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Reflow behavior of a Sn–8.5Zn–0.5Ag–0.01Al–0.1Ga (five-element) solder on the Ni/Cu substrate was investigated under different heating rates. Reflowed samples show decreased Zn and increased AgZn3 in the solder with a reduction in the heating rate. The Zn at the solder/substrate interface was found to be much lower than that in the Sn–Zn solder systems. Cu was observed to be diffused through the electroplated Ni layer and noticed only with the Ag–Zn compound in the solder. Ga was spotted at the interface in the Ag–Zn matrix, whereas Al was detected with the Zn at the interface. Small intermetallic compound (IMC) layer was formed at the interface; however, its amount enhanced with the reduction in the heating rate. Present study relates the reflow behavior of the five-element solder with the reactivity of different elements in the system and its influence on the formation of IMCs in the solder and at the solder/substrate interface.
Microstructural evolution occurred in 5Sn–95Pb/63Sn–37Pb composite flip-chip solder bump during electromigration. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) observations for 5Sn–95Pb/63Sn–37Pb composite flip-chip solder joints subjected to 5 kA/cm2 current stressing at 150 °C revealed a gradual orientation transformation of Pb grains from random textures toward (101) grains. We proposed that the combination of reducing the surface energy of Pb grain boundaries and resistance of the entire polycrystalline system are the driving force for the orientation transformation of Pb grains during an electromigration test.
This study investigated the polarity effect of electromigration (EM) on the interfacial intermetallic compounds (IMCs) (γ-Cu5Zn8, Cu6Sn5) formation at the anode and the cathode in a Cu/Sn-9Zn/Cu sandwich with a constant direct current density of 1.0 × 103 A/cm2 at 100 °C. The EM had different polarity effects on the nucleation and growth rates of the interfacial Cu5Zn8 IMC from those of Cu6Sn5 IMC. Upon current stressing, the growth rate of the Cu-Zn intermetallic compound (γ-Cu5Zn8) at the cathode interface was much faster than that at the anode. However, the nucleation and growth of the Cu6Sn5 IMC at the anode interface were enhanced, though retarded at the cathode, under the influence of electric current. The mechanism of EM-induced Cu6Sn5 IMC formation towards the anodic Cu is also discussed.
The electromigration behavior of the Cu/Au/SnAgCu/Cu combination was investigated under 103 A/cm2 of current stressing at ambient temperature. The Au layer, when it acts as a cathode, was consumed continuously, and no significant compound was found at the interface. Meanwhile, Cu6Sn5 was formed at the anodic Cu layer, and the thickness of the compound increased with increasing time. The Au atoms were found to be trapped in Cu6Sn5 within the solder matrix. The AuSn4 compound precipitated while attaching to Cu6Sn5 at the Cu6Sn5/solder interface. The thermomigration effect was found to be insignificant in this work as no obvious reaction occurred at the cathode/anode sides or in the solder matrix without current stressing.
The electrochemical corrosion behavior of Sn–XAg–0.5Cu alloys in 3.5% NaCl solution was examined using potentiodynamic polarization techniques. The Ag content in the alloy was varied from 1 to 4 wt%. The polarization curves obtained for the alloys show an active–passive transition followed by a transpassive region. Sn–XAg–0.5Cu alloys with higher Ag content (>2 wt%) show a strong tendency toward passivation. The passivation behavior has been ascribed to the presence of both SnO and SnO2 on the anode surface. Increase in Ag content from 1 to 4 wt% results in a decrease in the corrosion-current density (Icorr) and linear polarization resistance (LPR) of the alloy. Nevertheless, the corrosion potential (Ecorr) shifts toward negative values, and a decrease in corrosion rate is observed. The presence of Cl− ion initiates pitting and is responsible for the rupture of the passive layer at a certain breakdown potential. The breakdown potential (EBR) decreases and shifts toward more noble values with increase in Ag content in the alloy. Surface analyses by x-ray photoelectron spectroscopy (XPS) and Auger depth profile studies confirmed the formation of both Sn(II) and Sn(IV) oxides in the passive layer.
This study investigated the electromigration behavior between Cu and Sn–9Zn solder under a current density of 1.0 × 103 A/cm2 for up to 230 h. The experimental results indicated that Cu5Zn8 was formed at the interface between Cu and the cathode side of the Sn–9Zn solder as well as in the bulk near the anode. Consumption of Cu was also observed for the Cu plating on the cathode side and anodic side, but with less compound formation and Cu consumption at the anode. The intermetallic compound layer on the cathode side was always thicker than that on the anode side after the same current-stressing time. The effect of chemical potential overwhelms electromigration in inducing Zn diffusion when a counterflow of electrons and chemical potential gradient exists. Voids formed at the Cu5Zn8–solder interface inside the solder regardless of the direction of current flow.
