Novel Sn-rich Sn–Au–Ag solder alloys were designed based on the cluster-plus-glue-atom model and the [Sn11Au2]Sn3 and [AuSn8]Au were applied as the basic cluster formulas. The microstructure, melting behavior, wettability and the interfacial reaction of the designed Sn–Au–Ag solders were systematically investigated. The Sn–Au–Ag solders designed based on the [Sn11Au2]Sn3 cluster formula have a better ability to form the near-eutectic composition than those designed based on the [AuSn8]Au cluster formula. The [Sn11Au1Ag1]Sn3 and [Sn11Au0.5Ag1.5]Sn3 solders have near-eutectic compositions, with the melting temperatures of 206.89 and 207.25 °C, respectively. While the Sn–Au–Ag solders designed based on the [AuSn8]Au cluster formula are non near-eutectics. The microstructure of the Sn–Au–Ag bulk solders consists of the AuSn4 and Ag3Sn phases dispersed in β-Sn matrix. The Sn–Au–Ag solders have a better wettability on Ni substrate than on Cu substrate, while the Sn–Ag solder, i.e., [Sn11Ag2]Sn3 has a better wettability on Cu substrate than on Ni substrate.

•High anisotropy in β-Sn dominates different electromigration-induced failure mode.•Excessive dissolution of cathode Cu occurs if electrons flow in forward direction.•Voids initiate and propagate at cathode if electrons flow in reverse direction.•Failure modes are well explained in viewpoint of atomic diffusion flux in β-Sn.The effect of high diffusivity anisotropy in β-Sn grain on electromigration behavior of micro-bumps was clearly demonstrated using Sn-3.0Ag-0.5Cu solder interconnects with only two β-Sn grains. The orientation of β-Sn grain (θ is defined as the angle between the c-axis of β-Sn grain and the electron flow direction) is becoming the most crucial factor to dominate the different electromigration-induced failure modes: 1) the excessive dissolution of the cathode Cu, blocking at the grain boundary and massive precipitation of columnar Cu6Sn5 intermetallic compounds (IMCs) in the small angle θ β-Sn grain occur when electrons flow from a small angle θ β-Sn grain to a large one; 2) void formation and propagation occur at the cathode IMC/solder interface and no Cu6Sn5 IMCs precipitate within the large angle θ β-Sn grain when electrons flow in the opposite direction. The EM-induced failure mechanism of the two β-Sn grain solder interconnects is well explained in viewpoint of atomic diffusion flux in β-Sn.

To reveal the drop failure modes of the wafer level chip scale packages (WLCSPs) with Sn–3.0Ag–0.5Cu solder joints, board level drop tests were performed according to the JEDEC standard. Six failure modes were identified, i.e., short FR-4 cracks and complete FR-4 cracks at the printing circuit board (PCB) side, split between redistribution layer (RDL) and Cu under bump metallization (UBM), RDL fracture, bulk cracks and partial bulk and intermetallic compound (IMC) cracks at the chip side. For the outmost solder joints, complete FR-4 cracks tended to occur, due to large deformation of PCB and low strength of FR-4 dielectric layer. The formation of complete FR-4 cracks largely absorbed the impact energy, resulting in the absence of other failure modes. For the inner solder joints, the absorption of impact energy by the short FR-4 cracks was limited, resulting in other failure modes at the chip side.

Novel Sn-rich Au-Sn solder alloys were designed using the cluster-plus-glue-atom (CPGA) model, and the microstructure, melting behavior, wettability and mechanical properties of the designed Sn-Au-Ag(Ni) solders were systematically investigated. The cluster formula of [Sn11Au2]Sn3, a stable Sn-centered cluster (11 Sn atoms and 2 Au atoms) plus 3 Sn glue-atoms, is applicable to substitute Ag and Ni for Au. The [Sn11AuAg]Sn3 and [Sn11Au0.5Ag1.5]Sn3 solders are near-eutectics with a narrow melting temperature range of 206.89–207.25 °C; while the Sn-Au-Ni solders are non-eutectics with a wide melting temperature range of 215.33–255.44 °C, due to a large difference in chemical property between Ni and Au. The microstructure of the Sn-Au-Ag solders consists of AuSn4 and Ag3Sn phases dispersed in β-Sn matrix; while that of the Sn-Au-Ni solders consists of AuSn4 and Ni3Sn4 phases dispersed in β-Sn matrix. The wettability of the Sn-Au-Ag(Ni) solders on Ni substrate is better than that on Cu substrate because of the severe interfacial reaction on Cu substrate. The shear strength of the [Sn11Au0.5Ag1.5]Sn3 and [Sn11Au0.5Ni1.5]Sn3 solder joints is 50.59 MPa and 36.28 MPa, respectively, which is significantly higher than that of eutectic Au-30 at% Sn solder joints (24.5 MPa).

