Ying-Ta Chiu

Alloying modification of Sn–Ag–Cu solders by manganese and titanium

Abstract Effects of Mn and Ti additives on the microstructure and solidification behavior of Sn–1.0Ag–0.5Cu alloys (SAC105), as well as mechanical properties, were investigated in this study. Results show that alloying of Mn and Ti resulted in dramatically reduced undercooling, coarse eutectic structure and extended volume fraction of proeutectic Sn of which the dendritic size was refined. Those thermal and microstructural changes might be ascribed to the formation of MnSn 2 and Ti 2 Sn 3 intermetallic compounds (IMCs) which appeared respectively in the Mn-doped and Ti-doped samples. Nanoindentation tests demonstrated that the heterogeneous IMCs produced by alloying were harder and stiffer than the inherent IMCs, Ag 3 Sn and Cu 6 Sn 5 . Worthy of notice is that the elastic modulus of SAC105 alloys decreased with only a minor alloying addition due to the shrunken eutectic regions and coarsened eutectic microconstituents. With a larger quantity of additives, an ascending elastic modulus could be obtained because of the strengthening effect by hard heterogeneous compounds.

Ball Impact Reliability of Zn-Sn High-Temperature Solder Joints Bonded with Different Substrates

In this study, the high-speed deformation behavior of solder joints formed with Pb-free Zn-Sn and commercial Pb-Sn alloys bonded on different substrates was investigated by the ball impact test method. Overall, Zn-Sn joints exhibited greater impact strength but inferior impact toughness than Pb-Sn joints. This can be ascribed to the high hardness of Zn-Sn solders resulting in partial or overall interfacial fracture. In contrast, the joints with soft Pb-Sn solders all showed a ductile fracture feature. It is suggested that, for the joints revealing brittle fracture, the impact toughness (impact energy) increased with the plastic ability of interfacial intermetallic compounds, while for those showing a ductile fracture mode, the impact energy deteriorated with a hardened solder matrix resulting from substrate dissolution.

Time-dependent deformation behavior of interfacial intermetallic compound layers in electronic solder joints

This study evaluated room temperature creep properties of the intermetallic compounds (IMCs) formed at the interfaces between Pb-free solders (Sn-Ag-Cu and Sn-Zn) and the commonly used electronic substrates (Cu, Ni and Ag) using nanoindentation. The creep stress exponent (CSE), representing creep resistance, in the decreasing order was Cu, Ni and then Ag based IMCs, which was in good agreement with tendencies of the work hardening exponent (n) and the ratio of yield stress (Y) to Youngs modulus (E) as well.

Alloying design of Sn-Ag-Cu solders for the improvement in drop test performance

In addition to a reduced Ag content, it has been demonstrated that significant improvement of drop test performance of Sn-Ag-Cu solder joints can be achieved by alloying with Mn and Ti. This study aims to investigate the effects of Mn and Ti additives on the microstructure and solidification behavior of Sn-1.0Ag-0.5Cu alloys, as well as mechanical properties and thus to explain how the alloying elements affect drop test results. Results show that alloying of Mn and Ti results in coarse eutectic structure and greater amount of pro eutectic Sn of which the size was refined. The microstructural changes leads to a reduction in elastic modulus, which plays an important role for the enhancement of drop impact reliability. However, there exists an optimal value for the alloying content, since excess addition of Mn or Ti gives rise to the formation of massive intermetallic compounds (IMCs), MnSn2 and Ti2Sn3. Those heterogeneous IMCs are harder than the inherent IMCs, Ag3Sn and Cu6Sn5, according to the nanoindentation results and may cause the degradation in drop performance.

Effect of transition metals on the interfacial reactions in electroless Ni(P)-solder interconnections

The effect of transition metal (TM) additive of Ni, Co or Zn on the interfacial reactions of the solder joints between Sn-Ag-Cu (SAC) solder and the Cu/Ni(P)/Au substrate was investigated, especially when subsequent to multi-reflowing. (Cu,Ni)6Sn5 formed at the interface of all the joints but the SAC-Ni of which the interfacial compound was (Ni,Cu)6Sn5. The interfacial compounds of the SAC-Co and SAC-Zn contained a small amount of alloying elements of less than 3at%. Two P-rich layers, Ni3P and Ni-Sn-P were found have emerged at the interface of the SAC joints. Nanoindentation analysis indicates that the hardness and Youngs modulus of these two phases were slightly higher than Ni(P) substrate, which were in turn much greater than Cu-Ni-Sn compounds. Worthy of notice is that with TM additions the Ni-Sn-P phase between Ni3P and interfacial compounds was absent even after 10-time reflows. For the SAC-Co joints, the growth of Ni-containing intermetallic compounds (IMCs) within the solder gave rise to the excess Ni dissolution which caused a discrete Ni3P layer and over-consumed Ni(P) substrate underneath the grooves in-between (Cu, Ni)6Sn5 scallop grains at the interface.

