Analysis on interfacial reactions between Sn–Zn solders and the Au/Ni electrolytic-plated Cu pad

doi:10.1016/j.jallcom.2004.03.138

Journal of Alloys and Compounds

Volume 379, Issues 1–2, 6 October 2004, Pages 314-318

https://doi.org/10.1016/j.jallcom.2004.03.138 Get rights and content

Abstract

Interface reactions between Sn–Zn lead-free solders and the Au/Ni electrolytic-plated Cu pad after isothermal aging were investigated. The intermetallic compounds at the interface between solder and the Au/Ni electrolytic-plated Cu pad were examined by field-emission scanning electron microscopy and transmission electron microscopy. During the reflow, Zn firstly reacted with Au and then was transformed to the β-AuZn (cubic phase, JCPDS 30-0608) compound in the bottom side of the solder. As aging time increased, γ-Ni₅Zn₂₁ (cubic phase, JCPDS 06-0653) compounds were formed on the Ni layer. Finally, the Ni₅Zn₂₁ phase divided by three layer was formed with grains of columnar shape, and the thickness of the layers was changed by aging time.

Introduction

Microelectronic components have been evolved to become smaller, lighter, and more functional. As environmental pollution continues to be a worldwide concern, the apprehension about the hazard of lead has been also increased. As a measure to solve this problem, there have been tremendous efforts to develop lead-free solders [1], [2].

Recently, compared with the Sn–Ag solder, the Sn–Zn solder has been highly recommended as the substitution for Sn–Pb eutectic solder due to its low melting point. The Sn–Zn solder can be also used without replacing the existing manufacturing lines or electronic components. Moreover, Sn–Zn is advantageous at an economic point of view because Zn is a low cost metal. However, the Sn–Pb eutectic solder is difficult to handle in the practical uses due to its high activation characteristic. In addition, problems such as inferior wettability, controlling voids and oxidation, and coping with flow soldering still remain [3]. To test validate reliability of a Sn–Zn solder joint, various experiments have been conducted, but they are still lacking in meaningful results in this area of research. Furthermore, unlike the general reaction layer having a Sn-base solder, stable binary Au–Zn, Cu–Zn, and Ni–Zn intermetallic compounds (IMCs) are formed between the Sn–Zn solder and under bump metallurgy (UBM) by different experiment conditions. Many studies have been reported that the thickness and shape of IMCs change according to aging time and temperature [4], [5], [6], [7], [8], [9], [10].

This paper presents solder interface reactions by isothermal aging after low temperature Sn–Zn lead-free solders is soldered to the chip size package (CSP). Also, the growth reaction layer and microstructure of the IMCs in the Au/Ni electrolytic-plated Cu pad structure are investigated.

Section snippets

Experimental

In this study, 288 input/output (I/O) CSPs were used. The external dimension of the package was $13 mm ×13 mm ×1.0$ mm in length, height, and thickness. The ball size was 0.3±0.05 mm in diameter and 0.5 mm in pitch. The ball pad was a solder mask defined (SMD) type, and the solder resist was used with the thickness of 30±10 μm. Solder ball pad surface was formed by nickel and gold continuous plating on the copper pattern surface wired on bis-maleimide triazine (BT). Pad structures were made up of Au

Results and discussion

Fig. 1 shows cross-sectional micrographs of solder joints with and without 150 °C isothermal aging in Sn–7Zn and Sn–8Zn–3Bi solders. After aging, bands are observed in the bottom side of the solders, and spots corresponding to Zn-rich phases disappeared in peripheral regions of the bands. The tendency is more clear in the case of Sn–8Zn–3Bi of Fig. 1d. The solder consists of Sn, the matrix phase with bright gray, and Zn, the dispersed phase with dark gray as indicated in Fig. 2. It was found

Conclusion

Interfacial morphology between the Sn–Zn lead-free solders and the Au/Ni electrolytic-plated Cu pad was investigated according to isothermal aging of low temperature Sn–Zn solders soldered to the CSP. During the reflow, the AuZn phase was initially formed in the bottom side of the solder, and then as aging time increased, the Ni₅Zn₂₁ phase was formed on the Ni layer. Finally, the Ni₅Zn₂₁ phase divided by three layer grew with grains of columnar shape, and the thickness of the layers was changed

