This thesis investigated the introduction of megasound (MS) (1MHz) acoustic
technology as an enhanced agitation method of an electrolyte solution for the
electrochemical deposition of copper (Cu), used in electroplating processes. The thesis,
carried out at Merlin Circuit Technology Ltd, studied the possibility of improving
processing capabilities for use in Printed Circuit Board (PCB) industrial manufacture.
Prior laboratory experiments demonstrated increased metallisation of vertical
interconnect access (via) features in a Printed Circuit Board (PCB), which, if applied
within manufacturing, would enable increased connectivity throughout a PCB and result
in cost savings.
PCB manufacturing quality after MS-assisted Cu electroplating was assessed by
measurements of the topography of the electrodeposits, using scanning electron
microscopy and white-light interferometry. Cu plating rate changes were also measured
on the surface of the PCB and inside the vias.
After plating Cu with MS-assistance, the macro and microscale surface
composition was demonstrated to alter due to the direct influence of the acoustic waves.
Systematic characteristic of the surface was conducted by varying the settings of the
acoustic transducer device as well as the process parameters including electrical current
distribution, bath additive chemistry and solution temperature.
MS processing was shown to produce unique Cu artefacts. Their deleterious
formation was demonstrated to be influenced by acoustic standing waves and
microbubble formations at the electrolyte solution/PCB interface. Causes of these
artefacts, microfluidic streaming and cavitation, were also observed and controlled to
reduce the creation of these artefacts.
MS plating Cu down through-hole via (THV) and blind-via (BV) interconnects
was shown to produce measureable benefits. These include, for THVs, a 700 % increase
of Cu plating deposit thickness within a 175 μm diameter, depth-to-width aspect ratio (ar)
of 5.7:1, compared with processing under no-agitation conditions. For BVs, a 60 %
average increase in Cu deposition in 150 μm and 200 μm, ar 1:1, was demonstrated
against plating under standard manufacturing conditions - bubble agitation and panel
movement.Engineering and Physical Research Council (EPSRC) grant number EP/G037523/
