Abstract:
It is known to apply a pulse current during electrodeposition of nickel. In the invention, pulse current waveforms have ramp-down spikes leading to improvements in surface finishes of electroforms created by the process.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to nickel electroforms. 
     2. Description of Prior Art 
     Nickel electrodeposition processes are well-known and pulse currents with rectangular waveforms, instead of direct current, are commonly used to enhance deposition quality. The quality and repeatability of surface finishes provided by this process, especially to meet the requirements of modern micro-device products, has generated many proposals that are generally focussed on using different rectangular waveforms. It has however been proposed to use other types of waveforms in a Paper published in Surface Coatings &amp; Technology 115 (1999) 132-139 entitled ‘A study of surface finishing in pulse current electroforming of nickel by utilising different shaped waveforms’. However, repeatable extremely high quality surface finishes have not yet been attained. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to overcome or at least reduce this problem. 
     According to the invention there is provided a nickel electrodisposition process for creating electroforms having extremely high quality surface finishes, the process comprising applying pulses of direct current in which each pulse has a waveform with a ramp-down spike. 
     Each waveform may have a ramp-down spike in a rectangular waveform, in a triangular waveform, or, preferably, in a ramp down waveform. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Processes according to the invention will now be described by way of example with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic layout of apparatus for carrying out the processes; 
     FIG. 2 is a current time graph showing a first waveform of pulses applied during electroforming; 
     FIG. 3 is a current time graph showing a second waveform of pulses applied during electroforming; 
     FIG. 4 is a current time graph showing a third waveform of pulses applied during electroforming; 
     FIG. 5 illustrates the surface of an electroform after applying pulses of the first waveform; 
     FIG. 6 illustrates the surface of an electroform after applying pulses of the second waveform; 
     FIG. 7 illustrates the surface of an electroform after applying pulses of the third waveform; 
     FIG. 8 shows comparative illustrations of surface finishes provided by prior art processes and processes according to the invention; and 
     FIG. 9 is Table 1 showing comparisons of surface finishes using the described methods. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, in FIG. 1 a conventional electroforming bath  10  has a magnetic stirrer  11  and two electrodes  12  and  13 . The cathode  12  and anode  13  are supplied with pulsed current of different shaped waveforms from a pulse waveform generator  14  in a manner explained below. 
     The bath solution was nickel sulphamate 330 g/l, nickel chloride 15 g/l, boric acid 30 g/l and sodium dodecyl sulphate 0.2 g/l. The temperature was kept at 50±1°C. The initial pH of the electrolyte was 4.2, which is typical for electroforming. The cathode mandrel electrode was made of polished stainless steel and had dimensions of 100×3×1 mm. Electroforming processes were carried out using different shaped current pulses, as explained below. 
     The current pulses were each provided with repetitive ramp down spikes, which is a characteristic of embodiments of this invention. The preferred forms of each of the waveforms is shown in FIGS. 2 to  4 . In the Figures, i c  is the cathodic peak current density, t a  is the pause time, and t c  is the cathodic time. Typically in the Figures, the maximum i c  is 500 mA/cm 2 , and t c  and t a  are equal to Sins. The waveforms represent the applied conditions in each case. 
     FIGS. 5,  6  and  7  show the surface of the electroform generated using the waveforms of FIGS. 2,  3  and  4  respectively; the condition used was a fixed deposition thickness condition. The thickness of the electroforms produced for the different waveforms is about 15 μm. 
     In FIG. 8, the illustrations provide comparisons, in pairs, between the electroform surfaces deposited when ramp down spikes are not applied (see FIGS.  8 ( a ),  8 ( b ) and  8 ( c )) and when ramp down spikes are applied, see FIGS.  8 ( d ),  8 ( e ) and  8 ( f ). Thus, the refinement in grain structure is clearly illustrated by comparing FIGS.  8 ( a ) and  8 ( d ),  8 ( b ) and  8 ( e ), and  8 ( c ) and  8 ( f ). FIGS.  8 ( d ),  8 ( e ) and  8 ( f ) correspond to FIGS. 5,  6 , and  7  respectively. The improvements in surface finishing are clearly shown in Table 1.