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Effects of Electroplating Parameters on the Composition and Morphology of Sn-Ag Solder, The

Journal of Electronic Materials, Dec 2004 by Kim, J Y, Yu, J, Lee, J H, Lee, T Y

The Sn-Ag solder was electrodeposited from a bath that basically is composed of tin sulfate (SnSO^sub 4^), silver nitrate (AgNO^sub 3^), and thiourea (CH^sub 4^N^sub 2^S), acting as a complexing agent to silver. The composition and morphology of electrodeposited Sn-Ag solder were studied in terms of silver concentration in bath current density, duty cycle, and additives. It was possible to control silver content in the electrodeposit by means of varying silver concentration in bath and current density. The microstructure and surface morphology of the electrodeposit become finer and smoother with increasing current density. The pulse-current (PC) plating method was applied to compare to the conventional direct-current (DC) plating. Varying duty cycle in PC plating did not change the microstructure in general, but some improvement in surface roughness was observed compared to DC plating. However, the silver composition in the electrodeposit increased with decreasing the duty cycle at a constant current density. An addition of a surfaceactive agent helped to reduce the surface roughness and the variation of silver content in the electrodeposit. In an optimum condition, eutectic Sn-Ag solder bumps with a fine pitch of 30 µm and height of 15 µm were successfully electroplated. The composition of Sn-Ag bumps was analyzed by energy dispersive x-ray spectrometry (EDS) and wavelength dispersive x-ray spectrometry (WDS) methods, and the surface morphology was characterized by scanning electron microscopy (SEM) and a three-dimensional surface analyzer.

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Key words: Sn-Ag solder, electroplating, pulse plating, morphology, duty cycle, surface-active agent

INTRODUCTION

In flip-chip packaging, the electroplating method is used broadly to produce solder bumps that interconnect between a chip and a substrate. Especially for high-density interconnection, an electroplating method has merit because it can form fine-pitch solder bumps over other solder-bumping techniques. Recently, the Sn-Ag alloy has been recommended as one of the most promising candidates to replace the eutectic Sn-Pb alloy used in flip-chip applications.1,2

The Sn-Ag baths used for electroplating can be classified as either a cyanide (CN)-containing3-5 or a cyanide-free6-12 bath. The cyanide ion is the most important complexing agent in alkaline baths because the cyanide that forms stable complex ions with silver makes co-deposition of tin and silver possible by reducing considerably the electrode potential difference between silver and tin.13 However, because a cyanide-containing bath is chemically hazardous, producing various deadly colorless gases, such as hydrogen cyanide or cyanogen chloride at low pH, special care should be exercised for handling and waste treatment of cyanide-containing baths. Because of the toxicity of cyanide, serious efforts to substitute it with other ligands are in progress.

In Sn-Ag alloy plating, a uniform co-deposition of tin and silver to make the proper eutectic-solder composition is very difficult because the standard reduction-potential difference of silver and tin is very large. In addition, because the silver ion in the electrolyte exists as a monovalent ion, whereas tin exists either as adivalent or tetravalent ion, a tin ion tends to be preferentially reduced over the silver ion, and the amount of current needed for the reduction of tin ion is two or four times more than for the silver ion.12 Because the properties of an electrodeposit are significantly changing depending on operating conditions, a systematic study about the parameters affecting the properties of an electrodeposit is required. The parameters to be considered generally in an alloy electroplating include current density, concentration of the more noble metal ion, additive, temperature, agitation, pH, and so on.14 However, as shown in Fig. 1, the variation of current density, concentration of the more noble metal, and quantity of additives are found to influence more the composition and morphology of an alloy deposit than others.

Therefore, in this study, employing a cyanide-free acid Sn-Ag bath, which can be rather easily manufactured, the parameters, such as silver concentration in bath, current density, duty cycle of pulse-current (PC) plating and surface-active agent used as an additive, were investigated to establish an optimum condition for Sn-Ag electrodeposition. Subsequently, it was demonstrated to electroplate eutectic Sn-Ag solder bumps with a fine pitch of 30 µ and height of 15 µm under the optimum plating conditions.

EXPERIMENTAL PROCEDURES

The operating conditions of the Sn-Ag bath, which basically comprises tin sulfate (SnSO^sub 4^), silver nitrate (AgNO^sub 3^), and thiourea (CH^sub 4^N^sub 2^S), are shown in Table I. In this bath, thiourea was necessarily contained as not only a complexing agent but also an excellent dissolution regent for nitric silver in an acid medium. The Cr adhesion layer (50 nm) and Cu seed layer (300 nm) were sequentially sputtered for electroplating on a Si wafer, which was used as a substrate for electroplating. The Si wafer with the sputtered Cr and Cu layer was cleaned in acetone and ethanol before electroplating. All the Sn-Ag solder was electroplated to about 10-m thickness on a 1 cm^sup 2^ area in various electroplating conditions. The silver concentration in the bath was controlled by changing the quantity of silver nitrate. The current density was varied in the range of 1-3 A/dm^sup 2^. To investigate the effect of pulse current electroplating, duty cycles of 80%, 60%, and 40% in the frequency of 100 Hz/10 Hz were used. The effect of an additive was investigated by using a commercial surfaceactive agent. Solder-bumping sequence to produce a fine-pitch interconnection was described in a previous study.15 The composition of an electrodeposit was analyzed by energy dispersive x-ray spectrometry (EDS) and wavelength dispersive x-ray spectrometry (WDS). The microstructure and morphology of an electrodeposit were observed with scanning electron microscopy (SEM) and a three-dimensional surface analyzer.

Technology Industry

Effects of Electroplating Parameters on the Composition and Morphology of Sn-Ag Solder, The

Journal of Electronic Materials, Dec 2004 by Kim, J Y, Yu, J, Lee, J H, Lee, T Y

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