Enhanced copper electrodeposition onto printed circuit boards using pulsed current and eductor agitation
AbstractPrinted 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
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