Intermetallic reactions in reflowed and aged Sn-9Zn solder ball grid array packages with Au/Ni/Cu and Ag/Cu pads

During the reflowing of Sn-9Zn solder ball grid array (BGA) packages with Au/Ni/Cu and Ag/Cu pads, the surface-finished Au and Ag film dissolved rapidly and reacted with the Sn-9Zn solder to form a γ₃-AuZn₄/γ-Au₇Zn₁₈ intermetallic double layer and ε-AgZn₆ intermetallic scallops, respectively. The growth of γ₃-AuZn₄ is prompted by further aging at 100°C through the reaction of γ-Au₇Zn₁₈ with the Zn atoms dissolved from the Zn-rich precipitates embedded in the β-Sn matrix of Sn-9Zn solder BGA with Au/Ni/Cu pads. No intermetallic compounds can be observed at the solder/pad interface of the Sn-9Zn BGA specimens aged at 100°C. However, after aging at 150°C, a Ni₄Zn₂₁ intermetallic layer is formed at the interface between Sn-9Zn solder and Ni/Cu pads. Aging the immersion Ag packages at 100°C and 150°C caused a γ-Cu₅Zn₈ intermetallic layer to appear between ε-AgZn₆ intermetallics and the Cu pad. The scallop-shaped ε-AgZn₆ intermetallics were found to detach from the γ-Cu₅Zn₈ layer and float into the solder ball. Accompanied with the intermetallic reactions during the aging process of reflowed Sn-9Zn solder BGA packages with Au/Ni/Cu and Ag/Cu pads, their ball shear strengths degrade from 8.6 N and 4.8 N to about 7.2 N and 2.9 N, respectively.     

Intermetallic reactions in Sn-3.5Ag solder ball grid array packages with Ag/Cu and Au/Ni/Cu pads

During the reflow process of Sn-3.5Ag solder ball grid array (BGA) packages with Ag/Cu and Au/Ni/Cu pads, Ag and Au thin films dissolve rapidly into the liquid solder, and the Cu and Ni layers react with the Sn-3.5Ag solder to form Cu₆Sn₅ and Ni₃Sn₄ intermetallic compounds at the solder/pad interfaces, respectively. The Cu₆Sn₅ intermetallic compounds also appear as clusters in the solder matrix of Ag surface-finished packages accompanied by Ag₃Sn dispersions. In the solder matrix of Au/Ni surface-finished specimens, Ag₃Sn and AuSn₄ intermetallics can be observed, and their coarsening coincides progressively with the aging process. The interfacial Cu₆Sn₅ and Ni₃Sn₄ intermetallic layers grow by a diffusion-controlled mechanism after aging at 100 and 150°C. Ball shear strengths of the reflowed Sn-3.5Ag packages with both surface finishes are similar, displaying the same degradation tendencies as a result of the aging effect.     

Intermetallic formation in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with immersion Ag surface finish

The intermetallic compounds formed in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Ag/Cu pads are investigated. After reflow, scallop-shaped η-Cu₆Sn₅ and continuous planar η-(cu_0.9Ni_0.1)₆Sn₅ intermetallics appear at the interfaces of the Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder joints, respectively. In the case of the Sn3Ag0.5Cu specimens, an additional ε-Cu₃Sn intermetallic layer is formed at the interface between the η-Cu₆Sn₅ and Cu pads after aging at 150°C, while the same type of intermetallic formation is inhibited in the Sn3Ag0.5Cu0.06Ni0.01Ge packages. In addition, the coarsening of Ag₃Sn precipitates also abates in the solder matrix of the Sn3Ag0.5Cu0.06Ni0.01Ge packages, which results in a slightly higher ball shear strength for the specimens.     