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Microstructure, thermal analysis and damping properties of Ag and Ni nano-particles doped Sn-8Zn-3Bi solder on OSP-Cu substrate
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This paper investigates the effects of reaction time and temperature on the morphology and growth mechanism of intermetallic compound (IMC) layers of plain Sn–8Zn–3Bi solder and solders containing Ag, Ni nano-particles on Organic Solderability Preservative (OSP)–Cu substrate. At the interface of the plain Sn–8Zn–3Bi solder/OSP–Cu system, a scallop-shaped Cu₅Zn₈ IMC layer was clearly observed. However, after adding Ag nano-particles into the Sn–8Zn–3Bi solder, an additional island-type AgZn₃ IMC layer was found at the top surface of the scallop-shaped Cu₅Zn₈ IMC layer. Moreover, in the Sn–8Zn–3Bi–0.5Ni solder/OSP–Cu system, a ternary (Cu, Ni)–Zn IMC layer was well adhered at their interfaces. Furthermore, in solder ball regions a needle-shaped α-Zn and spherically-shaped Bi phases were clearly observed in the β-Sn matrix of plain Sn–8Zn–3Bi solder. However, in the solder alloys containing Ag and Ni nano-particles an additional very fine AgZn₃ IMC particles for Ag doped solder and Zn–Ni IMC particles for Ni doped solder was uniformly distributed as well as the needle-shaped α-Zn and spherically-shaped Bi phases in the β-Sn matrix. These IMC layer thicknesses were increased with the reaction time and temperature. However, after the addition of the Ag and Ni nano-particles, the IMC growth behavior was slower than that of the plain solder system.
Damping capacity of plain Sn–8Zn–3Bi solder alloy and solder alloys containing Ag, Ni nano-particles were measured by dynamic mechanical analyzer (DMA). The damping capacity of composite solder alloys exhibited higher values than that of plain Sn–8Zn–3Bi solder alloy which is essential to suppress the mechanical vibration and wave propagation. Furthermore, the composite solder alloys doped with Ag, Ni nano-particles improved the oxidation resistance than that of plain Sn–8Zn–3Bi solder alloy by the formation of fine Zn containing IMC particles which block the penetration of vapor and oxygen into the solder matrix.
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%) composition. It is considered to be the CuZn phase [26]. Thick and irregular regions with light-gray contrast and substantial thickness were formed close to the SB solder.
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Effect of bismuth and silver on the corrosion behavior of Sn-9Zn alloy in NaCl 3wt.% solution
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The effect of silver (Ag) and bismuth (Bi) on the corrosion resistance of Sn–9Zn alloy in NaCl 3 wt.% solution was investigated using electrochemical techniques. The results showed that the addition of Bi and Ag lead to the increase of corrosion rate and the corrosion potential E_corr is shifted towards less noble values. After immersion, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive of spectroscopy (EDS) analysis of the corroded alloy surface revealed the nature of corrosion products. EDS and XRD analyses confirmed the oxide of zinc (ZnO and Zn₅(OH)₈Cl₂H₂O) as the major corrosion product formed on the outer surface of in the tested three solder alloys.
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Intermetallic compounds (IMCs) growth on the Sn–8Zn–3Bi (–Cr) solder joints with Cu and electroplated Ni substrates was investigated after aging at 150 °C. It was found that the IMCs were the Cu₅Zn₈ and Ni₅Zn₂₁ at the solder/Cu and solder/Ni interface, respectively. The IMCs growth rate at the Sn–8Zn–3Bi–Cr/Cu and Ni interface was slower than that at Sn–8Zn–3Bi/Cu interface (about 1/2 times) and Sn–8Zn–3Bi/Ni interface (about 1/4 times) during aging. The reason may be that Cr reacts with Zn and forms the Sn–Zn–Cr phase which block the diffusion of Zn atom to the interface and slow down the IMCs growth rate.
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The European Union (EU) has promulgated directives, such as WEEE and RoHS, on 1 July 2006, to restrict the use of Pb in electronic products. Thus, there is an urgent demand for lead-free solders in the electronic industry [1–21]. A low processing temperature is desirable for preventing heat damage to electronic devices during soldering, and this is a reason for the adoption of other low melting temperature alloys, i.e., Sn–Ag binary alloy and Sn–Ag ternary alloys [22–24].
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Analysis on interfacial reactions between Sn–Zn solders and the Au/Ni electrolytic-plated Cu pad

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Mater. Sci. Eng. R

Mater. Sci. Eng. R

Acta Mater.

J. Alloys Compd.

Mater. Lett.

J. Electron. Mater.