Materials and Design 96 (2016) 251–256
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Composite electroplating to enhance the GMI output stability of melt-extracted wires Jingshun Liu a,⁎, Ze Li a, Hongxian Shen b, Faxiang Qin c, Sida Jiang b, Zhaoxin Du a, Manh-Huong Phan d, Jianfei Sun b a
School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Institute for Composites Science Innovation (InCSI), School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China d Department of Physics, University of South Florida, Tampa, FL 33620, USA b c
a r t i c l e
i n f o
Article history: Received 24 November 2015 Received in revised form 17 January 2016 Accepted 20 January 2016 Available online 22 January 2016 Keywords: Amorphous wires Wire-terminal-connection Ni/Cu composite electroplating Impedance output stability
a b s t r a c t Impedance output stability of Co-rich amorphous wires depends largely on the wire-terminal-connection, which can be accomplished by composite electroplating towards a robust sensor application. The present experimental results indicated that Ni/Cu composite layer composed of Ni, Cu plating and passivating treatment show a homogeneous microstructure, and excellent surface wettability, weldability and thermal expansion coefficient matching under variable temperature of wire-terminal-connecting. The properties of composite electroplated layer are mainly influenced by cathode current density, electroplating time and electrolyte temperature. The inhomogeneous distributed clustering-regions, disproportionated reaction, tendency of hydrogen brittleness and crack have been reduced by optimized coactions of intermolecular Van der Waals force, inner diffusion and mechanical bonding of electroplated layer. Therefore, the wire-terminal-connection is able to effectively improve impedance output stability by minimizing the disturbance of stray capacity, decreasing radio-frequency wave emission and electromagnetic signal attenuation, and suppressing the destabilization or concussion. © 2016 Published by Elsevier Ltd.
1. Introduction Magnetic amorphous microwires have drawn much research interests owing to their potential sensor applications based on GMI properties, especially in providing the effective detection with respect to weak magnetic fields [1–6]. Importantly, the impedance output stability or reliability of wires as one of the bottleneck problems, which severely hampers magnetic performance testing, micro-electro-mechanical systems (MEMS) packaging of sensitive materials and miniaturization of sensor application [7–9]. Compared with several influencing factors (including magnetic field, ambient temperature and circuit noise) on impedance output stability, the contact stability of wire-connection plays more significant role in governing the working reliability of the sensor signal processing circuit [10–12]. In literature, some wire-connection methods (including spot welding, miniature specimen holder or micro-bolt fixed) have been mainly attempted and conducted by different researchers during GMI measurement and annealing circuits [13,14]. But the mentioned methods have some stringent requirements for final fixture, which results in the unstable output of impedance data. Y. Honkura et al. [15, 16] in Aichi steel company of Japan implemented the stable wire⁎ Corresponding author. E-mail address:
[email protected] (J. Liu).
http://dx.doi.org/10.1016/j.matdes.2016.01.090 0264-1275/© 2016 Published by Elsevier Ltd.
bonding between amorphous wires and conductively ceramic electrodes by adoption of ultrasonic welding method, i.e., wires were put into resin-based mold, while coil bias and feedback loops were outside the mold around, then the 6-pin chip packages of sensitive components in small dimension were realized. However, the ultrasonic welding and molding technology is seriously limited by packaging accuracy and wire-connection operated process, even including large pressed stress and local crystallization resulted from ultrasonic welding. So it is necessary to explore novel connecting ways to solve above mentioned problems. Applied transition layer between the wires and solder is an effective method for improving the wettability such as physical (chemical) vapor deposition, electroplating and chemical plating, thus achieving stable connection in electronic package. Among them, electroplate possesses many advantages such as simple equipment [17], strong operability, flexibly controlling the coating thickness and high efficiency. Reasonable plating metal layer can effectively improve the surface wetting property and wire-connection quality of electronic circuit. Another point for holding up the working stability and reliability is thermal expansion coefficient matching between different coatings and matrix. In this regard, little work has been done, therefore it is worthwhile to explore the mechanism of wire-terminal-connection with plated metallic layer for improving the wire-connection mode and impedance output stability.
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In present work, we have explored the wire-terminal-connection by Ni/Cu composite electroplating based on the optimization of electroplated processing parameters; and revealed the correlation between wire-terminal-connecting and impedance output stability.