Intermetallic compounds formed during the reflow and aging of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder ball grid array packages

After reflow of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder balls on Au/Ni surface finishes in ball grid array (BGA) packages, scallop-shaped intermetallic compounds (Cu_0.70Ni_0.28Au_0.02)₆Sn₅ (IM1a) and (Cu_0.76Ni_0.24)₆(Sn_0.86In_0.14)₅ (IM1b), respectively, appear at the interfaces. Aging at 100°C and 150°C for Sn-3.8Ag-0.7Cu results in the formation of a new intermetallic phase (Cu_0.70Ni_0.14Au_0.16)₆Sn₅ (IM2a) ahead of the former IM1a intermetallics. The growth of the newly appeared intermetallic compound, IM2a, is governed by a parabolic relation with an increase in aging time, with a slight diminution of the former IM1a intermetallics. After prolonged aging at 150°C, the IM2a intermetallics partially spall off and float into the solder matrix. Throughout the aging of Sn-20In-2Ag-.5Cu solder joints at 75°C and 115°C, partial spalling of the IM1b interfacial intermetallics induces a very slow increase in thickness. During aging at 115°C for 700 h through 1,000 h, the spalled IM1b intermetallics in the solder matrix migrate back to the interfaces and join with the IM1b interfacial intermetallics to react with the Ni layers of the Au/Ni surface finishes, resulting in the formation and rapid growth of a new (Ni_0.85Cu_0.15)(Sn_0.71In_0.29)₂ intermetallic layer (IM2b). From ball shear tests, the strengths of the Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder joints after reflow are ascertained to be 10.4 N and 5.4 N, respectively, which drop to lower values after aging. An erratum to this article is available at .     

Intermetallic reactions in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes

The intermetallic compounds (IMCs) formed during the reflow and aging of Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes were investigated. After reflow, the thickness of (Cu, Ni, Au)₆Sn₅ interfacial IMCs in Sn3Ag0.5Cu0.06Ni0.01Ge was similar to that in the Sn3Ag0.5Cu specimen. The interiors of the solder balls in both packages contained Ag₃Sn precipitates and brick-shaped AuSn₄ IMCs. After aging at 150°C, the growth thickness of the interfacial (Ni, Cu, Au)₃Sn₄ intermetallic layers and the consumption of the Ni surface-finished layer on Cu the pads in Sn3Ag0.5Cu0.06Ni0.01Ge solder joints were both slightly less than those in Sn3Ag0.5Cu. In addition, a coarsening phenomenon for AuSn₄ IMCs could be observed in the solder matrix of Sn3Ag0.5Cu, yet this phenomenon did not occur in the case of Sn3Ag0.5Cu0.06Ni0.01Ge. Ball shear tests revealed that the reflowed Sn3Ag0.5Cu0.06Ni0.01Ge packages possessed bonding strengths similar to those of the Sn3Ag0.5Cu. However, aging treatment caused the ball shear strength in the Sn3Ag0.5Cu packages to degrade more than that in the Sn3Ag0.5Cu0.06Ni0.01Ge packages.     

Intermetallic Reactions in Sn-0.4Co-0.7Cu Solder BGA Packages with an ENIG Surface Finish

As-cast Sn-0.4Co-0.7Cu solder contains both (Cu_0.98Co_0.02)₆Sn₅ and (Co_0.85Cu_0.15) Sn₃ intermetallic phases in the matrix. After reflowing, the Au thin film in the electroless Ni/immersion Au (ENIG) surface-finished Sn-0.4Co-0.7Cu solder ball grid array (BGA) packages dissolved rapidly into the solder matrix to form AuSn₄ intermetallics, and a thin layer of (Cu_0.57Ni_0.35Au_0.08)₆Sn₅ intermetallic compound appeared at the solder/pad interface, growing very slowly during aging at 100°C. Increasing the aging temperature to 150°C caused the formation of a new intermetallic layer, (Ni_0.79Cu_0.21)₃Sn₄, at the (Cu_0.57Ni_0.35Au_0.08)₆Sn₅/Ni interface. The reflowed Sn-0.4Co-0.7Cu BGA packages have a ball shear strength of 6.8 N, which decreases to about 5.7 N and 5.5 N after aging at 100°C and 150°C, respectively. The reflowed and aged solder joints fractured across the solder balls with ductile characteristics in ball shear tests.     

