Kwang-Lung Lin

The interaction kinetics and compound formation between electroless NiP and solder

The interdiffusion between an electroless NiP deposit and liquid solder was investigated with the aid of an X-ray diffractometer and a scanning electron microscope to identify the intermetallic compounds formed. Sn was found to be the dominant diffusing atom in the interdiffusion process according to analysis by Auger electron spectroscopy. The intermetallic compounds Ni3Sn4 and Ni3Sn2 were formed between NiP and the solder in which phosphorus was also detected. The kinetics of the formation of intermetallic compounds and the activation energy for the diffusion of Sn were investigated.

High-Temperature Oxidation of a Sn-Zn-Al Solder

The oxidation of 91Sn-9(95Zn-5Al) solder in theliquid state, 250°C, was studied by thermalgravimetric analysis (TGA). The oxidation behavior of63Sn-37Pb, 91Sn-9Zn, 99.4Sn-0.6Al, and Sn was alsoinvestigated for comparison. The weight gains per unitsurface area descend in the order: 63Sn-37Pb > Sn> 91Sn-9Zn > 91Sn-9(95Zn-5Al) > 99.4Sn-0.6Al.The initial weight gains of the materials investigated increase linearly with reaction time, whileparabolic behavior exists after the linear stage. Therate constants of the oxidation reaction for the tworeaction stages were determined. Activation energies for oxidation of the five materials weredetermined in the range of 250 to 400°C. Theactivation energies, derived from the linear rateconstants for the early stages of oxidation, are 27.7kJ/mole for 99.4Sn-0.6Al, 23.3 kJ/mole for 91Sn-9Zn, 21.4kJ/mole for 91Sn-9(95Zn-5Al), 20.5 kJ/mole for63Sn-37Pb, and 19.8 kJ/mole for Sn. Thesurface-oxidation behavior was investigated further withelectron spectroscopy for chemical analysis (ESCA) and Auger electronspectroscopy (AES). AES profiles showed that oxides ofZn and Al formed on 91Sn-9Zn and 91Sn-9(95Zn-5Al)solders, while tin oxide is formed on 63Sn-37Pbsolder.

Manufacturing of Cu/electroless nickel/Sn-Pb flip chip solder bumps

A process for manufacturing Cu/electroless Ni/Sn-Pb solder bump is discussed in this paper. An attempt to replace zincation with a Cu film as an active layer for the electroless Ni (EN) deposition on Al electrode on Si wafer is presented. Cu/electroless Ni is applied as under bump metallurgy (UBM) for solder bump. The Cu film required repeated etches with nitric acid along with activation to achieve a satisfactory EN deposit. Fluxes incorporating rosin and succinic acid were investigated for wetting kinetics and reflow effectiveness of the electroplated solder bump. The solder plating current density and the reflow condition for achieving solder bumps with uniform bump height were described. The Cu/EN/Sn-Pb solder system was found to be successfully produced on Al terminal in this study that avoids using zincating process.

The electrochemical corrosion behaviour of Pb-free Al-Zn-Sn solders in NaCl solution

Abstract The corrosion behaviour of Pb-free X(5Al-Zn)- Y Sn solders, where X is 9-50 and Fis 91-50, has been investigated with regard to open-circuit potential, galvanic corrosion and potentiodynamic polarization in a 3.5% NaCl solution. The galvanic current densities for 9 (5Al-Zn)-91Sn in NaCl solution are 25, 20 and 12.5 μA/cm 2 with respect to Cu, Ni-Cu-P and Ni-P, respectively. The corrosion behaviour has been compared with that of 63Sn-37Pb solder. A passivation behaviour is observed for all of the investigated Al-Zn-Sn solders. The magnitudes of the passivation current densities depend on the compositions of the solders and the potentials applied. The polarization behaviour of eutectic 9 (5Al-Zn)-91Sn solder is very similar to that of 63Sn-37Pb solder. SnO 2 is formed at polarization potentials below −330 mV, while SnCl 4 forms when the potential is at 120 mV. ZnCl 2 replaces SnCl 4 on polarizing at 120 mV for an extended period of 1 h.

Interfacial reactions of lead-free Sn–Zn based solders on Cu and Cu plated electroless Ni–P/Au layer under aging at 150 °C

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 Cu 5 Zn 8 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.