The giant magnetoimpedance ratio of ΔZ/Z0 is defined as follows [19]:
1.1. Experiment description
where the applied axially external magnetic field Hex is supplied by a solenoid ranging from 0 to 4.5 Oe along the wire-axis, Z(Hex) and Z(H0) corresponds to the impedance values of the measured axially Hex and the initial magnetic field H0, respectively. According to the values of GMI ratio based on Eq. (1), the maximum ΔZ/Z0(%) at corresponding magnetic field is denoted by [ΔZ/Z0]max. In addition, the voltage signal output of wire-terminal-connection was also monitored by a mixed digital signal oscillograph (type: Agilent MSO9104A) with a bandwidth of 1 GHz in order to effectively validate GMI output characteristic. All measurements were together conducted at room temperature (25 °C).
Master alloy rod with a diameter of 8 mm and nominal composition of Co68.2Fe4.3B15Si12.5 (in at. %) was firstly fabricated by vacuum arcmelting and copper mold under a pure argon atmosphere and then re-melt in a precision melt-extraction facility. Melt-extraction process was conducted by using a copper wheel with a 60° V-shaped edge, fixed constant linear velocity of 25 m/s for wheel rim and stable feeding rate of 15 μm/s, and then continuous microwires with diameters of ~40 μm were fabricated. Generally, the as-prepared wires possess the superior magnetic properties due to the amorphous nature obtained by high solidification velocity. Ni/Cu composite electroplating was conducted on a home-made electrochemical reaction platform. The electrolyte composition of Cu electroplating consists of copper sulfate (CuSO4·5H2O: 250 g/L), sulfuric acid (H2SO4: 75 g/L), potassium chloride (KCl: 150 mg/L), glucose (50 mL/L) and sulfocarbolate acid (10 mL/L), and that of Ni electroplating consists of nickel sulfate (NiSO4: 300 g/L), nickel chloride (NiCl2: 50 g/L), boric acid (HBO3: 40 g/L) and lauryl sodium sulfate (C12H25-OSO3Na: 100 mg/L). And their optimized processing parameters for Ni electroplating (4.0 A/dm2, 230 s, 55 °C and pH = 4.5) and Cu electroplating (55.84 A/dm2, 60 s, 40 °C and pH = 7.0) were given in previous reports [11,12]. After that the chemical passivating treatment was also carried out by the solution treating for 30 s with a proportion of chromic anhydride (CrO3: 100 g/L), sulfuric acid (H2SO4: 35 g/L) and sodium chloride (NaCl: 2 g/L). Surface and cross-section morphology of composite electroplated wire-terminal were observed using a field emission scanning electron microscopy (SEM, Helios Nanolab600i) at 20 kV equipped with an energy dispersive spectroscopy (EDS) detector for composition linear scanning. The scarification tests including both friction force and acoustic signal ratio modes were performed on a MFT4000 multi-functional materials surface tester with a sliding length of 5 mm and loading speed of 20 N/min ranging from 0 to 20 N for evaluating the combination property of interface between electroplated layer and alloy matrix. Thermal expansion coefficient (TEC) was measured on a thermal expansion instrument (type: NETZSCH DIL 402C) with a wide range of − 60 °C ~ + 400 °C and heating-up speed of 5 °C/min in protective atmosphere of liquid nitrogen; the dimension of sample is Φ6 mm × 25 mm. Video-based contact angle measurement device (type: DataPhysics OCA 20LHT) was employed to observe the dynamic contact characteristics according to the drop shape of different interfaces in a high-vacuum pipe-still heater. GMI output stability of amorphous wires are evaluated according to the stability of oscillograph voltage wave form during GMI measurement in a magnetically shielded space, thus acquiring the effectively candidate wire-terminal-connection for practical wire-electronicpackaging applications of GMI sensor. The impedance measurement adopted the typical four-point method was conducted on an Agilent 4294A precision impedance analyzer, which was placed in a self-designed magnetically shielded space (MSS) at frequency of 100 kHz–15 MHz with a driving current amplitude (20 mA). And the microwire with 24 mm long and electroplated two-terminal length of 4 mm was connected on printed circuit board (PCB) using lead-free solder [18], which is composed of tin (Sn: 96.5%), silver (Ag: 3.0%), and copper (Cu: 0.5%) in wt.%, as denoted by SAC (the first word of each element). The wireterminal-connection conforms to the international standard for electronic packaging.
