Mechanical properties of sintered Ag–Cu die-attach nanopaste for application on SiC device

In this work, mechanical properties of Ag–Cu nanopaste that formulated by mixing Ag and Cu nanoparticles with organic compounds have been reported. Various weight percents of Cu nanoparticles (20–80 wt%) had been loaded in the nanopaste, in which an increasing trend for hardness, stiffness and Young’s modulus were recorded with the increment of Cu loading. When the nanopaste was used to bond two pieces of Cu substrates, a declining of bonding strength has been recorded with an increasing of Cu loading. For metallization studies, Ag and Au coatings on Cu substrate have displayed the highest (52.6 MPa) and the lowest (34.4 MPa) bonding strength, respectively. The values of bonding strength were found to have a close relationship with the interface microstructure between the nanopaste and metallization layer on the substrate. Finally, the nanopaste was used to attach a SiC die on a substrate with either Ag or Au coating. The entire bonding structure has undergone a thermal aging test, whereby the thermal-aged microstructure was in agreement with the microstructure of metallization studies.     

Fluxless bonding of silicon wafers to molybdenum substrates using electroplated tin-rich solder

A novel fluxless bonding process of silicon wafer on molybdenum substrate is successfully developed. Si-to-Mo bonding can be used for packaging power devices, especially when a device consists of an entire wafer. 300 Å Cr layer and 1,000 Å Au layer are first deposited on Si wafers and Mo substrates. The Cr/Au dual layer is used as underbump metallurgy and seed layer of electroplating. To reduce plastic shear strain on the solder in a bonded pair, thick Sn layer (70 μm) is electroplated over Mo substrates having Cr/Au structure, followed immediately by thin (0.1 μm) Ag layer. This Ag layer acts as the capping layer to prevent inner Sn from oxidation. The bonding process is performed in 50 millitorrs vacuum to inhibit oxidation. The bonding condition is 290 °C for 15 min without the use of any flux. The bonding layer thickness is controlled at 50 μm by small spacers placed between Si wafer and Mo substrate. Microstructure and composition of the joints are studied under scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Scanning acoustic microscopy (SAM) is also used to verify the quality of joints over the entire sample. Without using any flux, high quality and uniform bonding layer is achieved. The composition of the joint is more than 97 at.% Sn. No intermetallic compound layers exist in the joint. This novel fluxless bonding process should be valuable in packaging large high power devices.     

镀Au键合Ag线性能对键合质量的影响研究

利用扫描电镜、能谱仪、拉力-剪切力测试仪等研究了不同镀Au厚度的镀Au键合Ag线Free Air Ball(FAB)特性和不同力学性能的镀Au键合Ag线对键合强度及其可靠性的影响规律，研究结果表明：镀Au键合Ag线镀层厚度过小会造成Electronic-Flame-Off(EFO)过程中的FAB偏球及球焊点形状不稳定，镀层厚度过大会导致FAB变尖；高强度、低伸长率会造成焊点颈部产生裂纹而造成焊点的拉力偏低并在颈部断裂，低强度、高伸长率引起颈部晶粒粗大进而降低颈部连接强度；镀Au键合Ag线颈部应力集中或内部组织结构不均匀，在冷热冲击周期性形变作用下，球焊点颈部产生裂纹并引起电阻增加，进而导致器件失效.     

Thermal fatigue of Ag flake sintering die-attachment for Si/SiC power devices

Emerging SiC power semiconductor devices are expected to work under the high temperature condition of 250–300 °C while the operation of Si devices is limited up to 180 °C. The die-bonding materials for emerging SiC power devices hence need to have sufficient capability in such extreme operating environments. In this study, we investigated the thermomechanical reliability of the die-attach technology using Ag flake paste, which can be processed by low-temperature and low-pressure sintering. The Ag flakes start to sinter immediately after the organic dispersant layer is removed from the flake surface at 160 °C, and die-bonding consequently becomes possible. The tested Si die-attachments joining with the paste maintained high strength (23 MPa) up to 1,000 thermal cycles from ?40 to 180 °C. The stable microstructures without crack and no interfacial debonding assure the reliability of the Ag flake paste die-attach of Si. SiC die-attachments also maintained their high strength (24 MPa) up to 1,000 cycles of ?40 and 250 °C, though a slight degradation appeared after 1,000 cycles. The debondings at the sintered Ag flake paste layer/SiC wafer interface were affected to the joining strength with the Ag flake paste. The obtained results indicate that our Ag flake paste die-attach can be applied to both Si and SiC power devices capable of high temperature operations.     