The Deposition and Crystallization Behaviors of Electroless Ni‐Cu‐P Deposits

This study investigated the deposition and crystallization behaviors of electroless Ni-Cu-P deposits on aluminum substrates. The critical phosphorus content. 7 wt %, of transformation between crystalline and amorphous structure for Ni-Cu-P deposit has been identified. The concentration of copper salt (CuSO 4 ) in the deposition solution greatly affects the Cu content of Ni-Cu-P deposit and thus the Ni and P contents. An excess CuSO 4 concentration may impede the formation of amorphous Ni-Cu-P deposit. It was also found that the chemical composition and crystal structure of Ni-Cu-P deposits will vary during deposition. An amorphous Ni-21.66Cu-11.60P deposit exhibits transformation at 366.5 and 461.4°C when analyzed with differential scanning calorimeter. The Ni-Cu alloy and the Ni 5 P 2 compound form preferentially from the amorphous Ni-Cu-P deposit during crystallization at 366.5°C. The Ni 5 P 2 phase further transforms to Ni 3 P phase upon interaction with Ni at 461.4°C. The activation energies with respect to the two transformation reactions are 229.1 and 213.0 kJ/mol, respectively, for the crystallization of amorphous Ni-Cu-P deposit and the formation of Ni 3 P.

Damages and microstructural variation of high-lead and eutectic SnPb composite flip chip solder bumps induced by electromigration

The electromigration behavior of the high-lead and eutectic SnPb composite solder bumps was investigated at 150 °C with 5 × 10 3 A/cm 2 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.

Internal stress and adhesion of amorphous Ni–Cu–P alloy on aluminum

Abstract This study investigated the effect of saccharin on the internal stress and the adhesion of amorphous Ni–Cu–P deposited on aluminum. An amorphous Ni–Cu–P deposit with slight compressive stress can be produced when one adds 8–10 g/l saccharin into the Ni–Cu–P deposition solution. The stress relief mechanism was investigated. The addition of saccharin restrains the coalescence of the islands within Ni–Cu–P nodules and reverses the internal stress of the electroless Ni–Cu–P deposit from tensile to compressive. The adhesion strength of the Si/Ti/Al/Ni–Cu–P multilayer specimen obtained with 10 g/l saccharin is around 35 to 45 MPa, and the fracture occurs at the silicon substrate after the pull test. The shear strength of the Ti/Al/Ni–Cu–P bump (100×100 μm) on Si is 132.9±12.7 g, and the fracture occurs at the Ni–Cu–P deposit after the shear test. Moreover, the inhibition of coalescence of the fine islands within Ni–Cu–P nodules increases the brightness and the hardness of the deposit.

The Crystallization of an Electroless Ni‐P Deposit

The crystallization behavior of the electroless Ni‐P deposit was investigated in detail with the aid of SEM, TEM, XRD, and electron diffraction techniques. The as‐deposited film was shown to exhibit amorphous structure with XRD (x‐ray diffraction) and electron diffraction. The XRD and EDAX results show that heat‐treatment at temperatures above 300°C gives rise to Ni and phases. The fcc Ni and tetragonal grains of the film heated at 800°C are identified with electron diffraction. The occurrence of the maximum hardness achieved upon heating of the electroless Ni‐P plating is explained in terms of crystallization, grain coarsening, and defect change. The grain coarsening is also responsible for the random distribution of the phosphorus content across the deposit at temperatures above 600°C.

Solderability of Electroless Nickel Alloys Using Wetting Balance Technique

Solderability of electroless nickel-alloy deposits was investigated by the wetting balance technique. The wetting time, wetting force and dewetting can be obtained from the wetting curve. Four parameters, wetting time, percentage of theoretical force, stability of wetting and solderability index, were applied to determine the effects of the soldering variables including two kinds of flux, substrates of electroless nickel-alloy deposits and their solderability. In general, wetting time decreases with an increase in soldering temperature. The rosin-activated flux has a stronger fluxing effect than organic acid. Among the investigated items, solderability index and contact angle are quite sensitive to the base metal material. The electroless Ni–Cu–P deposit and the electroless Ni–P with Au-coated deposit exhibit good wetting behavior with contact angles of 25°<θ<45° when using RA flux. On the other hand, poor wettability, with contact angles of 65°<θ<90°, was observed for the deposits investigated in this work using OA flux.