes tend to position
ver-tically at the solder/pad interface (Fig.3a). However, the increase in the peak temperature causes them to form in the solder matrix (Fig.3b-d). These intermetallic flakes also tend to appear at a distance away from the solder/pad interface on the reflow profile having longer melting time (Fig.4 and 5). In this case, they will either stay in the interior of the solder matrix or move farther to the outer surface of the solder ball (Fig.5). Together with the migration of intermetallic compounds, the intermetallic flakes also coarsen into thicker plates (Fig.5) or even cubic blocks (Fig.5). It has been mentioned that the Au thin film on Au/Ni/Cu pads dissolves quickly at the onset of the reflow process. Subsequently, the exposed Ni layer will also dissolve into the liquid solder. However, the dissolution rate of Ni is much slower than that of the Au film (Ref 12). The dissolved Au and Ni atoms in liquid Sn-58Bi solder react predominantly with the Sn element to form (Au
0.66-Ni0.34)(Sn0.82Bi0.18)4intermetallic flakes (IM1) in between the
Bi lamellae. The increase in peak temperature (Tmax) or melting
time (Dtm) of the reflow profile causes these Au and Ni atoms to
diffuse to greater distances and precipitate farther away from
the solder/pad interface. It is the surface tension that causes them to coarsen into thicker plates or cubic blocks.
The specimens reflowed at the peak temperature of 180C for the melting time of 80 s have been further aged at temperatures ranging from 75 to 125C for various times from 100 to 700 h. It can be seen inFig.6 and 7 that a gray layer of intermetallic phase (IM2) appears at the solder/pad interface,
Fig. 4 Intermetallic flakes IM1 tend to form at a distance away from the solder/pad interface after reflowing at 160C for a longer melting time of 80 s
Fig. 5 Microstructure of the Sn-58Bi solder joints reflowed at the peak temperatures of 160C (a, b) and 190 C (c, d) for a longer melting time of 140 s
which grows with the increase of aging temperature and aging time. The chemical compositions of IM2 intermetallic com-pounds formed in the Sn-58Bi solder joints after aging under various conditions are listed in Table1. As such a gray interfacial intermetallic phase (IM2) grows, another thin intermetallic phase (IM3, in dark color) forms between IM2 layer and Ni/Cu pad. EDX analysis indicates that the gray and
dark intermetallic layers possess compositions of (Au
0.30-Ni0.70)(Sn0.90Bi0.10)4 and Ni3Sn4, respectively. It is evident
that the (Au0.66Ni0.34)(Sn0.82Bi0.18)4 intermetallic flakes (IM1)
in the solder matrix have migrated to the solder/pad interface, which is similar to the case reported for a Sn37Pb BGA package by Minor and Morris (Ref 9). FromTable1, in which the compositions of IM1 and IM3 are also included for
Fig. 6 Microstructure of the Sn-58Bi solder joints after aging at various temperatures for 100 h: (a) 75C, (b) 100 C, (c) 115 C, and (d) 125C
Fig. 7 Microstructure of the Sn-58Bi solder joints after aging at 100C for various times: (a) 100 h, (b) 300 h, (c) 500 h, and (d) 700 h
comparison, the interfacial intermetallic phase IM2 has higher Ni and Sn contents than the IM1 flakes in the solder matrix. It implies that the (Au0.66Ni0.34)(Sn0.82Bi0.18)4 intermetallics
(IM1) migrated toward the Ni layers on Cu pads have reacted simultaneously with the Ni atoms, causing its composition to change to (Au0.30Ni0.70)(Sn0.90Bi0.10)4. Further increasing the
aging time and temperature, the Sn atoms in the Sn-58Bi solder
matrix have diffused through the IM2 intermetallic layer to react with the Ni layer on Cu pads to form the Ni3Sn4 phase
(IM3).
The growth thicknesses (X) of IM2 and IM3 intermetallic layers formed at the solder/pad interfaces after aging at various temperatures are measured and presented as a function of the square root of aging time (t1/2) inFig.8. All plots show a linear relation, which indicate that the growth kinetics for both IM2 and IM3 intermetallic compounds is diffusion-controlled. The growth rate constants (K = X/t1/2) of both intermetallic layers are also calculated from Fig.8 and plotted in an Arrhenius diagram inFig.9. The slopes of both curves give the activation energies (Q) for the growth of IM2 and IM3 intermetallic compounds, which are 58.06 and 80.28 kJ/mol, respectively. Yeh and Huntington (Ref 13) reported that the activation energy for the lattice diffusion of Ni atoms in Sn is 54.2 kJ/mol. This value is quite consistent with the activation energy for the growth of IM2 intermetallics, which confirms the above inference that the diffusion of Ni atoms through the (Au0.66Ni0.34)(Sn0.82Bi0.18)4 phase causes the change of its
composition to (Au0.30Ni0.70)(Sn0.90Bi0.10)4. On the other hand,
the activation energy for the self-diffusion of Sn as measured by Lange and Hassner (Ref 14) is 93.8 kJ/mol, which is near the value for the intermetallic growth of the IM3 phase. It is also verified that the growth rate of Ni3Sn4intermetallic layer (IM3)
Table 1 Chemical Compositions (at.%) of Intermetallic Compounds Formed in Sn-58Bi Solder Joints After the Reflow and Aging Processes
IMC IM1
IM2
IM3 Process Reflow Aging 115 C, 100 h Aging 115 C, 300 h Aging 125 C, 100 h Aging 125 C, 300 h Aging
