A study of surface finishing in pulse current electroforming of nickel by utilizing different shaped waveforms

doi:10.1016/S0257-8972(99)00165-6

Surface and Coatings Technology

Volume 115, Issues 2–3, 18 July 1999, Pages 132-139

https://doi.org/10.1016/S0257-8972(99)00165-6 Get rights and content

Abstract

This paper studies experimentally and theoretically the effect of different types of shaped waveform on the surface finish of nickel electroforms in pulse current electroforming. Experimental results show that the quality of the electroforms, in terms of surface roughness improvement, influenced by the types of waveform is of the order of ramp and triangular both with relaxation time>rectangular>ramp and triangular sawtooth. The results show that at the same cathodic peak current density, pulse period, and a fixed electrodeposition time or electrodeposition thickness, the surface roughness can be reduced by as much as two to three times when a relaxation time was used in the cases of ramp and triangular waveforms. These results are supported by the theoretical predictions and the study of the surface morphology of the electroforms by scanning electron microscopy.

Introduction

In electrodeposition processes, the surface profile of a deposit may be exaggerated from the original surface profile of the cathode [1]. This arises because protrusions on a surface grow more rapidly than the surrounding surfaces for the current density is higher at protrusions. As a result, the surface roughness could be amplified during the deposition process. When the thickness of the deposited layer increases, the surface becomes rougher. Therefore, roughening of a deposit surface is of great concern in electroforming because electroforming normally generates thick deposited layer products [2]. Although for some applications, post-machining can be adopted to improve the surface finish of the deposited surfaces, it is not feasible in micro-electroforming owing to the small size of the micro-components [3]. Since there is an increasing demand for better surface finishing and tighter dimensional tolerance of micro-devices, many research works have been concentrated in improving the surface finishing of a deposit [3], [4], [5]. In electrodeposition, there are two methods commonly employed for the improvement of the surface finish of a deposit. The first one is to add additives to a bath, and the second method is to use a pulse current. In electroforming, it is very difficult to keep the amount of the additives constant throughout the entire process. Therefore, the second method is sometime preferred [6]. It has been reported that for the same processing conditions, the use of a pulse current on a rectangular waveform resulted in the formation of a deposit with a better surface finish than that formed by a direct current [7], [8], [9], [10], [11], [12].

As far as pulse current electroforming was concerned, most of the published works in the past have been focused on rectangular waveform, and relatively little effort has been paid to solve the surface finish problem by utilizing different types of shaped waveforms. Since the surface roughness development is directly influenced by the micro-current distribution at protrusions and the surrounding surfaces [1], [4], [10], [11], [12], this study therefore investigates the effect of different kinds of shaped waveform on the current distribution on some uneven surfaces. The different types of waveform studied were (i) rectangular waveform with relaxation time, (ii) ramp waveform with relaxation time, (iii) triangular waveform with relaxation time, (iv) ramp sawtooth waveform, and (v) triangular sawtooth waveform.

Section snippets

Experimental procedures

In the electroforming experiments, the composition of the bath solution was nickel sulphamate 330 g l⁻¹; nickel chloride 15 g l⁻¹; boric acid 30 g l⁻¹ and sodium dodecyl sulphate 0.2 g l⁻¹. The electrolyte was gently agitated by means of a magnetic stirrer, and the temperature was kept at 50±1°C. The initial pH of the electrolyte was at 4.2, which is a typical value used in electroforming. The cathode mandrel was made of stainless steel with dimensions of 100 mm×30 mm×1 mm, and it was ground finish on

Constant electrodeposition time condition

Fig. 2 shows that a much rougher surface finish was obtained when either a ramp sawtooth waveform or a triangular sawtooth waveform was employed, while some improvement was obtained when the rectangular waveform with relaxation time was used. The best surface finish, in terms of change of surface roughness, was obtained by using either a ramp waveform with relaxation time or a triangular waveform with relaxation time. The improvement over that without relaxation time could be as much as two to

Conclusions

The effects of different types of shaped waveform on the surface finish of nickel electroforms have been studied experimentally and theoretically. According to the experimental findings, at the same cathodic peak current density, pulse period, and a fixed electrodeposition time or electrodeposition thickness, the best surface finish was obtained when either a ramp or a triangular waveform, both with relaxation time, was used. Analytical equations for the development of protrusions at a fixed

Acknowledgements

Financial support for the project, under the code V668 from the Hong Kong Polytechnic University is acknowledged.

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A study of surface finishing in pulse current electroforming of nickel by utilizing different shaped waveforms

Abstract

Introduction

Section snippets

Experimental procedures

Constant electrodeposition time condition

Conclusions

Acknowledgements

Surf. Technol.

Surf. Technol.

Met. Finish.

Surf. Technol.

Surf. Technol.

J. Electrochem. Soc.

Trans. Inst. Met. Finish.

Proc. SPIE