Influence of multi-interfacial structure on mechanical and tribological properties of TiSiN/Ag multilayer coatings

The TiSiN/Ag multilayer coatings with bilayer periods of ~50, 65, 80, 115, 150, and 410 nm have been deposited on Ti6Al4 V alloy by arc ion plating. In order to improve the adhesion of the TiSiN/Ag multilayer coatings, TiN buffer layer was first deposited on titanium alloy. The multi-interfacial TiSiN/Ag layers possess alternating TiSiN and Ag layers. The TiSiN layers display a typical nanocrystalline/amorphous microstructure, with nanocrystalline TiN and amorphous Si₃N₄. TiN nanocrystallites embed in amorphous Si₃N₄ matrix exhibiting a fine-grained crystalline structure. The Ag layers exhibit ductile nanocrystalline metallic silver. The coatings appear to be a strong TiN (200)-preferred orientation for fiber texture growth. Moreover, the grain size of TiN decreases with the decrease of the bilayer periods. Evidence concluded from transmission electron microscopy revealed that multi-interfacial structures effectively limit continuous growth of single (200)-preferred orientation coarse columnar TiN crystals. The hardness of the coatings increases with the decreasing bilayer periods. Multi-interface can act as a lubricant, effectively hinder the cracks propagation and prevent aggressive seawater from permeating to substrate through the micro-pores to some extent, reducing the friction coefficient and wear rates. It was found that the TiSiN/Ag multilayer coating with a bilayer period of 50 nm shows an excellent wear resistance due to the fine grain size, high hardness, and silver-lubricated transfer films formed during wear tests.     

Influence of Thermal Cycling on the Microstructure and Shear Strength of Sn3.5Ag0.75Cu and Sn63Pb37 Solder Joints on Au/Ni Metallization

The influence of thermal cycling on the microstructure and joint strength of Sn3.5Ag0.75Cu （SAC） and Sn63Pb37 （SnPb） solder joints was investigated. SAC and SnPb solder balls were soldered on 0.1 and 0.9 μm Au finished metallization, respectively. After 1000 thermal cycles between -40℃ and 125℃, a very thin intermetallic compound （IMC） layer containing Au, Sn, Ni, and Cu formed at the interface between SAC solder joints and underneath metallization with 0.1 μm Au finish, and （Au, Ni, Cu）Sn4 and a very thin AuSn-Ni-Cu IMC layer formed between SAC solder joints and underneath metallization with 0.9 μm Au finish. For SnPb solder joints with 0.1 μm Au finish, a thin （Ni, Cu, Au）3Sn4 IMC layer and a Pb-rich layer formed below and above the （Au, Ni）Sn4 IMC, respectively. Cu diffused through Ni layer and was involved into the IMC formation process. Similar interfacial microstructure was also found for SnPb solder joints with 0.9μm Au finish. The results of shear test show that the shear strength of SAC solder joints is consistently higher than that of SnPb eutectic solder joints during thermal cycling.     

Microstructural stability of Ag sinter joining in thermal cycling

As a heat-resistant die attach technology processed at low temperatures, three Ag filler-based sinter joining materials have been proposed. Among these, Ag flake pastes exhibited the greatest potential. Joining was carried out by sintering Ag nanoparticles/flakes in air at 200 °C for 60 min. All of the joined samples survived up to 1,000 thermal cycles in a temperature range from ?40 to 180/250 °C with a 30 min dwell time. In particular, the joining strengths with the Ag micron and, Ag nano-thick flake pastes maintained excellent strength. Neither thermal fatigue cracks nor large voids were observed in the Ag sintered layers. Thus, low-temperature and low-pressure sinter joining with Ag flakes is expected to have an application in high power semiconductor devices for ultra-high temperature operation.     