Sn–8Zn–3Bi solder paste and Sn–3.2Ag–0.5Cu solder balls were reflowed simultaneously at 240 °C on Cu/Ni/Au metallized ball grid array substrates. The joints without Sn–Zn–Bi addition (only Sn–Ag–Cu) were studied as a control system. Electrical resistance was measured after multiple reflows and aging. The electrical resistance of the joint (R1) consisted of three parts: the solder bulk (Rsolder bulk, upper solder highly beyond the mask), interfacial solder/intermetallic compound (Rsolder/IMC), and the substrate (Rsubstrate). R1 increased with reflows and aging time. Rsolder/IMC, rather than Rsolder bulk and Rsubstrate, seemed to increase with reflows and aging time. The increase of R1 was ascribed to the Rsolder/IMC rises. Rsubstrate was the major contribution to R1. However Rsolder/IMC dominated the increase of R1 with reflows and aging. R1 of Sn–Zn–Bi/Sn–Ag–Cu samples were higher than that of Sn–Ag–Cu samples in various tests.
Sn–8Zn–3Bi solder paste and Sn–3.2Ag–0.5Cu solder balls were reflowed simultaneously on Cu/Ni/Au metallized ball grid array (BGA) substrates. The correlation between microstructural evolution and the electrical resistance of the joints under various testing conditions of reflow cycles and heat treatment was investigated. The electrical resistance of the Sn–Ag–Cu joints without Sn–Zn–Bi was also conducted for comparison. The average resistance values of Sn–Ag–Cu and Sn–Ag–Cu/Sn–Zn–Bi samples changed, respectively, from 7.1 (single reflow) to 7.3 (10 cycles) mΩ and from 7.2 (single reflow) to 7.6 (10 cycles) mΩ. Furthermore, the average resistance values of Sn–Ag–Cu and Sn–Ag–Cu/Sn–Zn–Bi samples changed, respectively, from 7.1 (aging 0 h) to 7.8 (aging 1000 h) mΩ and from 7.2 (aging 0 h) to 7.9 (aging 1000 h) mΩ. It was also noticeable that the average resistance values of Sn–Ag–Cu/Sn–Zn–Bi samples were higher than those of Sn–Ag–Cu samples in each specified testing condition. The possible reasons for the greater resistance exhibited by the Sn–Zn–Bi incorporated joints were discussed.
The electromigration behavior of the high-lead and eutectic SnPb composite solder bumps was investigated at 150 °C with 5 × 103 A/cm2 current stressing for up to 1711 h. The diameter of the bumps was about 125 μm. The underbump metallization (UBM) on the chip side was sputtered Al/Ni(V)/Cu thin films, and the Cu pad on the board side was plated with electroless Ni/Au. It was observed that damages occurred in the joints in a downward electron flow (from chip side to the substrate side), while those joints having the opposite current polarity showed only minor changes. In the case of downward electron flow, electromigration damages were observed in the UBM and solder bumps. The vanadium in Ni(V) layer was broken under current stressing of 1711 h while it was still intact after current stressing of 1000 h. The electron probe microanalyzer (EPMA) elemental mapping clearly shows that the Al atoms in the trace migrated through the UBM into the solder bump during current stressing. Voids were found in the solder bump near the UBM/solder interface. The Sn-rich phases of the solder bumps showed gradual streaking and reorientation upon current stressing. This resulted in the formation of uniaxial Sn-rich phases in the middle of the solder bump, while the columnar and fibrous Sn-rich phases were formed in the surrounding regions. The formation mechanism of electromigration-induced damage to the UBM structure and solder bump were discussed.
The soldering reaction and interfacial microstructure formed between liquid Sn–Zn–Ag solder and Cu at the early stage of soldering at 250 °C for 15 s were studied primarily with the aid of transmission electron microscope (TEM). To achieve the early stage reaction information, the soldered specimens, 5 mm × 5 mm × 500 μm solder on 10 mm × 10 mm × 20 μm Cu, were rapidly quenched in liquid nitrogen after soldering. The results of TEM interfacial analysis show that a Cu–Zn reaction zone, consisting of β′–CuZn and γ–Cu5Zn8, formed near Cu while a Ag–Zn zone, consisting of γ–Ag5Zn8 and ϵ–AgZn3, formed near solder. The innermost layer adjacent to the Cu substrate is an amorphous Cu-Zn diffusion region that contains dispersed β′–CuZn nanocrystalline cells. The β′–CuZn also precipitates in the γ–Ag5Zn8 and ϵ–AgZn3 layer due to the supersaturation of Cu.