ΔZ Z ðH ex Þ−Z ðH0 Þ 100%; ð%Þ ¼ Z0 Z ðH 0 Þ
ð1Þ
2. Results and discussion 2.1. Wire-terminal-connection by composite electroplating 2.1.1. Mechanism of composite electroplating Based on the previously experimental study, the wire-terminalconnection with Cu electroplating can significantly improve the surface wetting property [11], but it is difficult to achieve the stability at the changing condition of ambient temperature [20]. The Cu electroplated layer shows a large heat conductivity, low resistivity, slight electromagnetic influence, good electroplating flexibility and excellent wetting property, while Cu electroplated layer, with a relatively larger TEC than that of CoFeBSi matrix, is easily affected by ambient temperature due to TEC mismatching. According to the matching rule of TEC, thermal stress can be expressed as [21]: Z σ ðT Þ ¼
E1 ΔαðT ÞdT
ð2Þ
where E1 is elastic modulus of solder, Δα(T) is the TEC difference between solder and matrix metal under different temperature, which can be defined as [21]: ΔαðT Þ ¼ α 1 ðT Þ−α 2 ðT Þ:
ð3Þ
Fig. 1. Temperature dependence of thermal expansion coefficient (TEC) of CoFeBSi alloy, Cu and Ni, and inset indicates the schematic diagram of Ni/Cu compound electroplated layer.
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In practice, the large thermal stress σ(T) is adverse for electroplated layer and matrix, and the Δα(T) should be as small as possible from Eq. (2). Accordingly, it is necessary to seek a kind of transition electroplated layer between Cu plating layer and metallic substrate, namely the difference of their TEC coefficients is approximately small among substrate, transition layer and electroplated layer. Fig. 1 shows the temperature dependence of thermal expansion coefficient (TEC) of CoFeBSi alloy, Cu and Ni. Therefore, the basic principle of Ni/Cu composite electroplated layer is described in the inset of Fig. 1. Specifically, Ni layer as the transition layer could satisfy the intermediate TEC value between substrate and Cu electroplated layer at wide temperature range. By optimizing Ni electroplating, TEC value falls in between the Cu layer and CoFeBSi alloy matrix, the matching nature of Ni electroplated layer and alloy matrix is superior for its adsorption and combining ability with the substrate and Cu plating layer. But the Ni electroplated shows poor wettability and weak magnetism; the layer thickness should be strictly controlled to be as thin as possible. Therefore, Ni/Cu composite electroplated layer with good TEC matching and wettability properties can effectively enhance the stability of wireterminal-connection and actually wire-electronic-packaging under the temperature changing condition.
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passivating treatment (30 s) of Cu electroplated layer. Accordingly, Fig. 2 a) and b) shows the surface and cross-section morphologies of Ni/Cu electro-plated layer at the optimized current density and electroplating time. The Ni/Cu electroplated layer is continuous and uniform while the combinations of Ni–Cu layer and Ni/Cu-matrix layer exhibit high tightness. Moreover, its linear scanning element distribution of chemical composition including Co, Fe, B and Si in radial direction of microwire exhibits homogenous. Obviously, the curve of Ni linear scanning element distribution shows prominency with a thickness of 1.5 μm at two-side, and that of Cu shows a thickness of 5 μm at twoterminal, as exhibited in Fig. 2 c). 2.3. Wetting and bonding property of composite electroplating On the other hand, the wetting mechanism is closely related to the wetting angle θ and spreading coefficient SL/S between different electroplated interfaces and solder, both θ and SL/S are generally formulated as the corresponding expressions: [22] cos θ ¼
σ S−G −σ S−L σ L−G
ð4Þ
2.2. Microstructure of composite electroplating
SL=S ¼ Raðσ S−G −σ S−L Þ−σ L−G ¼ σ L−G ðRa cos θ−1Þ
ð5Þ
We conduct the related analysis of the influencing and wetting properties of the Cu and the Ni electroplated layers under different parameters (i.e. current density of cathode, electroplating duration, and the temperature and pH values of electrolyte) based on the microstructural evolution and the dynamic characteristics. The optimized processing parameters of Ni/Cu composite electroplated layers based on our previous research reports [11,12] satisfy the TEC matching rule, and they consist of Ni electroplating (4.0 A/dm2, 230 s, 55 °C, pH = 4.5), Cu electroplating (55.84 A/dm2, 60 s, 40 °C, pH = 7.0) and chemical
where σS-G, σS-L and σL-G are the interface tensions of solid–gas, solid– liquid and liquid–gas, respectively. Ra means the roughness concentration of solid interface. From Eq. (4), as for σS–G N σS–L, θ b 90°, it means that the smaller wetting angle of the interface, the better wetting characteristics. And it is the absolutely wetting and spreading state when θ = 0° and SL/S = 0. On the other hand, as for SL/S b 0, it indicates the liquid is hard to spread on solid surface. According to Eq. (5), the SL/S of rough surface is much larger than that of smooth surface, and it can easily spread on the rough surface.
