Su-Yueh Tsai

Thermal stability and mechanical properties of Ni-W-P electroless deposits

Abstract The ternary Ni–W–P alloy coatings were deposited by electroless plating on 420 stainless steel to evaluate the thermal property and related mechanical characteristics of the coating assemblies. The thermal stability of electroless Ni–W–P deposits, analyzed by differential scanning calorimetry (DSC), could be enhanced by the co-deposition of tungsten as compared to binary electroless Ni–P films. The phase and composition of Ni–W–P and Ni–P were evaluated by X-ray diffraction technique and electron probe microanalysis, respectively. The co-deposition of tungsten enabled the deposits to exhibit an amorphous structure with lower phosphorus content. Adhesion strength of the deposits on the substrates were assessed by a scratch test. The introduction of the tungsten in the Ni–P coating effectively reduced the linear crack length and delayed the cracking formation. Surface hardness of binary alloy coating could be modified with the introduction of the tungsten element.

Corrosion behaviour of electroless nickel plating modified TiN coating

Abstract TiN films were prepared by reactive r.f. magnetron sputtering on low carbon steel substrates. An electroless Ni-P plating was introduced as the interlayer in the modification of the TiN coating. The corrosion behaviour of the TiN coating assembly was evaluated using the salt spray test and the polarization technique. For a thicker Ni-P interlayer (greater than 5 μm), no appreciable corrosion was observed up to hundreds of hours after salt spray testing. The polarization curve indicates a barrier-type passive film in the TiN/Ni 3 P/Fe coating assembly. The corrosion currents for TiN/Fe, Ni 3 P/Fe and TiN/Ni 3 P/Fe are 200, 40–70 and 6–7 μA cm -2 respectively. It appears that the electroless Ni-P interlayer modified TiN coating has better anticorrosion resistance than pure TiN coating on low carbon steel.

Wear behaviour in TiN coatings modified by an electroless Ni interlayer on mild steel

Abstract An electroless nickel deposit is introduced as an interlayer between a carbon steel substrate and r.f.-magnetron-sputtered TiN films, and a compound coating assembly TiN/Ni 3 P/Fe is derived after appropriate heat treatment. The pin-on-disc test is employed to evaluate the wear behaviour of the fabricated compound coating. The depth profiles of the wear track are measured with an α-step and the morphology of the wear track is investigated with the aid of a scanning electron microscope. It is observed that the track depth in TiN/Ni 3 P/Fe is about an order of magnitude smaller than that in the Fe substrate, and the employment of an electroless Ni interlayer in TiN coating provides beneficial anti-wear resistance.

High Frequency Characteristics and Electrical Properties of Multilayer FeCoHfO/AlO_x Films

For the high frequency application in mobile communication technology, the operating frequency is over 3GHz, hence the ferromagnetic resonance frequency (fres) of magnetic films must be over 3 GHz. In this study, FeCoHfO alloy is chosen as a promising material system for this application. Inserting an insulator (e.g., AlOx) layer into ferromagnetic layers is a good way to enhance the resistivity and therefore to reduce the eddy current loss. A significant reduction of hard axis anisotropic field was observed when the number of multilayer reached five. With this optimum condition of [FeCoHfO (400 nm)/AlOx (10 nm)]3, favorable magnetic properties (effective anisotropy field of 110 Oe), high frequency characteristics (permeability ∼ 100 at 100 MHz and ferromagnetic resonance frequency over 3 GHz) and high electrical resistivity ρ ∼ 1074 μΩcm were obtained.

Synthesis and Application of Novel Lead-Free Solders Derived from Sn-based Nanopowders

Two kinds of lead-free solders derived from tin-based nanopowders were fabricated. One was the lead-free solders with Sn-Ag-Cu nanopowders, and the other was nano-sized Cu₆Sn₅ doped Sn-Ag-Cu solders. The lead-free solders with Sn-Ag-Cu nanopowders were synthesized by chemical precipitation with NaBH₄. The isolated particle exhibited a near spherical shape with particle sizes around 5 nm. The primary particle size of Sn-Ag-Cu nanopowders was in the range of 40 nm. Microstructural characteristics of particle growth were evaluated by TEM and FE-SEM. The primary particles after precipitation were (Ag,Cu)₄Sn with a size of 4.9 nm. It was also revealed that (Ag,Cu)₄Sn transformed into (Ag,Cu)₃Sn, when the total amount of Sn contributed from both (Ag,Cu)₄Sn and Sn covering the (Ag,Cu)₄Sn overtook that of (Ag,Cu)₃Sn. The nano-sized Cu₆Sn₅ doped Sn-Ag-Cu solder paste was produced by mixing Cu₆Sn₅ nanopowder into commercial SnAg solder paste. To realize the effect of Cu₆Sn₅ nanopowder doping, the ball shear strength of the joint was further investigated. The fracture behavior of Sn-Ag-Cu solder joints was probed with respect to the fracture surfaces, interfacial morphologies, and ball shear strength. It was demonstrated that the creep strain rate sensitivity of nano- Cu₆Sn₅ doped composite solder was higher than that of commercial Sn-Ag-Cu solder, although the creep hardness of both solders was nearly identical

Employment of sputtered Ni-Zn films for under bump metallization with Sn-3.0Ag-0.5Cu solder attached during liquid reactions