Microstructural damage analysis of SnAgCu solder joints and an assessment on indentation procedures

In solder joint reliability, solder/pad adjoining interface is crucial, the quality of which is determined by the metallization. In this paper, microstructural analyses of SnAgCu alloy and soldered joints are conducted in direct connection with the metallization. Solder balls, solder paste and cast SnAgCu are reflowed on Cu, Ni/Au and Cu/Ni(V)/Au. Substrate influence on bulk and interfacial metallurgy is examined and a correlation between interfacial microstructure and the corresponding damage paths is established. Damage localizes at the bonding interfaces with a strong influence of intermetallic layers and primary crystals. Crack propagation is studied with Cu and Ni/Au substrates and the cracking mechanism in principal directions is scrutinized. In BGA production, different reflow parameters are investigated, and an optimum bumping procedure is established. Nano-indentation is used for the mechanical characterization of the solder alloy. An assessment on indentation parameters for soft solders is conducted and the influence of Ag content on material properties of SnAgCu is presented.     

Effect of bismuth–tin alloy particle diameter on bonding strength of copper nanoparticles/bismuth–tin solder hybrid joints

The effect of the diameter of Bi–Sn alloy particles on the bonding strength of hybrid joints formed between SiC chips and direct-bonded copper (DBC) plates using a Cu nanoparticles/Bi–Sn solder was studied. The bonding strength was the highest at 40 MPa for a Bi–Sn alloy particle diameter of 10 µm. Further, the bonding strength was dependent on the area of the bonding layer adhering to the SiC-side fracture surface, as determined by the die-shear test. Ni, which was deposited on the SiC chips and DBC plates before the bonding process, remained near the interfacial area of the bonding layer in the joints formed using the 5 µm particles. In contrast, Ni diffused all over the bonding area, with the exception of the interfacial area where Cu–Sn compounds were formed, in the joints produced using the larger alloy particles. The distribution of Sn in the bonding layer became more uniform and the segregation of Bi at the interface became more pronounced as the particle size was reduced. Further, with an increase in the particle size, the Ag layers deposited on the surfaces of the SiC chips and DBC plates diffused into the bonding layer after the first firing step at 473 K, which was performed before the secondary firing step at 623 K. These results imply that the diameter of the Bi–Sn solder particles in hybrid joints affects the interfacial structure, as it governs the wetting behavior of the Bi–Sn solder and hence has a determining effect on the bonding strength.     

Studies of CrNAu as an improved die attach metallization system

The increased incidence of premature integrated circuit die attach failure in ceramic packages has recently become a problem of major concern. It appears that the conventional “back-side gold” process has become marginal from the standpoint of bond strenght and reliability when used in conjunction with the conventional “eutectic die attach” process. This study shows that this trend may be due, in part, to the basic metallurgical instability of the “back-side gold” process, i.e. heat treatment of this metallization in the eutectic bonding temperature range (375–425°C) is shown to result in rapid and extensive growth (about 10 nm min^?1) of silicon oxide over the gold metallization. The resultant oxide film is known to degrade profoundly the bondability of the gold metallization. The movement of the silicon through the gold layer is thought to proceed via grain boundary interdiffusion. Thus, dice which were initially easily bondable can become non-bondable by extended storage at room temperature prior to the die attach step in processing.In this study we describe a metallization scheme (CrNAu) which can easily withstand eutectic bonding temperatures for short periods with minimal oxide growth. Additionally, data comparing shear force strength and thermal shock testing between the two metallization will be presented. It is shown that use of the CrNAu system results in considerable die attach yield improvement and greater strength and reliability over the conventional “back-side gold” process. The use of Auger electron spectroscopy for such an investigation is shown to be a very definitive tool.     

Structural,optical and electrical properties of Ag doped PbS thin films: role of Ag concentration

The present work reports on the chemical synthesizes of (0–8 at.%) silver (Ag)-doped PbS thin films with tunable opto-electrical properties. From the X-ray diffraction analyses, it was understood that the preferred growth orientation of Ag:PbS films was dependent on the Ag doping concentration. The variation in the Ag:PbS films orientation was reflected in the film morphology as observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM studies revealed that the variation in Ag concentration lead to different grain shapes for different grain orientations. The AFM study showed that the RMS roughness of the undoped PbS film has been reduced considerably due to silver doping. From the optical studies, a widening in the optical band gap was revealed after Ag-doping due to the quantum confinement effect. It was obtained that 4 at.% Ag-doped PbS thin films display an optimum band gap value of 1.45 eV. As for electrical characterization result, the resistivity reduces and the carrier density improved with 4 at.% Ag concentration. Based on all the data, it was concluded that the 4 at.% Ag-doped PbS thin film showed the best morphological, optical and electrical behavior, which recommend it as an active layer for solar cell devices.     