Metallurgical Perspective on Alloying Modification of Sn-Ag-Cu Solders

This study investigates the effect of alloying additives on the solidification behavior and microstructural characteristics from a metallurgical perspective. Results show that alloying of Mn and Ti resulted in dramatically reduced undercooling coarse eutectic structure and extended volume fraction of proeutectic Sn of which the dendritic size was refined. Those thermal and microstructural changes might be ascribed to the formation of MnSn2 and Ti2Sn3 intermetallic compounds (IMCs) appeared respectively in the Mn-doped and Ti-doped samples. Instead of forming heterogeneous IMCs, adding of the low temperature solute elements, Bi and Ga, decreases the melting point, extends the solidus/liquidus range and causes a mixed normal-coarse structure with a varying solute concentration. Unlike Bi which is only detected in the Sn matrix, Ga also dissolves in eutectic Ag3Sn and even transforms it into a new intermetallic phase, Ag72Ga28 within the coarse eutectic cells. It is believed that this could give rise to a specific two-stage nonequilibrium eutectic solidification feature.

Impact test performance of Zn-based die-attach joints for power devices

Fracture behavior under high speed deformation of the high temperature solder joints with Pb-free Zn-Sn and commercial Pb-Sn alloys bonded on different surface finishes was studied by ball impact test (BIT) method. All in all, Zn-Sn joints exhibited greater impact strength but inferior impact toughness than Pb-Sn joints. This can be ascribed to the high hardness of Zn-Sn solders resulting in partial or overall interfacial fracturing. In contrast, the joints with soft Pb-Sn solders all showed a ductile fracture feature. It can be suggested that for the joints revealing brittle fracturing the impact toughness increased with the plastic ability of interfacial intermetallic compounds (IMCs), while those showing ductile fracture mode, the impact energy deteriorated with a hardened solder matrix resulting from substrate dissolution.

Time dependent deformation behavior of interfacial intermetallic compounds in electronic solder joints

This study evaluated room temperature creep properties of the intermetallic compounds (IMCs) formed at the interfaces between Pb-free solders (Sn-Ag-Cu and Sn-Zn) and the commonly used electronic substrates (Cu, Ni and Ag) using nanoindentation. The creep stress exponent (CSE), representing creep resistance, in the decreasing order was Cu, Ni and then Ag based IMCs, which was in good agreement with tendencies of the work hardening exponent (n) and the ratio of yield stress (Y) to Youngs modulus (E) as well.

Size and substrate effects on microstructure and shear properties of solder joints

Considering that the mechanical behavior plays a very important role in the packaging reliability, this study examined the shear deformation properties of pure Sn joints with different substrate materials and various gap sizes. The proeutectic Sn dendrites were refined and the shear strength was increased with a shrunken sample gap. The interfacial intermetallic compounds (IMCs) were thickened slightly when the sample gap was less than 100 mum. A decrease in joint thickness transformed the fracture paths of the Cu/Sn/Cu samples from solder bulk to the boundary between interfacial IMC and the solder. As for the Cu/Sn/Ni sandwich structured joints, they exhibited an asymmetrical structure which resulted from the interaction of substrate elements and thus inferior shear strength compared to the Cu/Sn/Cu samples with the same thickness, as well as the difference in fracture mode.

Ball impact responses of Sn-Ag-Cu solder joints doped with Ni or Ge

In this work, we present ball impact test (BIT) results conducted at an impact velocity of 500 mm/s on Sn-4Ag-0.5Cu, Sn-1Ag-0.5Cu, Sn-1Ag-0.5Cu-0.05Ni, Sn-1.2Ag-0.5Cu-0.05Ni, and Sn-1Ag-0.5Cu- 0.05Ge package-level solder joints, bonded on substrate pads of immersion tin (IT) and direct solder on pad (DSOP) surface finishes. Differences of BIT results with respect to multi-reflow are also reported.