TECHNICAL PAPER Electrodeposition of copper into high aspect ratio PCB micro-via using megasonic agitation S. Costello • N. Strusevich • D. Flynn • R. W. Kay • M. K. Patel • C. Bailey • D. Price • M. Bennet • A. C. Jones • M. P. Y. Desmulliez Received: 12 July 2012 / Accepted: 26 January 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract This paper presents the use of micro-particle imaging velocimetry (micro-PIV) to analyse fluid flow and hence ion replenishment in PCB micro-via during the electroplating process. The cross section of a PCB via is fabricated in PMMA to allow optical access to the sample. Fluid flow within two 1:1 aspect ratio blind micro-vias, one with straight side walls and the other with tapered side walls were compared. Flow is also analysed in a 1:1 aspect ratio through via. Flow rates measured using micro-PIV are used to validate simulated flow models. The results show that there are increased flow rates within the blind via with tapered side walls. This goes some way to explaining the improved electroplating results obtained in industry when tapered vias are used. Initial experimental results using megasonic streaming to remove bubbles from blind micro- via and promote ion transportation within high aspect ratio PCB micro-via to enhance electrodeposition are also reported. 1 Introduction Through Silicon Vias (TSVs) techniques have been in research and development since the late 1990s with a view to achieving higher microelectronic package interconnect densification (Zhang et al. 2007). More recently, several commercial products, notably image sensors such as mobile phone cameras, have appeared that now use this enabling technology for 3D-integration. Filling of TSV’s with high aspect ratio (HAR) vias up to 10:1 and typical via diameters in the range of tens of microns have been achieved using sputtering, chemical vapour deposition or electroplating (Wolf et al. 2008). Many of the techniques used to successfully fill high aspect ratio TSV’s in the microelectronics industry are however too expensive for the PCB industry. Although electrodeposition remains the most cost effective way to achieve high density intercon- nects in PCBs, there are technical issues in filling increasingly high aspect ratio and large volume vias of diameter ranging from 40 up to 150 lm. For vias of these dimensions, the pinching of the mouth due to current crowding effects closes the via and prevents the replen- ishing of electrolyte solution, slowing down thereby the electroplating process as shown in Fig. 1a. The resulting lack of fluid circulation in the via is further compounded by the depletion in metal ions inside the blind via resulting in increasing pinching, diminishing of replenishing, and finally cutting-off the electrolyte inside the via from the main volume. Therefore the electroplating process is stopped before the via filling is completed leading to the formation of seam voids as shown in Fig. 1b. Electrodeposition also relies on the presence of a con- tinuous conductive seed layer within the via. In the PCB industry, seed layers are typically formed using electroless copper plating, conductive polymer, carbon or graphite S. Costello Á D. Flynn Á R. W. Kay Á M. P. Y. Desmulliez MicroSystems Engineering Centre (MISEC), School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK N. Strusevich (&) Á M. K. Patel Á C. Bailey School of Computing and Mathematical Sciences, University of Greenwich, Old Royal Naval College, Park Row, London SE10 9LS, UK e-mail: sn06@gre.ac.uk D. Price Merlin Circuit Technology Ltd., Hawarden Industrial Park, Manor Lane, Deeside CH5 3QZ, UK M. Bennet Á A. C. Jones School of Chemistry, University of Edinburgh, Joseph Black Building, Edinburgh EH9 3JF, UK 123 Microsyst Technol DOI 10.1007/s00542-013-1746-7