Electrical characteristics and detailed interfacial structures of Ag/Ni metallization on polycrystalline thermoelectric SnSe

SnSe is a promising thermoelectric material with a high figure of merit in single crystal form, which has stimulated continuous research on polycrystalline SnSe. In this study, we investigated a metallization techniques for polycrystalline SnSe to achieve highly efficient and practical SnSe thermoelectric modules. The Ag/Ni metallization layers were formed on pristine polycrystalline SnSe using various deposition technique: sputter coating Ni, powder Ni and foil Ni by spark plasma sintering. Structural analysis demonstrated that the microstructure and contact resistance could be different according to the metallization process, despite using the same metals. The Ag/Ni metallization layer using foil Ni acted as an effective diffusion barrier and minimized electrical contact resistance(2.3 × 10~(-4) Ω cm~2). A power loss in the thermoelectric module of only 5% was demonstrated using finite element simulation.     

Characteristics of Laser Reflow Bumping of Sn3.5Ag and Sn3.5Ag0.5Cu Lead-Free Solder Balls

ead-free Sn3.5Ag and Sn3.5Ag0.5Cu solder balls were reflowed by laser to form solder bumps. Shear test was performed on the solder bumps, and SEM/EDX （scanning electron microscopy/energy dispersive X-ray spectrometer） was used to analyze the formation of intermetallic compounds （IMCs） at interface region. A finite element modeling on the temperature gradient and distribution at the interface of solder bump during laser reflow process was conducted to elucidate the mechanism of the IMCs growth direction. The results show that the parameters window for laser reflow bumping of Sn3.5Ag0.5Cu was wider than that of Sn3.5Ag. The shear strength of Sn3.5Ag0.5Cu solder bump was comparable to that of Sn3.5Ag solder bump, and was not affected obviously by laser power and irradiation time when appropriate parameters were used. Both laser power and heating time had a significant effect on the formation of IMCs. A continuous AuSn4 intermetallic compound layer and some needle-like AuSn4 were observed at the interface of solder and Au/Ni/Cu metallization layer when the laser power is small. The formation of needle-like AuSn4 was due to temperature gradient at the interface, and the direction of temperature gradient was the preferred growth direction of AuSn4. With increasing the laser power and heating time, the needle-like AuSn4 IMCs dissolved into the bulk solder, and precipitated out once again during solidification along the grain boundary of the solder bump.     

Modified Ni/Pd/Au-finished DBA substrate for deformation-resistant Ag–Au joint during long-term thermal shock test

An Ag–Au joint developed by Ag paste joining on electroless nickel/electroless palladium/immersion gold (ENEPIG)-plated direct-bonded aluminum (DBA) substrate was employed in SiC (Silicon carbide) power modules. The ENEPIG plating was modified to possess high mechanical strength by thickening the Ni layer to 20 μm. The reliability of SiC/DBA die-attached module by Ag–Au joining was evaluated during a long-term thermal shock test (TST) from ??50 to 250 °C up to 2000 cycles. The shear strength of as-sintered Ag–Au joint was evaluated to be 37.6 MPa, but it showed a significant decrease after 1000 thermal cycles and maintained stably from 1000 cycles to 2000 cycles. Based on the microstructural evolution via EBSD observation, it was confirmed, by modifying ENEPIG, the Ag–Au interfacial deformation derived from Al plastic deformation was successfully suppressed even after 2000 cycles. In this case, the mechanism for shear strength degradation was found to be the interfacial delamination between Ag paste layer and ENEPIG substrate. This can be ascribed to the large thermal stress caused by different coefficients of thermal expansion (CTE) of power module components. This study not only realized the deformation-free structure for Ag–Au joint during long-term thermal cycling but also provided fundamental insights for fracture behaviors of Ag–Au sinter joint.