The early dissolution behavior of Cu in a molten Sn–Zn–Ag solder was studied at 250 °C by fast quenching the dissolving specimen in liquid nitrogen. The atomic level dissolution behavior of Cu in the molten solder was revealed by high-resolution transmission electron microscopy. The dissolution of Cu occurs through channel dissolution and thermal vibrational dissolution. The dissolution channel has a dimension of less than 0.5 nm. The formation of channels, and thus the channel zone, is initiated by preferential removal of Cu atoms from the surface vacant site of Cu lattice. Relict strips of lattice between channels subsequently dissolve into the molten solder with the aid of thermal vibration and the interaction with liquid Zn atoms. The dissolved atoms form an atomic cluster zone. These clusters are the intermediate state of the dissolution of Cu from the channel zone into the molten Sn–Zn–Ag solder. The clusters convert into an amorphous structure prior to further formation of compound.
Sn–8Zn–3Bi solder paste and Sn–3.2Ag–0.5Cu solder balls were reflowed simultaneously on Cu/Ni/Au metallized ball grid array (BGA) substrates to investigate the interfacial bonding behaviors for multiple reflow cycles at two different soldering temperature of 210 and 240 °C. The different intermetallic compounds that formed at the interface after one reflow cycle were respectively the island-shaped Ag–Au-Cu-Zn (near the solder) compounds and the Ni–Sn–Cu-Zn (near the metallized pad) compounds in 210 or 240 °C soldering systems. Layered Ag–Au–Cu–Zn, Ag5Zn8, and Ag–Zn–Sn compounds were also observed within the solder near the interface after single reflow cycle. After ten reflow cycles, the Ag–Au–Cu–Zn compounds significantly decomposed, while the Ag3Sn and Ni–Sn–Cu–Zn compounds coarsened obviously.
The interfacial reactions of Sn–Zn based solder on Cu and Cu/Ni–P/Cu–plating substrates under aging at 150 °C were investigated in this study. The compositions of solders investigated were Sn–9Zn, Sn–8.55Zn–0.45Al, and Sn–8.55Zn–0.45Al–0.5Ag solders in weight percent. The experimental results indicated that the Cu substrate formed Cu5Zn8 with the Sn–9Zn solder and Al–Cu–Zn compound with Al–containing solders. However, it was detected that Cu6Sn5 formed at the Sn–9Zn/Cu interface and Cu5Zn8 formed at the Al–containing solders/Cu interface after aging for 1000 h. When it contacted with the Cu/Ni–P/Au substrate, the Sn–9Zn solder formed Au–Zn compound, and the Al–containing solders formed Al–Cu–Zn compound at the interface. After a long aging time, the intermetallic compounds existing between solders and the Cu/Ni–P/Au metallization layers almost did not grow. It was found that the interdiffusion between solders and Cu/Ni–P/Au was slower than that with Cu under aging. Furthermore, the addition of Ag to Sn–Zn solder resulted in the formation of AgZn3 particles at the interface.
In the Sn–9Zn–xAg (x ranges from 0.5 to 3.0%) solder system, the Ag–Zn intermetallics start crystallizing as β–AgZn at about 300 °C. Subsequently, the solder experiences two peritectic transformations upon solidification. This results in the multiphase structural feature. The complicated crystallization process can be briefly described as L(liquid) → L + AgZn → L + AgZn + Ag5Zn8 → AgZn + Ag5Zn8 + AgZn3.
The shear strength, intermetallic compound formation, and failure mechanism of high-lead solder (5Sn–95Pb) bump on flip chip under bump metallurgy, Al/Ni(V)/Cu, were investigated after thermal cycling, multiple reflow, and high-temperature aging. Two kinds of intermetallic compound, Cu3Sn and AlxNiy, were found at the interface. The Cu3Sn was formed between the solder and Ni(V) layer while AlxNiy was formed between Ni(V) and Al layer. The formation of the Cu3Sn compound will not affect the shear strength, 27–30 g, of the solder bump even after a high temperature long time aging test. However, the shear strength after the 30th reflow drops to less than 25 g, ascribed to the formation of a brittle compound, AlxNiy. The failure modes of the solder bump upon shear test were also discussed.