Fig. 2. Surface (a) and cross-section (b) morphology of Ni/Cu composite electroplated layer at the optimized current density and electro-plating time and its linear scanning distribution of chemical composition in radial direction of microwire (c).
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Fig. 3. Contrast of surface wetting morphology based on OCA images of CoFeBSi alloy before & after Ni/Cu composite electroplating: (a) non-electroplating surface (І); and (b) electroplated surface (ІІ).
Fig. 3 reveals the contrast between wetting surface morphology of Ni/Cu composite electroplated layer and CoFeBSi alloy. The SAC leadfree solder spreads obviously on composite electroplated surface. Compared with the conventional connection mode, the wetting angles decrease from 85.5° for CoFeBSi smooth surface to 42.2° for Ni/Cu electroplating rough surface, namely, the appropriate roughness can effectively improve the wetting property.
In addition, the bonding strength (so-called tensile strength) test between electroplated layer and the substrate alloy is applied to judge if it can satisfy the device packaging requirements. Generally, scarification test as the direct and effective inspection method is adopted. Fig. 4 shows the analysis of bonding properties of Cu, Ni and Ni/Cu composite electroplated layers by using scarification test. The results of scarification tests under dual mode of friction force and acoustic signal ratio show that the electroplating method could obtain good combined performances between electroplated metallic layer and CoFeSiB alloy matrix. Acoustic signal detection can directly show the cohesive failure fracture of electroplating layers, and the acoustic emission signal increases suddenly as shown in Fig. 4 b). The critical load of Cu and Ni uni-electroplated layers obtained under optimization parameters achieve 5.86 N and 12.26 N, respectively, and the composite electroplated Ni/Cu layer composed of outer Cu layer and inner Ni layer showing better performance: 1) the critical load of outer Cu layer is around 3.91 N, which is slightly lower than that of uni-electroplated Cu layer; 2) the critical load of inner Ni electroplated layer combined with matrix reaches to 14.43 N, which is much higher than that of uni-electroplated Ni layer. In a word, the surface wetting and bonding properties are improved significantly by composite electroplated treatment. This method mainly reduces clustering regions with inhomogeneous distribution, disproportionate reaction and tendency of hydrogen brittleness and crack, finally achieves the stable connection by coactions of van der Waals attraction, inner diffusion and mechanical bonding. Moreover, it can be realized that wire-terminal-connection has practical significance for the magnetic sensor application of sensitive materials under adverse conditions (mechanical vibration, ambient temperature change).
3. Giant magnetoimpedance output stability based on Ohm contact theory 3.1. Existence of impedance output stability
Fig. 4. Comparison of combination or bonding property for dual mode: friction force (a) and acoustic signal ratio (b) between the different electroplated layers and their matrix in scarification testing.
Fig. 5 illustrates the comparisons of impedance and GMI (Z and ΔZ/ Z0) output stabilities of CoFeSiB wire before and after the Ni/Cu composite electroplating. GMI output characteristic of CoFeSiB wire with Ni/Cu composite layer has similarly changing trend in contrast to single electroplating (Cu or Ni layer). There exists an obvious fluctuation in external magnetic field of 1.5–4.0 Oe, especially for 1.5 Oe, 3 Oe and 4 Oe. Additionally, with the aid of the amplification of local grid, the grid lines exhibit the non-linear and unstable fluctuation with the increase of the external magnetic field at range of 8–12 MHz. Compared with the local mesh magnification in circle region of non-electroplated wire (as shown in Fig. 5 b)), the impedance variation and impedance output stability have been effectively improved to a certain extent (as shown in Fig. 5 d)) after wire-terminal-connection with Ni/Cu composite electroplated layer. In order to confirm the existence of the impedance fluctuation change before and after the electroplating, we use the highperformance oscilloscope probe with weak signal attenuation characteristics to detect the internal changes of the voltage signal and the reliability. The compared analysis of oscilloscope waveform outputs is shown in Fig. 6. We found that the output voltage waveform change appears fluctuations especially in both peaks and trough position of sine signal for non-electroplated layer wire-terminal-connection when the alternating current (AC) excitation frequency equal to 10 MHz [23]. The maximum wave amplitude is about 120 mV and the waveform exhibits irregular change. Differently, the output voltage waveform almost keeps regular sine waveform for the wave peak and trough positions without obvious fluctuation during wire-terminal-connection tests with composite electroplated layer.
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Fig. 5. Impedance Z output stabilities and the corresponding GMI ratio ΔZ/Z0 output stabilities versus frequency (0.1–15 MHz) and magnetic field (0–4.25 Oe). a) and c) are the impedance outputs of non-electroplated wire and Ni/Cu composite electroplated wire; b) and d) are the GMI outputs of non-electroplated wire and Ni/Cu composite electroplated wire, respectively.