Microstructure characterization of Sn-Ag solder joints between stud bumps and metal pads

The microstructure of the flip-chip solder joints fabricated using stud bumps and Pb-free solder was characterized. The Au or Cu stud bumps formed on Al pads on Si die were aligned to corresponding metal pads in the substrate, which was printed with Sn-3.5Ag paste. Joints were fabricated by reflowing the solder paste. In the solder joints fabricated using Au stud bumps, Au-Sn intermetallics spread over the whole joints, and the solder remained randomly island-shaped. The δ-AuSn, ε-AuSn₂, and η-AuSn₄ intermetallic compounds formed sequentially from the Au stud bump. The microstructure of the solder joints did not change significantly even after multiple reflows. The AuSn₄ was the main phase after reflow because of the fast dissolution of Au. In the solder joints fabricated using Cu stud bumps, the scallop-type Cu₆Sn₅ intermetallic was formed only at the Cu interface, and the solder was the main phase. The difference in the microstructure of the solder joints with Au and Cu stud bumps resulted from the dissolution-rate difference of Au and Cu into the solder.     

Selective formation of intermetallic compounds in Sn-20In-0.8Cu ball grid array solder joints with Au/Ni surface finishes

After Sn-20In-0.8Cu solder balls are reflowed on a ball grid array (BGA) substrate (substrate A) with an Au/Ni surface finish, scallop-shaped intermetallic compounds with a composition of 0.83[Cu₆(Sn_0.87In_0.13)₅] + 0.17[Ni₃(Sn_0.87In_0.13)₄] are formed at the solder/pad interface. The distribution of the intermetallics is not altered by gravity or by multiple reflows of the solder joints. As another substrate (substrate B) is further attached onto the primary reflowed BGA assembly to form a sandwich structure subjected to subsequent multiple reflows, the Cu₆(Sn_0.87In_0.13)₅ interfacial intermetallic scallops remain still on the side of substrate A while many Au(In_0.91Sn_0.09)₂ intermetallics of cubic shape appear near the solder/Ni interface on the side of substrate B. When the Sn-20In-0.8Cu solder balls are assembled simultaneously in between two substrates (A and B), Au(In_0.91Sn_0.09)₂ intermetallic cubes of equal proportion are observed to form on both sides of the assembly. In summarizing the results, it is proposed that the diffusion of Cu atoms in the Sn-20In-0.8Cu solder toward the Ni layers after Au thin-film dissolution on Au/Ni surface finishes led to the formation of Cu₆(Sn_0.87Zn_0.17)₅ intermetallic compounds, which prevailed over the gravitational effect so that no intermetallic sedimentation in the liquid solder would occur. The appearance of Au(In_0.91Sn_0.09)₂ at the Ni/Sn-20In-0.8Cu interfaces was hindered by the preferential formation of Cu₆(Sn_0.87Zn_0.17)₅ until the Cu atoms in the Sn-20In-0.8Cu solder matrix were consumed to a lower content via the attachment of a second substrate to the assembly.     

Reflow and burn-in of a Sn-20In-0.8Cu ball grid array package with a Au/Ni/Cu pad

The intermetallic compounds formed after reflow and burn-in testing of a Sn-20In-0.8Cu solder ball grid array (BGA) package are investigated. Along with the formation of the Cu₆(Sn_0.78In_0.22)₅ precipitates (IM1) in the solder matrix, scallop-shaped intermetallic compounds (IM2) with a compositional mixture of Cu₆(Sn_0.87In_0.13)₅ and Ni₃(Sn_0.87In_0.13)₄ appear at the interfaces between the solder balls and Au/Ni/Cu pads. A significant number of intermetallic particles (IM3), with a composition of (Au_0.80Cu_0.20)(In_0.33Sn_0.67)₂, can also be found in the solder matrix. After aging at 115°C for 750 h, an additional intermetallic compound layer (IM4) with a composition of (Ni_0.91Cu_0.09)₃(Sn_0.77In_0.23)₂ is formed at the interface between IM2 and the Ni layer. The ball shear strength of the Sn-20In-0.8Cu BGA solder after reflow is 4.5 N and will rise to maximum values after aging at 75°C and 115°C for 100 h. With a further increase of the aging time at both temperatures, the joint strengths exhibit a tendency to decline linearly at about 1.7×10⁻³ N/h.     