This study investigated the characteristics of the intermetallics that appear in Sn–Zn–Ag solder alloys, particularly their behavior in molten solder during cooling and remelting. The results indicated that the intermetallics, which deplete the Zn-rich phase, were present in the form of inhomogeneous dendrites and consisted of two intermetallic phases, ε–AgZn3 and γ–Ag5Zn8. These Ag–Zn intermetallics formed as the primary dendrites upon cooling from temperatures slightly below 300 °C. These intermetallics transformed into coarse nodules with a stable, high Ag-content phase when isothermally heated at 250 °C. These massive intermetallic particles tended to settle at the bottom of the melt due to low buoyancy. Isothermal heating at slightly above 300 °C resulted in the rapid melting of these intermetallics. Subsequent quenching caused numerous fine dendritic intermetallics to form throughout the solder.
The type and magnitude of stress in electroless Ni–Cu–P deposits on Al were manipulated by controlling the concentration of saccharin in the plating solution. Tensile, zero, and compressive stress of the electroless Ni–Cu–P deposits was obtained with 0, 8, and 10 g/l saccharin for studying the effect of stress on the diffusion and crystallization behavior of the deposit. The effect of stress on the diffusion behavior of Cu, Ni, and Al elements during annealing was investigated. Interdiffusion between Al and Ni in an amorphous Ni–Cu–P/crystal Al diffusion couple is abated by the effects of amorphous structure, atomic affinity, and backstress. Therefore, the effect of stress on diffusion is manifested by Cu elemental diffusion. The tensile stress promotes the formation of Ni3P and the diffusion of Cu into the substrate.
A uniform deposition of a Sn–Zn alloy deposit was achieved by pulse plating. Apparently, the relative composition of Sn and Zn in the deposit was affected by the bath compositions and pulse condition. A pulse-plating condition of 99.9 ms on-time and 1.0 ms off-time gave rise to a eutectic Sn–Zn deposit, with a eutectic temperature of 198.8 °C (as analyzed by differential scanning calorimetry) and a uniform composition distribution across the deposit. A mechanism for explaining the pulse-deposition behavior of the Sn–Zn eutectic deposit was proposed. A longer off-time period, 99.9 ms versus 0.1 ms, resulted in a nodular, yet thinner deposit.
The interfacial intermetallic formation at 150 °C between Cu and various solders, including Sn–9Zn, Sn–8.55Zn–1Ag, and Sn–8.55Zn–1Ag–XAl was investigated. The Al contents X of the quaternary solder alloys investigated were 0.01–0.45 wt.%. The compositions and the growth kinetics of intermetallic compounds (IMCs) were investigated. The IMC consisted of three layers for Sn–9Zn/Cu, Sn–Zn–Ag/Cu, and Sn–Zn–Ag–XAl/Cu specimens after aging for 100–600 h. These three layers included the Cu3(Zn, Sn) phase adjacent to the solder, the Cu6(Sn, Zn)5 phase in the middle, and the Cu–rich phase near to Cu. For long–term aging time over 1000 h, the Cu6(Sn, Zn)5 phase grew, while the Cu3(Zn, Sn) phase diminished. Al segregation formed in the IMC for all of the Sn–Zn–Ag–XAl/Cu specimens after aging.Cracks formed, when aged for 1000 h, at the solder/IMC interface or within the IMC layer for the following solders: Sn–9Zn, Sn–8.55Zn–1Ag, Sn–8.55Zn–1Ag–0.1Al, Sn–8.55Zn–1Ag–0.25Al, and Sn–8.55Zn–1Ag–0.45Al. The crack was not detected up to 3000 h for the Sn–8.55Zn–1Ag–0.01Al/Cu couple, of which the IMC growth rate was the slowest among all solders.
The microstructure, melting point, and mechanical properties of Sn–8.55Zn–0.45Al–XAg lead-free solders were investigated. The Ag content of the solders investigated was 0–3 wt.%. The results indicate that the AgZn3 and Ag5Zn8 compounds are formed at the addition of Ag to Sn–8.55Zn–0.45Al solders. The adding of Ag also results in the formation of hypoeutectic structure, increasing the melting point of the solders and decreasing the ductility. Results of thermal analysis reveal that the Sn–8.55Zn–0.45Al–XAg solder has eutectic temperature at 198 °C when the addition of Ag is 0.5 wt.%. The eutectic solder exhibits greater tensile strength and higher ductility than the 63–Sn–37Pb solder.