The interfacial reactions between Sn-based solders and two common substrate materials, Cu and Ni, are the focuses of this paper. The reactions between Sn-based solders and Cu have been studied for several decades, and currently there are still many un-resolved issues. The reactions between Sn-based solders and Ni are equally challenging. Recent studies further pointed out that Cu and Ni interacted strongly when they were both present in the same solder joint. While this cross-interaction introduces complications, it offers opportunities for designing better solder joints. In this study, the Ni effect on the reactions between solders and Cu is discussed first. The presence of Ni can in fact reduce the growth rate of Cu3Sn. Excessive Cu3Sn growth can lead to the formation of Kirkendall voids, which is a leading factor responsible for poor drop test performance. The Cu effect on the reactions between solders and Ni is then covered in detail. The knowledge gained from the Cu and Ni effects is applied to explain the recently discovered intermetallic massive spalling, a process that can severely weaken a solder joint. It is pointed out that the massive spalling was caused by the shifting of the equilibrium phase as more and more Cu was extracted out of the solder by the growing intermetallic. Lastly, the problems and opportunities brought on by the cross-interaction of Cu and Ni across a solder joint is presented.

Interfacial reaction issues for lead-free electronic solders

Composite lead-free solders, containing micro and nano particles, have been widely studied. Due to grain boundary drag or Zener drag, these particles can refrain the solder microstructure from coarsening in services, especially for Cu6Sn5, Ag3Sn intermetallic compounds and the β-Sn phases. Due to dispersion hardening or dislocation drag, the mechanical properties of the composite solder alloys were enhanced significantly. Moreover, these particles can influence the rate of interfacial reactions, and some particles can transform into a layer of intermetallic compound. Wettability, creep resistance, and hardness properties were affected by these particles. A systematic review of the development of these lead-free composite solders is given here, which hopefully may find applications in microbumps to be used in the future 3D IC technology.

Structure and properties of lead-free solders bearing micro and nano particles

Effect of thermal aging on the interfacial structure of SnAgCu solder joints on Cu.

High strength and high electrically conductivity Cu-Ni-Si alloys are important candidate materials for extending the life of currently used elastic-conductor materials. Phase transformation behaviors and properties of a synthesized Cu-6.0Ni-1.0Si-0.5Al-0.15Mg-0.1Cr alloy were investigated systematically. After homogenized for 4 h at 940 °C, hot rolled by 80% at 850 °C, solution treated for 6 h at 970 °C, cold rolled by 50%, and annealed for 60 min at 450 °C, the studied alloy approached physical properties of 1097.5 MPa in tensile strength and 26.4%IACS in electrical conductivity. Transmission electron microscope observations showed that four precipitation phases including δ-Ni2Si, β-Ni3Si, β-NiAl, and γ′-Ni3Al were formed in the studied alloy, which was subjected to different annealing temperatures. A detailed diagram for isothermal decompositions has been established. The high strength of the studied alloy was primarily attributed to the Orowan precipitation strengthening, and secondary attributed to the solid solution strengthening, the Hall-Petch type grain boundary strengthening, and the work hardening. Multi-precipitation phases of Ni2Si, γ′-Ni3Al, β-Ni3Si played significantly into the strengthening effects, and the low electron diffraction influence of Mg and Cr contributed to both solution strengthening and maintaining a high electrical conductivity. Multi-phase precipitation strengthening and using trace amount alloying elements for solution strengthening will be a new strategy to develop high strength and high electrical conductivity copper alloys.

Phase transformation behaviors and properties of a high strength Cu-Ni-Si alloy

Ni-P and Ni-Cu-P modified carbon catalysts for methanol electro-oxidation in KOH solution

Wettability and interfacial reactions of Sn–Ag–Cu/Cu and Sn–Ag–Ni/Cu solder joints

The ball shear strength of BGA solder joints during isothermal aging was studied with Sn–35Ag–075Cu solder on three different pads (Cu, electroless Ni–P/Cu, immersion Au/Ni–P/Cu) at temperature between 70 and 170°C for times ranging from 1 to 100 days The reliability of solder ball attachment was characterized by mechanical ball shear tests As a whole, the shear strength of BGA joints decreased with increasing temperature and time The shear strength for both the immersion Au/Ni–P/Cu and electroless Ni–P/Cu pads was consistently higher than that of the Cu pad for all isothermal aging conditions The fracture surface showed various characteristics depending on aging temperature, time, and the types of BGA pad The P-rich Ni layer formed at the interface between (Cu, Ni)6Sn5 and Ni–P deposits layer after aging, but fracture at this interface was not the dominant site for immersion Au/Ni–P/Cu and electroless Ni–P/Cu pad

Effect of Isothermal Aging on Ball Shear Strength in BGA Joints with Sn-3.5Ag-0.75Cu Solder

The ball shear strength of BGA solder joints during isothermal aging was studied with Sn-3.5Ag-0.75Cu solder on three different pads (Cu, electroless Ni-P/Cu, immersion Au/Ni-P/Cu) at temperature between 70 and 170°C for times ranging from 1 to 100 days. The reliability of solder ball attachment was characterized by mechanical ball shear tests. As a whole, the shear strength of BGA joints decreased with increasing temperature and time. The shear strength for both the immersion Au/Ni-P/Cu and electroless Ni-P/Cu pads was consistently higher than that of the Cu pad for all isothermal aging conditions. The fracture surface showed various characteristics depending on aging temperature, time, and the types of BGA pad. The P-rich Ni layer formed at the interface between (Cu, Ni) 6 Sn 5 and Ni-P deposits layer after aging, but fracture at this interface was not the dominant site for immersion Au/Ni-P/Cu and electroless Ni-P/Cu pad.

Effect of isothermal aging on ball shear strength in BGA joints with Sn-3.5Ag-0.75Cu solder : Lead-free electronics packaging

Characteristic evaluation of electroless nickel-phosphorus deposits with different phosphorus contents

In the present work, the wettability and interfacial tension between Cu-substrate and Sn-Bi solder were examined. The variables of this experiment used the content of Bi, types of flux and soldering temperature. Through analyzing the experimental result concerning wettability, it was found that the role of Bi was diminishing wettability of Cu substrate during soldering. The increasing Bi contents made liquid solder-Cu substrate interfacial tension (γ s1 ) go up. Consequently, the increasing liquid solder-Cu substrate interfacial tension (γ s1 ) led to the degradation of wettability in solder. The rates of wetting can be controlled by the interfacial energy between liquid solder and substrate. This paper describes the effect of Bi element on the wettability of Sn-Bi solders.

The effect of Bi concentration on wettability of Cu substrate by Sn-Bi solders : Special issue on basic science and advanced technology of lead-free electronics packaging

Chang-Chae Shur

Papers

Wettability and interfacial reactions of Sn–Ag–Cu/Cu and Sn–Ag–Ni/Cu solder joints

Effect of Isothermal Aging on Ball Shear Strength in BGA Joints with Sn-3.5Ag-0.75Cu Solder

Effect of isothermal aging on ball shear strength in BGA joints with Sn-3.5Ag-0.75Cu solder : Lead-free electronics packaging

Characteristic evaluation of electroless nickel-phosphorus deposits with different phosphorus contents

The effect of Bi concentration on wettability of Cu substrate by Sn-Bi solders : Special issue on basic science and advanced technology of lead-free electronics packaging