Growth of intermetallic compounds in the Sn-9Zn/Cu joint

We have studied the microstructure of the Sn-9Zn/Cu joint in soldering at temperatures ranging from 230°C to 270°C to understand the growth of the mechanism of intermetallic compound (IMC) formation. At the interface between the Sn-9Zn solder and Cu, the results show a scallop-type ε-CuZn₄ and a layer-type γ-Cu₅Zn₈, which grow at the interface between the Sn-9Zn solder and Cu. The activation energy of scallop-type ε-CuZn₄ is 31 kJ/mol, and the growth is controlled by ripening. The activation energy of layer-type γ-Cu₅Zn₈ is 26 kJ/mol, and the growth is controlled by the diffusion of Cu and Zn. Furthermore, in the molten Sn-9Zn solder, the results show η-CuZn grains formed in the molten Sn-9Zn solder at 230°C. When the soldering temperature increases to 250°C and 270°C, the phase of IMCs is ε-CuZn₄.     

Intermetallic compounds formed during the reflow of In-49Sn solder ball-grid array packages

The intermetallic compounds formed at the interfaces between In-49Sn solder balls and Au/Ni/Cu pads during the reflow of In-49Sn solder, ball-grid array (BGA) packages are investigated. Various temperature profiles with peak temperatures ranging from 140°C to 220°C and melting times ranging from 45 sec to 170 sec are plotted for the reflow processes. At peak temperatures below 170°C, a continuous double layer of intermetallics can be observed, showing a composition of Au(In,Ni)₂/Au(In,Ni). Through selective etching of the In-49Sn solders, the intermetallic layer is made up of irregular coarse grains. In contrast, a number of cubic-shaped AuIn₂ intermetallic compounds appear at the interfaces and migrate toward the upper domes of In-49Sn solder balls after reflow at peak temperatures above 200°C for longer melting times. The upward floating of the AuIn₂ cubes can be explained by a thermomigration effect caused by the temperature gradient present in the liquid solder ball. The intermetallic compounds formed under various reflow conditions in this study exhibit different types of morphology, yet the ball shear strengths of the solder joints in the In-49Sn BGA packages remain unaffected.     

Interfacial Reactions of Sn-8wt.%Zn-3wt.%Bi Solder with Cu,Ag, and Ni Substrates

Sn-Zn-Bi alloys are promising Pb-free solders. Interfacial reactions between the Sn-8wt.%Zn-3wt.%Bi (Sn-13.80at.%Zn-1.62at.%Bi) alloy and the Cu, Ag, and Ni substrates are examined. Two different kinds of substrates, the bulk plate and the electroplating layer, are used, and the reactions are carried out at 250°C and 220°C. Although the Zn content is only 13.8 at.%, two Zn-Cu compounds, γ-Cu₅Zn₈ and ε-CuZn₅ phases, are formed in the Sn-13.80at.%Zn-1.62at.%Bi/Cu couples. The ε-CuZn₅ phase is scallop shaped, and the γ-Cu₅Zn₈ phase is planar. In the Sn-13.80at.%Zn-1.62at.%Bi/Ag couples, three Zn-Ag compounds are observed, and they are ε-AgZn₃, γ-Ag₅Zn₈, and ζ-AgZn phases. In the Sn-13.80at.%Zn-1.62at.%Bi/Ni couples, a Zn-Ni compound, γ-Ni₅Zn₂₁ phase, is formed. Similar results are found in the couples prepared with an electroplating layer: the reaction phases are the same, but the growth rates are different.     

Intermetallic reactions in a Sn-20In-2.8Ag solder ball-grid-array package with Au/Ni/Cu pads

The intermetallic compounds formed during the reflow and aging of Sn-20In-2.8Ag ball-grid-array (BGA) packages are investigated. After reflow, a large number of cubic-shaped AuIn₂ intermetallics accompanied by Ag₂In precipitates appear in the solder matrix, while a Ni(Sn_0.72Ni_0.28)₂ intermetallic layer is formed at the solder/pad interface. With further aging at 100°C, many voids can be observed in the solder matrix and at the solder/pad interface. The continuous distribution of voids at the interface of specimens after prolonged aging at 100°C causes their bonding strength to decrease from 5.03 N (as reflowed) to about 3.50 N. Aging at 150°C induces many column-shaped (Cu_0.74Ni_0.26)₆(Sn_0.92In_0.08)₅ intermetallic compounds to grow rapidly and expand from the solder/pad interface into the solder matrix. The high microhardness of these intermetallic columns causes the bonding strength of the Sn-20In-2.8Ag BGA solder joints to increase to 5.68 N after aging at 150°C for 500 h.     