     

Synthesis of Au-Ag alloy nanoparticles with Au/Ag compositional control in SiO2 film matrix

Au-Ag alloy nanoparticles with tunable atomic ratios have been generated in SiO2 film matrix using a new two layer (TL) approach. Two successive overlapping coating layers of similar thickness were deposited on silica glass substrates using Au- and Ag-incorporated inorganic-organic hybrid silica sols, respectively. The Au and Ag concentrations in the individual layers were varied to obtain the desired Au-Ag alloys of different compositions. Four sets of such TL coating assemblies were prepared from the following pair of sols: (i) 4 equivalent mol.% Au-96% SiO2 and 2 equivalent mol.% Ag-98% SiO2, (ii) 3 equivalent mol.% Au-97% SiO2 and 2 equivalent mol.% Ag-98% SiO2, (iii) 3 equivalent mol.% Au-97% SiO2 and 3 equivalent mol.% Ag-97% SiO2, and (iv) 2 equivalent mol.% Au-98% SiO2 and 3 equivalent mol.% Ag-97% SiO2 and subjected to UV (2.75 J/cm2) and heat-treatments (450-550 degrees C) in air and H2-N2 atmospheres for the generation of Au-Ag alloy nanoparticles of approximate compositions Au.66Ag0.33, Au0.6Ag0.4, Au0.5Ag0.5, and Au0.4Ag0.6, respectively. After UV-treatment, individual Au and Ag nanoparticles were formed in the respective layers. The heat-treatment (450-550 degrees C) induces interlayer diffusion of Au and Ag to each other with the generation of Au-Ag alloy nanoparticles, and as a result, Au-Ag alloy surface plasmon resonance (SPR) absorptions were observed in between the Ag- and Au-SPR absorption positions in the visible spectra. The expected alloy compositions are formed through several intermediate alloy nanoparticles, which can also be arrested by controlling the annealing parameters. The alloy formations were monitored by UV-VIS, FTIR, XRD, EDAX, and TEM studies.     

Influence of cerium oxide (CeO2) nanoparticles on the microstructure and hardness of tin–silver–copper (Sn–Ag–Cu) solders on silver (Ag) surface-finished copper (Cu) substrates

Nano-sized, non-reacting, non-coarsening CeO₂ particles with a density close to that of solder alloy were incorporated into Sn–3.0 wt%Ag–0.5 wt%Cu solder paste. The interfacial microstructure and hardness of Ag surface-finished Cu substrates were investigated, as a function of reaction time, at various temperatures. After the initial reaction, an island-shaped Cu₆Sn₅ intermetallic compound (IMC) layer was clearly observed at the interfaces of the Sn–Ag–Cu based solders/immersion Ag plated Cu substrates. However, after a prolonged reaction, a very thin, firmly adhering Cu₃Sn IMC layer was observed between the Cu₆Sn₅ IMC layer and the substrates. Rod-like Ag₃Sn IMC particles were also clearly observed at the interfaces. At the interfaces of the Sn–Ag–Cu based solder-Ag/Ni metallized Cu substrates, a (Cu, Ni)–Sn IMC layer was found. Rod-like Ag₃Sn and needle-shaped Cu₆Sn₅ IMC particles were also observed on the top surface of the (Cu, Ni)–Sn IMC layer. As the temperature and reaction time increased, so did the thickness of the IMC layers. In the solder ball region of both systems, a fine microstructure of Ag₃Sn, Cu₆Sn₅ IMC particles appeared in the β-Sn matrix. However, the growth behavior of the IMC layers of composite solder doped with CeO₂ nanoparticles was inhibited, due to an accumulation of surface-active CeO₂ nanoparticles at the grain boundary or in the IMC layers. In addition, the composite solder joint doped with CeO₂ nanoparticles had a higher hardness value than the plain Sn–Ag–Cu solder joints, due to a well-controlled fine microstructure and uniformly distributed CeO₂ nanoparticles. After 5 min of reaction on immersion Ag-plated Cu substrates at 250 °C, the micro-hardness values of the plain Sn–Ag–Cu solder joint and the composite solder joints containing 1 wt% of CeO₂ nanoparticles were approximately 16.6 and 18.6 Hv, respectively. However after 30 min of reaction, the hardness values were approximately 14.4 and 16.6 Hv, while the micro-hardness values of the plain Sn–Ag–Cu solder joints and the composite solder joints on Ag/Ni metallized Cu substrates after 5 min of reaction at 250 °C were approximately 15.9 and 17.4 Hv, respectively. After 30 min of reaction, values of approximately 14.4 and 15.5 Hv were recorded.     