This study aims to investigate the feasibility of sputtered Ni-xZn films for application as the under bump metallization (UBM). The interfacial reaction under liquid reactions of Sn-3.0Ag-0.5Cu (SAC305) joints with Ni-Zn films will be evaluated. Various kinds of Ni-Zn films were fabricated by sputtering. The surface roughness and residual stress of the Ni-Zn film was evaluated using atomic force microscope (AFM) and the curvature technique. The X-ray diffractometry (XRD) was used to further identify the structure and phases in the films. Detailed morphology of the interfacial reaction in SAC305/Ni-xZn joints was performed by a field-emission scanning electron microscope (FE-SEM) with low angle backscattered electrons detector (LABE). Different structure growth in sputtered deposition might lead to distinct IMC formation. The microstructure evolution and phase formation in the SAC305/Ni-7Zn and SAC305/Ni-20Zn joints varied with reflow time. This study demonstrated that the binary Ni-Zn film might be a potential alternative for under-bump metallization application. In addition, the influence of Zn on Ni-Zn films UBM during liquid reactions was discussed and probed.

Solid state reaction of Sn3.0Ag0.5Cu solder with Cu(Mn) under bump metallization

In flip chip technology, Cu thin-film is a widely used under bump metallization (UBM). However, the major disadvantages of Cu UBM are fast consumption of copper, rapid growth of IMCs and easy formation of Kirkendall voids [1, 2]. Many efforts have been focused on suppression of Kirkendall voids which are detrimental to solder joints reliability in the microelectronics industry [3, 4]. In this study, a novel Cu(Mn) UBM design was provided by sputtering to improve the conventional Cu metallization. For the higher Mn concentration (10 at.% Mn) in the Cu(Mn) UBM, a new Sn-rich phase formed between Cu6Sn5 and the Cu(Mn) UBM. However, a layer of crack formed after aging. For the lower Mn concentration (2 at.% Mn), the growth of Cu3Sn and Kirkendall voids was significantly suppressed during thermal aging. The interfacial reaction was analyzed by a field emission electron probe microanalyzer (FE-EPMA) to evaluate the composition distribution. Kinetic analysis and X-ray color mapping gave the evidence that Mn diffusion into Cu3Sn slowed down the diffusivity of Cu in Cu3Sn layer. The Mn-enriched Cu3Sn might serve as a diffusion barrier to reduce interfacial reaction and Kirkendall voids formation. These results suggest the Cu-Mn UBM with a low Mn concentration is beneficial to retarding the Cu pad consumption in solder joints.

Morphological and microstructural evolution of Sn-patch in SnAgCu solder with Ni(V)/Cu under bump metallization

In flip chip technology, the Ni(V)/Cu multi-metallic thin-films is a widely used under bump metallization (UBM), for doping 7 wt.% V into the Ni target can eliminate the magnetism of Ni during sputtering [1,2]. It was noted that V in the Ni(V) layer did not react with solders and intermetallic compounds (IMC) during reflow and aging process, yet a Sn-rich phase, as the so-called “Sn-patch”, would form in the Ni(V) layer [3]. The possible reason of Sn-patch formation may be the fast Sn diffusion from the solder matrix to the Ni(V) layer. However, the formation mechanism of Sn-patch and the detailed composition variation and structure evolution in Sn-patch were not fully discussed yet. In this study, Sn-patch would be analyzed by a field emission electron probe X-ray microanalyzer (FE-EPMA) and a transmission electron microscope (TEM) to elucidate the composition redistribution and the microstructure evolution, respectively. There existed concentration redistribution of the constituent element in Sn-patch, and its microstructure also varied with the aging time. On the basic of the detailed characterization by FE-EPMA and TEM, it was revealed that Sn-patch was consisted of crystalline Ni and amorphous Sn-rich phase after reflow, while V2Sn3 formed with amorphous Sn-rich phase during aging. A possible formation mechanism of Sn-patch was proposed, which could be employed to explain the corresponding composition variation and structure evolution associated with the Sn-patch formation.

Role of Cu contents in the isothermal metallurgical reaction for the flip chip Sn-3.0Ag-[0.5 or 1.5]Cu joints with Cu/electroless Ni-P/immersion Au bonding pad during aging

Cu/electroless Ni/immersion Au metallization is usually used as bonding pad on the substrate side of flip chip solder joints. Sn-Ag-Cu solder is one of the promising candidates to replace the conventional Sn-Pb solder. Isothermal interfacial reaction in Sn-3.0Ag-(0.5 or 1.5)Cu solder joints with electroless Ni/immersion Au surface finish after aging at 150degC was investigated in this study. Intermetallic compounds (IMC) of (Cu_1-y,Ni_y)₆Sn₅, (Ni_1-x,Cu_x)₃Sn₄ and P-rich layer formed between the solder and the EN layer in both Sn-Ag-Cu joints during aging. For the Sn-3.0Ag-0.5Cu joints after more than 2000 h aging, (Ni_1-x,Cu_x)₃Sn₄ IMC gradually grew. Through the investigation of cross-sectional microstructural observation and quantitative analysis, the interfacial microstructure and the elemental distribution between the solder and bonding pad could be clearly revealed. It was demonstrated that Cu content in the solders near the solder/IMC interface played an important role in the growth of (Ni_1-x,Cu_x)₃Sn ₄ and (Cu_1-y,Ni_x)₆Sn₅