Effect of reaction time on mechanical strength of the interface formed between the Sn-Zn(-Bi) solder and the Au/Ni/Cu bond pad

In this work, the shear strengths and the interfacial reactions of Sn-9Zn, Sn-8Zn-1Bi, and Sn-8Zn-3Bi (wt.%) solders with Au/Ni/Cu ball grid array (BGA) pad metallization were systematically investigated after extended reflows. Zn-containing Pb-free solder alloys were kept in molten condition (240°C) on the Au/electrolytic Ni/Cu bond pads for different time periods ranging from 1 min. to 60 min. to render the ultimate interfacial reaction and to observe the consecutive shear strength. After the shear test, fracture surfaces were investigated by scanning electron microscopy equipped with an energy dispersive x-ray spectrometer. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. The solder ball shear load for all the solders during extended reflow increased with the increase of reflow time up to a certain stage and then decreased. It was found that the formation of thick Ni-Zn intermetallic compound (IMC) layers at the solder interface of the Au/electrolytic Ni/Cu bond pad with Sn-Zn(-Bi) alloys deteriorated the mechanical strength of the joints. It was also noticed that the Ni-Zn IMC layer was larger in the Sn-Zn solder system than that in the other two Bi-containing solder systems.     

Interfacial microstructure and bonding strength of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder BGA packages with immersion Ag surface finish

Although it has been verified that tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn-3Ag-0.5Cu-0.5Ce solder, no detailed studies have been conducted on interfacial reactions and mechanical properties of Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with an immersion Ag surface finish. The intermetallic compounds formed during the reflow and aging of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder ball grid array (BGA) packages were investigated. Because more heterogeneous nucleation sites, provided by CeSn₃ intermetallics and Zn atoms, formed in the Sn-3Ag-0.5Cu-0.5Ce-xZn solder matrix, and Cu and Zn have a stronger affinity than Cu and Sn, the Cu-Sn intermetallics growth in Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with Ag/Cu pads was suppressed. The 0.2% Zn addition for inhibiting rapid whisker growth in RE-doped Sn-Ag-Cu solder joints is more appropriate than 0.5 wt.% additions, as excess Zn addition causes poor oxidation resistance and inferior bonding strength.     

Reliability investigation and interfacial reaction of ball-grid-array packages using the lead-free Sn-Cu solder

The interfacial reactions between two Sn-Cu (Sn-0.7Cu and Sn-3Cu, wt.%) ball-grid-array (BGA) solders and the Au/Ni/Cu substrate by solid-state isothermal aging were examined at temperatures between 70°C and 170°C for 0 to 100 days. For the Sn-0.7Cu solder, a (Cu,Ni)₆Sn₅ layer was observed in the samples aged at 70–150°C. After isothermal aging at 170°C for 50 days, the solder/Ni interface exhibited a duplex structure of (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄. For the Sn-3Cu solder, only the (Cu,Ni)₆Sn₅ layer was formed in all aged samples. Compared to these two Sn-Cu solders, the Cu content in the (Cu,Ni)₆Sn₅ layer formed at the interface increased with the Cu concentration in the Sn-xCu solders. And, the shear strength was measured to evaluate the effect of the interfacial reactions on the mechanical reliability as a function of aging conditions. The shear strength significantly decreased after aging for 1 day and then remained nearly unchanged by further prolonged aging. In all the samples, the fracture always occurred in the bulk solder. Also, we studied the electrical property of Cu/Sn-3Cu/Cu BGA packages with the number of reflows. The electrical resistivity increased with the number of reflows because of an increase of intermetallic compound (IMC) thickness.     

Effects of Ce Addition on the Microstructure and Mechanical Properties of Sn-58Bi Solder Joints

The effects of a rare-earth element on the microstructure, mechanical properties, and whisker growth of Sn-58Bi alloys and solder joints in ball grid array (BGA) packages with Ag/Cu pads have been investigated. Mechanical testing indicated that the elongation of Sn-58Bi alloys doped with Ce increased significantly, and the tensile strength decreased slightly, in compar- ison with undoped Sn-58Bi. In addition, the growth of both fiber- and hillock-shaped tin whiskers on the surface of Sn-58Bi-0.5Ce was retarded in the case of Sn-3Ag-0.5Cu-0.5Ce alloys. The growth of interfacial intermetallic compounds (IMC) in Sn-58Bi-0.5Ce solder joints was slower than that in Sn-58Bi because the activity of Ce atoms at the interface of the Cu₆Sn₅ IMC/solder was reduced. The reflowed Sn-58Bi and Sn-58Bi-0.5Ce BGA packages with Ag/Cu pads had a ball shear strength of 7.91 N and 7.64 N, which decreased to about 7.13 N and 6.87 N after aging at 100°C for 1000 h, respectively. The reflowed and aged solder joints fractured across the solder balls with ductile characteristics after ball shear tests.     