3.2. Explanation for correlation of wire-terminal-connection and impedance output stability The related mechanism analyses of improvement for GMI output stability using wire-terminal-connection with Ni/Cu composite electroplated layer were described as follows: 1) the SAC solder joint with composite electroplated layer possesses superior bonding strength and wetting characteristics; 2) it can effectively reduce the radio frequency emission of noise signal, attenuation of high frequency electromagnetic signal and interference of stray or parasitic capacitance; 3) it also can effectively restrain disturbance, vibration and impact caused by unstable contact phenomenon at a relatively high frequency (f ≥ 10 MHz); 4) the unstable impedance output probably resulted
Fig. 6. Oscillograph voltage signal outputs of different wire-terminal-connections as f = 10 MHz: (a) non-electroplated wire; (b) Ni/Cu composite electroplated wire. And (c) illustrates the effect of formation of RLC parallel resonant circuit on impedance output characteristic for wire-connecting of non-electroplated wire.
from both alternating magnetization under the action of alternating magnetic field and mechanical vibration near parallel resonance frequency of ferromagnetic microwire during impedance measurement. Herein, magnetic resonance absorption occurs when the exciting current frequency is consistent with mechanical vibration frequency, which influences the changes of complex permeability and impedance output value; 5) the formation of resistance inductance capacity (as denoted by RLC) parallel resonant circuit: CoFeBSi wire with poor wetting behavior brings on existence of wire-connecting microgaps between surface and solder joint for traditional connection (wire-terminal-connection with non-electroplated layer); with the wire's resistor Rwire and inductor Lwire, the interfered capacitance Cgap, formed by the welding plate with a certain area, forms RLC parallel resonant circuit under the action of alternating current, as shown in Fig. 6 c). Moreover, Z value changes instantaneously when the excitation frequency fp near resonance frequency f0, which results in ΔZ/Z0 fluctuation change. According to the Ohm contact theory [21], there are different contact resistances between different metals. The magnitude level of parasitic capacitance achieves ~ pF, which is almost equal to the magnitude of parasitic capacitance caused by unstable phenomenon of oscilloscope probe contact. In addition, the obvious impedance fluctuations are resulting from the TEC mismatching between composite electroplated layer and microwire under the changing ambient temperature, including rosin joint areas on the end of wire due to the large difference of TEC, the rapid increase of contact resistance for spot welding (rosin joint) during high-frequency measuring process and the temperature rise of microwire. Adopting Ni/Cu composite electroplated layer can effectively avoid TEC mismatching problem to improve the stable and reliable electronic circuit connection. Therefore, Ni/Cu composite electroplated layer connection or bonding on both wire-terminals mostly becomes one of the effective choices as practically electronic packaging for GMI sensor.
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4. Conclusions In summarizing, wire-terminal-connection accomplished by Ni/Cu composite electroplating effectively improved the stability of GMI output for amorphous wires. Accordingly, the optimal processing parameters of Ni/Cu composite electroplated layer with TEC matching are Ni electroplating (4.0 A/dm2, 230 s, 55 °C and pH = 4.5), Cu electroplating (55.84 A/dm2, 60 s, 40 °C and pH = 7.0), and chemical passivating treatment (30 s) of Cu electroplated layer. When wetting angles decrease from 85.5° of alloy surface to 42.2° of Ni/Cu electroplating surface, the stable and reliable electronic circuit connection was achieved. Notably, scarification test under dual mode of friction force and acoustic signal shows that the excellent bonding properties between different electroplated layers and CoFeSiB alloy matrix. The critical load of outer Cu layer and inner Ni electroplated layer for Ni/Cu composite electroplated layer are ~3.91 N and ~ 14.43 N, respectively. Moreover, wire-terminal-connection with Ni/Cu composite electroplated layer could effectively avoid the formation of RLC parallel resonant circuit and Z instantaneous fluctuations. Meanwhile, Ni/Cu composite electroplated layer can reduce the radio frequency emission of noise signal, the attenuation of high frequency electromagnetic signal and the interference of stray/parasitic capacitance, even can effectively avoid the disturbance, vibration and impact caused by unstable contact phenomenon and mechanical vibration at relatively high frequencies. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (NSFC) under grant Nos. 51401111 and 51561026, Natural Science Foundation of Inner Mongolia Autonomous Region of China under grant No. 2014BS0503. FXQ would like to thank the support from NSFC under grant No. 51501162.
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