Enhanced copper electrodeposition onto printed circuit boards using pulsed current and eductor agitation
Printed Circuit Board (PCB) manufacture involves an electrolytic copper deposition stage for consolidation of conductive circuit paths. Miniaturisation trends requiring increased circuit densities and high aspect ratio through-holes are restricted by the phenomenon of non-uniform copper electrodeposit thickness which can affect electrical impedance properties of the circuitry, cause electrical discontinuity between adjoining circuit layers and inhibit component-lead insertion. This thesis considers means of enhancing the electrodeposition process to alleviate the aforementioned problems. Following a comprehensive review of process technology, both a novel electrolyte agitation method utilising eductors and a Periodic Pulse Reverse (PPR) current technique were investigated experimentally within a pilot tank containing 350 litres of electrolyte. Eductor agitation was analysed/optimised using high-speed photography and a mass transport mapping technique. Data for agitation configurations were verified by statistical analysis of thickness distributions across high surface-area panels. PPR current was initially studied with a small-scale pulse unit and Assaf Cell throwing power test, followed by pilot tank trials using a full-size pulse rectifier in conjunction with eductor agitation and a proprietary electrolyte containing additives. Through-hole throwing power, deposit morphology and microstructure were investigated under various low-frequency pulse conditions and anodic-to-cathodic current density ratios. Eductor agitation and PPR current were compared against the more widely used air agitation and direct current techniques. The effects of air agitation on electrolyte conductivity and commercially produced PCBs were also considered. Optimum agitation conditions were achieved using eight eductors inclined at 37.5° from the horizontal and spaced equidistantly along longitudinal tank walls. Such conditions decreased the standard deviation of copper thickness measured on high surface area panels and lessened edge-effects. Consistent agitation levels up to ten times greater than static solution were recorded, providing enhanced deposition rates; by comparison, air agitation achieved levels of around seven times in uniform regions. Conductivity data showed good correlation with a theoretical approach; air agitation was found to reduce conductivity in proportion to the voidage fraction of gas bubbles and by 20-30% in electrolyte adjacent to air sparge pipes. PPR current provided superior deposits compared to direct current. Through-hole throwing power ratios between 1:1 and 1.3:1 (hole-thickness: surfacethickness) were recorded at mean cathodic current densities between 3.3-4 A/dm2 using pulse timings of 15,1,20,1,25,1 and 30,1 ms (cathodic: anodic) and current density ratios between 2.6:1 and 3:1 (peak-anodic: peak-cathodic); optimum conditions for boards produced in the pilot tank were provided by the 20,1 ms timing. The 25,1 ms timing exhibited high throwing power between 2.5-3.5 A/dm2 under Assaf evaluation but was unable to maintain a uniform thickness distribution in through-holes across a PCB surface. Deposit microstructure and microhardness recorded using PPR current varied according to pulse parameters. Controlling factors and their influence upon results were discussed. Parameters critical to optimisation of agitation and PPR current were attributed to electrochemical effects during deposition. The merits, limitations and potential application of these techniques were examined in relation to PCB manufacture and future priorities were considered.