Mechanisms for interfacial reactions between liquid Sn-3.5Ag solders and cu substrates

Intermetallic compounds formed during the soldering reactions between Sn-3.5Ag and Cu at temperatures ranging from 250°C to 375°C are investigated. The results indicate that scallop-shaped η-Cu₆(Sn_0.933 Ag_0.007)₅ intermetallics grow from the Sn-3.5Ag/Cu interface toward the solder matrix accompanied by Cu dissolution. Following prolonged or higher temperature reactions, ɛ-Cu₃ (Sn_0.996 Ag_0.004) intermetallic layers appear behind the Cu₆(Sn_0.933 Ag_0.007)₅ scallops. The growth of these interfacial intermetallics is governed by a kinetic relation: ΔX=tⁿ, where the n values for η and ɛ intermetallics are 0.75 and 0.96, respectively. The mechanisms for such nonparabolic growth of interfacial intermetallics during the liquid/solid reactions between Sn-3.5Ag solders and Cu substrates are probed.     

The thermal property of lead-free Sn-8.55Zn-1Ag-XAl solder alloys and their wetting interaction with Cu

The wetting behaviors between the quaternary Sn-8.55Zn-1Ag-XAl solder alloys and Cu have been investigated with the wetting balance method. The Al contents, x, of the quaternary solder alloys investigated were 0.01–0.45 wt.%. The results of differential scanning calorimeter (DSC) analysis indicate that the solders exhibit a solid-liquid coexisting range of about 7–10°C. The solidus temperature of the quaternary Sn-8.55Zn-1Ag-XAl solder alloys is about 198.2°C, while the liquidus temperatures are 205–207°C. The experimental results showed that the wettability of the Sn-8.55Zn-1Ag-XAl solder alloys is improved by the addition of Al. The mean maximum wetting force of the solders with Cu is within 0.75–1.18 mN and the mean wetting time is around 1.0–1.1 sec, better than the ∼1.3 sec of eutectic Sn-9Zn and Sn-8.55Zn-1Ag solder alloys. The addition of Al also depresses the formation of ε-Ag-Zn compounds at the interface between Sn-8.55Zn-1Ag-XAl solders and copper.     

Effects of Ce and La Additions on the Microstructure and Mechanical Properties of Sn-9Zn Solder Joints

The effects of rare-earth elements on the microstructure and mechanical properties of Sn-9Zn alloys and solder joints in ball grid array packages with Ni/Au(ENIG) surface finishes have been investigated. Metallographic observations showed that (Ce_0.8Zn_0.2)Sn₃ and (La_0.9Zn_0.1)Sn₃ intermetallic compounds appeared in the solder matrix of Sn-9Zn-0.5Ce and Sn-9Zn-0.5La alloys, respectively. Both fiber- and hillock-shaped tin whiskers were inhibited in the Sn-9Zn-0.5Ce solder, while tin fibers were still observed on the surface of oxidized (La_0.9Zn_0.1)Sn₃ intermetallics in Sn-9Zn-0.5La after air exposure at room temperature. Mechanical testing indicated that the tensile strength of Sn-9Zn alloys doped with Ce and La increased significantly, and the elongation decreased, in comparison with the undoped Sn-9Zn. The bonding strengths of the as-reflowed Sn-9Zn-0.5Ce and Sn-9Zn-0.5La solder joints were also improved. However, aging treatment at 100°C and 150°C caused degradation of ball shear strength in all specimens. During the reflowing and aging processes, AuZn₈ intermetallic phases appeared at the interfaces of all solder joints. In addition, Zn-rich phases were observed to migrate from the solder matrix to the solder/pad interfaces of the aged specimens.