Solder size effect on early stage interfacial intermetallic compound (IMC) evolution in wetting reaction between Sn–3.0Ag–0.5Cu solder balls and electroless nickel electroless palladium immersion gold (ENEPIG) pads at 250 °C was investigated. The interfacial IMCs transformed from initial needle- and rod-type (Cu,Ni)6Sn5 to dodecahedron-type (Cu,Ni)6Sn5 and then to needle-type (Ni,Cu)3Sn4 at the early interfacial reaction stage. Moreover, these IMC transformations occurred earlier in the smaller solder joints, where the decreasing rate of Cu concentration was faster due to the Cu consumption by the formation of interfacial (Cu,Ni)6Sn5. On thermodynamics, the decrease of Cu concentration in liquid solder changed the phase equilibrium at the interface and thus resulted in the evolution of interfacial IMCs; on kinetics, larger solder joints had sufficient Cu flux toward the interface to feed the (Cu,Ni)6Sn5 growth in contrast to smaller solder joints, thus resulted in the delayed IMC transformation and the formation of larger dodecahedron-type (Cu,Ni)6Sn5 grains. In smaller solders, no spalling but the consumption of (Cu,Ni)6Sn5 grains by the formation of (Ni,Cu)3Sn4 grains occurred where smaller discrete (Cu,Ni)6Sn5 grains formed at the interface.

•The dissolution and growth of interfacial Cu6Sn5 were real-time in situ observed.•Cu6Sn5 dissolved into the solder with decreasing aspect ratios in the heating stage.•Cu6Sn5 re-precipitated on the existing Cu6Sn5 grains in the cooling stage.•It is the concentration gradient that causes the dissolution and growth of IMC.•The growth behavior was well explained by a model based on concentration gradient.Synchrotron radiation real-time imaging technology was used for in situ study of dissolution and growth behavior of interfacial Cu6Sn5 intermetallic compound (IMC) in Sn/Cu solder interconnect during reflow soldering. The pre-formed Cu6Sn5 grains dissolved into the liquid solder with decreasing aspect ratio in the heating stage, maintained a thin layer of scallop-type in the dwelling stage, and re-precipitated on the existing Cu6Sn5 grains at a faster growth rate with increasing aspect ratio in the cooling stage. The Cu concentration gradient at the interface is responsible for the aspect ratio variation (corresponding to dissolution and re-precipitation of interfacial Cu6Sn5 grains), which is also supported by the simulation of atomic diffusion in the solder based on Fick׳s second law. The growth behavior was well explained by a proposed model based on the Cu concentration gradient.

The diffusion behavior of Zn atoms and the interfacial reaction in Cu/Sn-9 wt.%Zn/Cu interconnects undergoing liquid–solid electromigration were investigated under a current density of 5.0 × 103 A/cm2 at 230°C. A reverse polarity effect was revealed, in which the interfacial intermetallic compounds (IMCs) at the cathode grew continuously and were remarkably thicker than those at the anode. This behavior resulted from the directional migration of Zn atoms from the anode towards the cathode, which was induced by the positive effective charge number (Z*) of the Zn atoms rather than by back-stress. Consequently, at the anode, dissolution and massive spalling of the Cu-Zn IMCs occurred, and the depletion of Zn atoms resulted in the transformation of initial interfacial Cu5Zn8 IMC into (Cu6Sn5 + CuZn); at the cathode, the interfacial Cu5Zn8 IMC gradually transformed into (Cu5Zn8 + CuZn); in the solder, the Zn content reduced continuously from 9 wt.% to 0.9 wt.%. A growth model is proposed to explain the reverse polarity effect, and the average Z* of Zn atoms in Cu5Zn8 was calculated to be +0.25 using this model.

