Scallop-shaped Intermetallic Compound Research Articles

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(Sn0.87In0.13)5] + 0.17[Ni3(Sn0.87In0.13)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(Sn0.87In0.13)5 interfacial intermetallic scallops remain still on the side of substrate A while many Au(In0.91Sn0.09)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(In0.91Sn0.09)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(Sn0.87Zn0.17)5 intermetallic compounds, which prevailed over the gravitational effect so that no intermetallic sedimentation in the liquid solder would occur. The appearance of Au(In0.91Sn0.09)2 at the Ni/Sn-20In-0.8Cu interfaces was hindered by the preferential formation of Cu6(Sn0.87Zn0.17)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.

Aging treatment characteristics of solder bump joint for high reliability optical module

The joint strength and fracture surfaces of Sn–37 mass% Pb and Au stud bumps for photo diode packages after isothermal aging testing were studied experimentally. Al/Au stud bumps and Cu/Sn–37 mass% Pb solders were adopted, and aged for up to 900 h to analyze the effect of intermetallic compound (IMC). The joint strength decreased with aging time. The diffraction patterns of Cu 6Sn 5, scallop-shaped IMCs, and planar-shaped Cu 3Sn were characterized using transmission electron microscopy (TEM). The formation of Kirkendall voids and the growth of IMCs at the solder were found to be a possible mechanism for joint strength reduction.

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 (Cu0.70Ni0.28Au0.02)6Sn5 (IM1a) and (Cu0.76Ni0.24)6(Sn0.86In0.14)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 (Cu0.70Ni0.14Au0.16)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 (Ni0.85Cu0.15)(Sn0.71In0.29)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.

Microstructure and strength of bump joints in photodiode packages

The joint strength and fracture surfaces of Sn-Pb and Au stud bumps for photodiode packages after isothermal aging were studied experimentally. Aluminum/gold stud bumps and Cu/Sn-Pb solders were adopted and aged for up to 900 h to analyze the effect of intermetallic compound (IMC) formation. The joint strength decreased with aging time. The diffraction patterns of Cu6Sn5, scallop-shaped IMCs, and planar-shaped Cu3Sn were characterized by transmission electron microscopy (TEM). The IMCs between Au stud bumps and Al pads was identified as AlAu2. The formation of Kirkendall voids and the growth of IMCs at the solder joint were found to be a possible mechanism for joint strength reduction.

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(Sn0.78In0.22)5 precipitates (IM1) in the solder matrix, scallop-shaped intermetallic compounds (IM2) with a compositional mixture of Cu6(Sn0.87In0.13)5 and Ni3(Sn0.87In0.13)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 (Au0.80Cu0.20)(In0.33Sn0.67)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 (Ni0.91Cu0.09)3(Sn0.77In0.23)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.