Au 13.05 7.45 6.21 4.34 5.75 0
Ni 6.84 10.61 15.58 15.39 13.64 42.89
Sn 65.63 73.36 70.61 73.47 72.71 57.11
Bi 14.48 8.58 7.60 6.80 7.89 0
IM1: (Au0.66Ni0.34)(Sn0.82Bi0.18)4gray intermetallic flakes in the solder matrix after reflowing; IM2: (Au0.30Ni0.70)(Sn0.90Bi0.10)4gray intermetallic layer in the solder/pad interface after aging; IM3: Ni3Sn4dark intermetallic layer in the solder/pad interface after aging
Fig. 8 Thickness (X) of the intermetallic layers formed at the sol-der/pad interfaces of Sn-58Bi BGA packages after aging at various temperatures versus the square root of time (t1/2): (a) IM2 (Au 0.30-Ni0.70)(Sn0.90Bi0.10)4and (b) IM3 Ni3Sn4
Fig. 9 Arrhenius plot of the reaction constants (K) for the growth of intermetallic layers formed at the solder/pad interface of Sn-58Bi BGA packages after the aging processes: IM2 (Au 0.30-Ni0.70)(Sn0.90Bi0.10)4; IM3 Ni3Sn4
is determined by the diffusion of Sn atoms from Sn-58Bi solder through the IM2 phase containing more than 70 at.% Sn as indicated inTable1. Ball shear strengths of the Sn-58Bi solder
joints after the aging processes are measured and demonstrated in Fig. 10. It can be seen that the bonding strength of the reflowed specimen (Tmax= 180C, Dtm= 80 s) is 9.1 N, Fig. 10 Ball shear strength of the Sn-58Bi solder joints after aging
at various temperatures from 75 to 125C (reflow condition: Tmax= 180C, Dtm= 80 s)
Fig. 11 Ball shear strengths (S) of the Sn-58Bi solder joints as a function of IM2 thicknesses (X) after various aging times and tem-peratures
Fig. 12 Typical fractrography of the aged Sn-58Bi solder joints after ball shear tests: (a, b) as reflow; (c, d) 115C, 100 h; and (e, f) 115 C, 300 h
which is higher than the value of a traditional Sn37Pb package (8.7 N) reflowed under the optimized condition (Tmax= 225
C, Dtm= 60 s) (Ref 15). The ball shear strength drops
drastically to values between 7.1 and 5.8 N after aging at temperatures between 75 and 125C for 100 h. Longer aging times (over 700 h) cause the ball shear strengths to decrease further to values between 6.4 and 4.6 N. In order to clarify the cause for the degradation of solder joints, the ball shear strengths (S) of Sn-58Bi BGA packages after aging at various temperatures and times are plotted versus the thickness (X) of IM2 intermetallic layers at the solder/pad interfaces. Figure 11 shows that a linear relation exists in this case: S = 7.13- 0.33X. It implies that the growth of IM2 intermetallic compounds leads to the embrittlement of interfaces between Sn-58Bi solder balls and Au/Ni/Cu pads. This result can be reconfirmed from the fractography of the aged Sn-58Bi solder joints after ball shear tests as shown inFig.12. It reveals that the fracture occurs to the reflowed Sn-58Bi BGA package through the solder ball with ductile dimple characteristics (Fig.12a, b). On the contrary, the aged specimens show brittle fracture after ball shear tests. Most areas on the fracture surface of aged specimens contain cleavage planes (Fig.12c-f). EDX analysis indicates that the composition is (Au0.30Ni0.70)(Sn0.90Bi0.10)4
for those fracture zones with cleavage, which corresponds to the interfacial IM2 intermetallic phase. However, many fine particles are also found in certain regions (the upper left region, see fractrography inFig.12d), which possess a composition of the IM3 phase (Ni3Sn4). The results indicate that the aged
Sn-58Bi solder joints fracture mainly along the IM2 intermetallic layer. The fact that there are less fracture paths along the IM3 layer than along the IM2 layer, implies that the latter is much more brittle than the former.
4. Conclusions
Sn-58Bi solder BGA packages with Au/Ni/Cu pads are reflowed at a peak temperatures of 180C for a melting time of 80 s results in a ball shear strength of 9.1 N. Fractography after ball shear testing reveals ductile fracture through the solder ball. After the reflow processes, a small number of flake-shaped (Au0.66Ni0.34)(Sn0.82Bi0.18)4 intermetallic compounds (IM1)
appear in the solder matrix. Further aging at temperatures between 75 and 125C leads to the migration of IM1 intermetallic flakes from the solder matrix to the solder/pad interface, as well as the change of its composition to the (Au0.30Ni0.70)(Sn0.90Bi0.10)4 phase (IM2). As the aging time
increases, an additional intermetallic phase (IM3) with the composition of Ni3Sn4begins to form at the interface between
IM2 and Ni layer on the Cu pad. Aging processes cause the bonding strength of the reflowed specimen to drop drastically from 9.1 N to lower values between 6.4 and 4.6 N. A linear relation between ball shear strengths (S) and IM2 thicknesses
(X) has been obtained: S = 7.13- 0.33X, which implies that the degradation of Sn-58Bi solder joints in this case attributed to the embrittlement of interface caused by the growth of IM2 intermetallic compounds. Fractography of the aged solder joints after ball shear tests reveals brittle cleavage fracture mainly along the IM2 intermetallic layer.
Acknowledgment
Sincere thanks go to National Science Council, Taiwan, for sponsoring this research under Grant No. NSC-93-2216-E002-024. References
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