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

Kim, J 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.

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.

RESULTS AND DISCUSSION

Figure 2 shows the surface morphology of the SnAg solder electroplated at 1 A/dm^sup 2^, 2 A/dm^sup 2^, and 3 A/dm^sup 2^ in the bath containing silver concentrations of 0.001 M, 0.003 M, and 0.005 M, respectively. The microstructure and surface morphology of the electrodeposit shows a tendency to be finer and smoother as the current density increases. This is due to an increase of the cathodic overpotential on the electrodeposit by an increase in current density. Theoretically, the nucleation rate on the electrodeposit is exponentially proportional to a cathodic overpotential.16 Dendritic growth occurred at 1 A/ dm^sup 2^ in the bath containing a silver concentration, of 0.005 M, as shown Fig. 3. The occurrence of dendritic growth means that, by an increase of silver concentration the limiting current density for the silver electrodeposit located at an area of a low current density moved to an area of a higher current density. Therefore, the electrodeposition occurred nearly in the limiting current density for the silver electrodeposit, which had difficulty keeping a uniform growth rate. The relation between composition and current density of the Sn-Ag solder electroplated in the preceding baths is shown in Fig. 4. For an accurate composition, EDS and WDS methods were applied in the same specimen. Silver composition in an electrodeposit decreased with an increase of current density. The standard deviation at 1 A/ dm^sup 2^ with a silver concentration of 0.005 M was large because the electrodeposit has a nonuniform surface of dendritic growth. The analysis of an x-ray diffraction pattern is shown in Fig. 5. It indicates that the electrodeposit was composed of pure tin and an intermetallic compound of Ag^sub 3^Sn. The intensity of the Ag^sub 3^Sn peak decreases with an increase of current density. This agrees well with the results of the composition analysis shown in Fig. 4.

The PC plating was introduced to investigate the relationship between the composition and morphology of the electrodeposit compared to direct-current (DC) plating. The main effect of PC plating in the electrodeposit is to reduce surface roughness and the size of the microstructure with an increase of limiting current density and cathodic overpotential.18 In this study, duty cycles of 80%, 60%, and 40% in frequency of 100 Hz were employed in the bath with a silver concentration of 0.003 M. As shown in Fig. 6, the surface morphology was hardly changed, but it reduced the surface roughness as compared with DC plating. However, the silver composition on the electrodeposit at a lower duty cycle increased surely at a constant current density, as indicated in Fig. 7. At an off cycle, the concentration of silver in the diffusion layer was recovered to keep the support of silver ions, and thereby, the silver composition in the deposit was increased. In the meantime, the dendritic growth, as in Fig. 3, occurred on all electrodeposits at 1 A/dm^sup 2^ regardless of duty cycle.

Figure 8 shows the surface morphology of the electroplated solder from the bath containing a surface-active agent. The results of WDS analysis of the corresponding electroplated solder are shown in Fig. 9. The silver composition on the electrodeposit , was generally lower than the electroplated solders plated without the agent. This means that the surface-active agent plays a role in reducing the limiting current density in the silver electrodeposit. A little increase of silver composition at 3 A/dm^sup 2^ is attributed to an increased current efficiency of silver over tin. Figure 10 exhibits the surface roughness measurements of the electroplated solder in three different conditions. In this graph, the electroplated solder plated in a bath with a surface-active agent was noted to be the smoothest among others.

Using an optimum plating condition, Sn-Ag solder bumps were successfully electroplated as shown in Fig. 11. Figure Ua shows a SEM image of a Ni underbump metallization electroplated to 1.5 µm in height. Figure 11b shows Sn-Ag solder bumps electroplated to 15-µm height through a patterned photoresist with 15 µm in diameter and 30 µm in pitch.

CONCLUSIONS

In the Sn-Ag alloy bath, the surface morphology and composition of the electrodeposit was influenced by current density, silver concentration in the bath, applied current type, and addition of additive. The increase of current density made the surface morphology of the electrodeposit to be finer and smoother and decreased the silver composition in the electrodeposit. The silver concentration in the bath should be controlled precisely because the dendritic growth of silver can occur easily because of the increase of limiting current density. During PC plating, the surface morphology was hardly improved, but the silver composition in the electrodeposit was increased with the decrease of duty cycle. An addition of a surface-active agent in the Sn-Ag alloy bath improved the surface morphology of the electrodeposit by increasing cathodic overpotential and decreasing the limiting current density of silver. Therefore, in the design of the Sn-Ag alloy bath, the control of plating parameters, such as current density, silver concentration, proper current type, and an addition of additive, is critical to produce the electrodeposit with proper composition and morphology.

ACKNOWLEDGEMENTS

This work was supported by the Center for Electronic Packaging Materials of Korea Science Engineering Foundation. Special thanks go to J.Y. Song and Y.C. Sohn (KAIST) for suggestions, guidance, and assistance during this work and to Dr. S.S. Kang (Hanbat National University) for WDS analysis.

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J.Y. KIM,1 J. YU,1 J.H. LEE,2,4 and T.Y. LEE3

1.-Center for Electronic Packaging Materials, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701 Korea. 2.-Department of Materials Science and Engineering, Hongik University, Seoul 121-791 Korea. 3.-Department of Materials Engineering, Hanbat National University, Daejeon 305-719 Korea. 4.- E-mail: jhlee@wow.hongik.ac.kr

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