Pulse electroplating of copper for printed circuit technology
This thesis is concerned with the study of square wave unipolar pulse and bipolar pulse reverse electroplating of copper for printed circuit board(PCB) production. The aim of this research programme was to develop an acid copper electrolyte, preferably with additives under certain pulse conditions for electroplating of PCBs having hole diameters as small as 0.3mm. The aim was to obtain a surface to hole thickness ratio of 1:1. The use of additives in the electrolyte is preferred for commercial purposes. The literature review comprises an examination of the technology of copper electroplating, comparison between different electrolytes and their uses in PCB industries, study of the theoretical aspects of pulse current and its effectiveness in comparison to direct current(DC) and also an up-to-date bibliographical literature survey on pulse electroplating of metals and alloys with particular emphasison copper. The experimental work was concerned with studying and understanding of the pulse unit behaviour, substrate pre-treatments, effect of chloride ion concentration,comparisons between different electrolytes under DC, unipolar pulse and bipolar pulse reverse using polarisation curves (potential-current density curves). Furthermore, comparisons between unipolar pulse, bipolar pulse reverse and DC in terms of hardness, tensile strength, efficiency, morphology and surface topography have been made. A systematic behaviour was observed using a pulse unit and power supply as a means of power source unlike the results obtained with a pulse unit and a potentiostat The examination of the behaviour was made by polarisation studies. They indicated that the electronics within the pulse unit and potentiostat causes an inconsistant or surge in polarisation curves. The polarisation studies of DC, unipolar pulse and bipolar pulse reverse were carried out by a step-wise method using a power supply for DC and unipolar pulse and two power supplies for bipolar pulse reverse. To compare different pulse conditions, the Tafel region from the calculated polarisation curves, potential-logarithmic converted current density curves, were used to obtain the Wagner number and from that an enhanced Wagner number ratio(RWa), which is the ratio of pulse Wagner number to DC Wagner number, was derived. It was found that the electrolyte containing no additives, 'electrolyte I, gave the highest RWa values and the deposits had long columnar structures. The electrolyte containing all the additives, 'electrolyte V, provided higher RWa than DC but 23% and 47% lower RWa than electrolyte I for unipolar pulse and bipolar pulse reverse, respectively, were obtained. A cathodic/anodioc n-timer atio of lOms/0.2masn dp eakc athodiclanodirca tio of 1/4 produced the best coating thickness distributions under bipolar pulse reverse for electrolyte V at an average current denisty of 3.O Adm-2. The surfacea appearance obtained was lustrous being similar to that of DC. The electrolyte V produced a finer grained deposit structure than electrolyte I and hence it is more appropriate for use on PCBs since it permits the deposit to survive the solder float test and also provides an efficient electrical continuity. The optimum chloride ion concentration was found to be 50ppm for both DC and unipolar pulse conditions. The hardness, roughness, tensile strength and morphology were found to vary dependingu pon the pulse conditions employed for both unipolar pulse and bipolar pulse reverse. The surface appearances obtained under unipolar pulse were similar to that of DC for the electrolytes mentioned above but for bipolarp ulser evese, the surface appearance varied depending upon the pulse parameters used.