The effect of electromigration (EM) on the interfacial reaction in a line-type Cu/Sn/Ni-P/Al/Ni-P/Sn/Cu interconnect was investigated at 150°C under 5.0 × 103 A/cm2. When Cu atoms were under downwind diffusion, EM enhanced the cross-solder diffusion of Cu atoms to the opposite Ni-P/Sn (anode) interface compared with the aging case, resulting in the transformation of interfacial intermetallic compound (IMC) from Ni3Sn4 into (Cu,Ni)6Sn5. However, at the Sn/Cu (cathode) interface, the interfacial IMCs remained as Cu6Sn5 (containing less than 0.2 wt.% Ni) and Cu3Sn. When Ni atoms were under downwind diffusion, only a very small quantity of Ni atoms diffused to the opposite Cu/Sn (anode) interface and the interfacial IMCs remained as Cu6Sn5 (containing less than 0.6 wt.% Ni) and Cu3Sn. EM significantly accelerated the dissolution of Ni atoms from the Ni-P and the interfacial Ni3Sn4 compared with the aging case, resulting in fast growth of Ni3P and Ni2SnP, disappearance of interfacial Ni3Sn4, and congregation of large (Ni,Cu)3Sn4 particles in the Sn solder matrix. The growth kinetics of Ni3P and Ni2SnP were significantly accelerated after the interfacial Ni3Sn4 IMC completely dissolved into the solder, but still followed the t1/2 law.

Au-20 wt% Sn eutectic solder is used as bumps in flip chip package of power LED (Light Emitting Diode) due to its excellent properties. The Au/Sn dual-layer films were fabricated on Si wafer by pulse electroplating of Au and Sn sequentially, and the solid–solid interfacial reaction during aging and the eutectic reaction during reflow soldering were investigated in the present work. After storage at room temperature for 1 week, three phases of AuSn, AuSn2 and AuSn4 were sequentially formed at the Au/Sn (10 μm/10 μm) interface, and the thickness of this reaction region was about 5 μm. Firstly, AuSn4 was formed at the Au/Sn interface, and then AuSn and AuSn2 were formed at the Au/AuSn4 interface. After aging at 150 °C for 5 and 10 h, a similar layered structure of AuSn/AuSn2/AuSn4 was also observed. Due to the faster diffusion of Au to Sn layer, all the Sn elements were consumed after aging at 150 °C for 15 h and AuSn4 layer gradually transformed into AuSn and AuSn2 layers. For the specimen of Au/Sn (9 μm/6 μm) films on Si chip, a bamboo-shoot-like microstructure of Au5Sn/AuSn/AuSn2 was formed in the reaction region after reflowed at 280 °C for 10 s; while a typical two-phase (Au5Sn and AuSn) eutectic microstructure was formed after reflowed at 280 °C for 60 s.

The effect of Ag content on the wetting behavior of Sn-9Zn-xAg on aluminum and copper substrates during soldering, as well as the mechanical properties and electrochemical corrosion behavior of Al/Sn-9Zn-xAg/Cu solder joints, were investigated in the present work. Tiny Zn and coarsened dendritic AgZn3 regions were distributed in the Sn matrix in the bulk Sn-9Zn-xAg solders, and the amount of Zn decreased while that of AgZn3 increased with increasing Ag content. The wettability of Sn-9Zn-1.5Ag solder on Cu substrate was better than those of the other Sn-9Zn-xAg solders but worse than that of Sn-9Zn solder. The wettability of Sn-9Zn-1.5Ag on the Al substrate was also better than those of the other Sn-9Zn-xAg solders, and even better than that of Sn-9Zn solder. The Al/Sn-9Zn/Cu joint had the highest shear strength, and the shear strength of the Al/Sn-9Zn-xAg/Cu (x = 0 wt.% to 3 wt.%) joints gradually decreased with increasing Ag content. The corrosion resistance of the Sn-9Zn-xAg solders in Al/Sn-9Zn-xAg/Cu joints in 5% NaCl solution was improved compared with that of Sn-9Zn. The corrosion potential of Sn-9Zn-xAg solders continuously increased with increasing Ag content from 0 wt.% to 2 wt.% but then decreased for Sn-9Zn-3Ag. The addition of Ag resulted in the formation of the AgZn3 phase and in a reduction of the amount of the eutectic Zn phase in the solder matrix; therefore, the corrosion resistance of the Al/Sn-9Zn-xAg/Cu joints was improved.