GZO/Ag/GZO多层薄膜制备、结构与光电特性的研究

采用射频磁控溅射和离子束溅射联合设备在玻璃衬底上制备出了具有良好附着性、低电阻率和高透过率的GZO/Ag/GZO(ZnO掺杂Ga_2O_3简称GZO)多层薄膜.X射线衍射谱表明GZO/Ag/GZO多层薄膜是多晶膜,GZO层具有ZnO的六角纤锌矿结构,最佳取向为(002)方向;Ag层是立方结构,具有(111)取向.在GZO层厚度一定的情况下,研究了Ag层厚度的变化对多层膜结构以及光电特性的影响.研究发现,当Ag层厚度为10nm时,3层膜的电阻率为9×10~(-5)Ω·cm,在可见光范围内平均透过率达到89.7%,薄膜对应的品质因子数值为3.4×10~(-2)Ω~(-1).     

Microstructure and interfacial reaction of Sn–Zn–x(Al,Ag) near-eutectic solders on Al and Cu substrates

Sn–Zn–x(Al,Ag) near-eutectic solders, namely Sn–8.3Zn–0.73Ag, Sn–8.4Zn–0.44Al and Sn–7.4Zn–0.26Al–0.68Ag (in wt%) with melting points of 200.74, 198.00 and 197.32 °C, respectively, as well as the Sn–9Zn eutectic solder, were used to join Al and Cu substrates. The addition of Ag led to the formation of dendritic AgZn₃ phases, while the addition of Al obviously refined the microstructure of Sn–Zn eutectic, as well as the AgZn₃ phases. The Sn–Zn–Al solder possessed the best wettability on both Cu and Al substrates among the four solders. Al_4.2Cu_3.2Zn_0.7 intermetallic compound (IMC) layers formed at the Sn–Zn–x(Al,Ag)/Cu interfaces while Al-rich (Zn) solid solutions at the Sn–Zn–x(Al,Ag)/Al interfaces of all the as-soldered joints. The shear strength of the Al/Sn–Zn–Al/Cu solder joints was the highest among the four solder joints. The declining degree of the shear strength of the Sn–Zn–x(Al,Ag) solder joints in 3.5 % NaCl solution was in agreement with the corrosion-resistance order of the bulk solders. The Al/Sn–Zn–Ag/Cu joint thus owned the best corrosion resistance.     

Epitaxy of noble metals and (111) surface superstructures of silicon and germanium part I: Study at room temperature

We have studied the growth mode and orientation at room temperature of Ag and Au deposits on the (111) cleavage faces of Si and Ge. Two-dimensional epitaxial layers are formed. In addition, we have observed by LEED and RHEED new superstructure patterns which depend strongly on the initial structural state of the substrates.     

Microstructural variation and high-speed impact responses of Sn–3.0Ag–0.5Cu/ENEPIG solder joints with ultra-thin Ni–P deposit

Electroless Ni–P/electroless Pd/immersion Au (ENEPIG) with ultra-thin Ni–P deposit serve as a potential replacement of traditional ENEPIG surface finish because of its superior electrical performance in flip chip solder joints interconnection. However, the interfacial reaction and mechanical reliability of solder joints in ENEPIG with ultra-thin Ni–P layer is not yet well evaluated. In this study, we investigated the characteristic microstructure of interfacial intermetallic compounds and high-speed impact responses of Sn–3.0Ag–0.5Cu/ENEPIG attachments with 4.8, 0.3, and 0.05 μm Ni–P deposit. ENEPIG with Ni–P layer of 0.3 μm exhibited the eutectic structure dispreading in the solder alloys and layer-type P-rich IMCs at solder/metallization interface, while there was (Cu,Ni)₆Sn₅ precipitation in the solder but no P-rich IMCs layer formed in ENEPIG with 0.05 μm Ni–P layer. Slower interfacial reaction rate in ENEPIG with 0.3 μm Ni–P layer was attributed to the effect of electroless Ni–P diffusion barrier layer, which would further provide better impact resistivity than that of ENEPIG with 0.05 μm Ni–P deposit. Moreover, breach in P-rich IMCs and underneath (Cu,Ni)₆Sn₅ patch were observed in ENEPIG with 0.3 μm Ni–P layer. The growth mechanism was closely related to the Ni diffusion from surface finish and element redistribution.