Ag₃Sn plate formation in the solidification of near ternary eutectic Sn–Ag–Cu alloys

Abstract

Near-ternary eutectic Sn-Ag-Cu alloys are leading candidates for Pb-free solders. These alloys have three solid phases: ␤-Sn, Ag 3 Sn, and Cu 6 Sn 5 . Starting from the fully liquid state in solidifying near-eutectic Sn-Ag-Cu alloys, the equilibrium eutectic transformation is kinetically inhibited. The Ag 3 Sn phase nucleates with minimal undercooling, but the ␤-Sn phase requires a typical undercooling of 15 to 30°C for nucleation. Because of this disparity in the required undercooling for nucleation, large, platelike Ag 3 Sn structures can grow rapidly within the liquid phase, before the final solidification of the solder joints. At lower cooling rates, the large Ag 3 Sn plates can subtend the entire cross section of solder joints and can significantly influence the mechanical deformation behavior of the solder joints under thermomechanical fatigue conditions. In this paper, it is demonstrated that the Ag 3 Sn plate formation can be inhibited, an important factor in assuring the reliability of solder joints composed of these alloys.The electronics industry will make substantial progress toward a full transition to Pb-free soldering technology in the near future. At present the leading candidate alloys are near-ternary eutectic Sn-Ag-Cu alloys. The ternary-eutectic composition is now thought to be close to the composition Sn-3.5Ag-0.9Cu 1,2 with a melting point of 217°C. The near-eutectic, commercially available alloys are exemplified by the Sn-3.8Ag-0.7Cu and Sn-4.0Ag-0.5Cu compositions. The electronics industry has begun to study both the processing behaviors and the thermomechanical fatigue properties of these alloys in detail to understand their applicability in the context of current electronic card reliability requirements. 3 The near-eutectic ternary Sn-Ag-Cu alloys yield three phases upon solidification: ␤-Sn, Ag 3 Sn, and Cu 6 Sn 5 . Attempts to characterize the solidification behavior and define the ternary eutectic composition in the Sn-Ag-Cu system have been reported by several investigators. 1,2,4 It has been previously reported 5,6 that relatively large Ag 3 Sn plates can form during solidification of the nearternary eutectic Sn-Ag-Cu alloys. Using differential thermal analysis (DTA) methods, 1 it was reported that Ag 3 Sn plate nucleation and ensuing growth may occur with minimal undercooling. The ␤-Sn phase required significantly larger undercoolings to induce nucleation and bring about final solidification. 1,4 The physical requirement for large undercoolings to promote the solidification of Sn-bearing solders has been previously reported. 7,8 These findings on the nucleation behavior of Sn-based solder systems are similar to that of the present study. The results of our work are consistent with the nucleation of the Ag 3 Sn phase with minimal undercooling in the Sn-Ag-Cu system. Under manufacturing process conditions, solder joints, composed of Sn-3.8Ag-0.7Cu alloy, required 15 to 30°C undercooling for the nucleation of the ␤-Sn phase. This determination was made by implanting thermocouples in b...

The Failure Mechanism of Recrystallization‐Assisted Cracking of Solder Interconnections

“…It is particularly noteworthy that the Cu 6 Sn 5 or the Ag 3 Sn phase can nucleate with minimum undercooling in the liquid SnAgCu interconnections [40][41][42] but the nucleation of the tin-rich phase results in a significantly wide range of undercooling that can extend up to 60 ⁰C [43][44][45][46][47][48]. The large amounts of undercooling indicate apparent difficulties in the nucleation of tin crystals in the liquid, which can be one of the reasons why there are very often only few orientations of the Sn-rich phase observed on a cross-section of solder interconnections.…”

Measurement of Sn and In Solidification Undercooling and Lattice Expansion Using In Situ X-Ray Diffraction

“…Indium, on the other hand, displays very little undercooling and solidifies close to its equilibrium melting point on all of the surfaces studied. Reducing the amount of undercooling of Sn and Sn-based solder alloys prior to solidification is an area of great interest to improve mechanical properties of lead-free joints, [4][5][6][7][8][9][10][11][12][13][14] and future work is planned to use the in situ diffraction experiments developed here to investigate the influence of various inoculants on reducing undercooling in Sn and Sn-based solder alloys.…”

The Microstructure, Thermal Fatigue, and Failure Analysis of Near-Ternary Eutectic Sn-Ag-Cu Solder Joints

“…4,5) As a consequence of this disparity in the required undercooling for nucleation, large, plate-like Ag 3 Sn structures can grow rapidly within the liquid phase, during cooling, before the final solidification of solder joints, as reported previously. [6][7][8] In addition, when large Ag 3 Sn plates are present, adverse effects on the plastic deformation properties of the solder, 7) and plastic strain localization at the boundary between the Ag 3 Sn plates and bounding -Sn phase were observed. 6) In a study involving the thermo-mechanical fatigue testing of CBGA (ceramic ball grid array) solder joints, 8) strain localization was noted to occur at the boundary between the Ag 3 Sn plates and the -Sn phase, as well as preferred crack growth along the -Sn/Ag 3 Sn interface.…”

“…[6][7][8] In addition, when large Ag 3 Sn plates are present, adverse effects on the plastic deformation properties of the solder, 7) and plastic strain localization at the boundary between the Ag 3 Sn plates and bounding -Sn phase were observed. 6) In a study involving the thermo-mechanical fatigue testing of CBGA (ceramic ball grid array) solder joints, 8) strain localization was noted to occur at the boundary between the Ag 3 Sn plates and the -Sn phase, as well as preferred crack growth along the -Sn/Ag 3 Sn interface. This study also described the effective control of large Ag 3 Sn plate formation by increasing the cooling rate and/or reducing the Ag content in Sn-Ag-Cu alloys.…”

“…The growth of large Ag 3 Sn plates in Sn-Ag-Cu alloys has been extensively studied in terms of cooling rate, [8][9][10] alloy content, 8,9) and minor alloying elements. 11,12) The cooling rate during the reflow of Sn-Ag-Cu alloys was found to be a critical factor in controlling the formation of large Ag 3 Sn plates in SAC joints.…”