Electroplating for high aspect ratio vias in PCB manufacturing: enhancement capabilities of acoustic streaming

Abstract

This paper considers the copper electrodeposition processes in microvias and investigates whether the quality of the electroplating process can be improved by acoustic streaming using megasonic transducers placed into a plating cell. The theoretical results show that acoustic streaming does not take place within the micro-via (either through or blind-via's) but it does help improve cupric ion transport in the area close to the mouth of a via. This replenishment of cupric ions at the mouth of micro-via leads to better quality filling of the micro-via through diffusion compared to basic conditions. Experiments showing the improved quality of the filling of vias are also presented.

Morphology and acoustic artefacts of copper deposits electroplated using megasonic assisted agitation

“…It is difficult to induce fluid convection down THVs of high aspect ratio (10:1) and small diameter size (<0.3 mm) because of the increased influence of viscous forces over inertial forces. Previous simulations have highlighted that the increases in forced streaming currents by MS acoustics are weak so as to not significantly contribute to fluid transport down high aspect ratio vias (Strusevich et al , 2013). Regardless, increases to plating efficiencies have been observed (Costello et al , 2013).…”

“…Compared to US, MS offers less aggressive agitation because of the reduction in cavitation forces and higher plating efficiency due to the reduction of the diffusion layer (Strusevich et al , 2013). Studies have provided in-depth analysis of electrochemical behaviour during the plating trials; however, they have offered a limited analysis of the changes in topography of the electrodeposits.…”

Copper electroplating of PCB interconnects using megasonic acoustic streaming

“…The Cu electroplating stopped in the BV at the point where the Cu seed layer failed to deposit in the electroless process prior to electroplating. For this reason, despite the enhancement to electrolyte replenishment observed with MS, plating failed to occur at the bottom of high ar BVs and was unable to reproduce results obtained in [10].…”

“…The BV MS plating investigations in [10] reported a uniform filling of Cu down a 0.1 mm diameter, ar 3:1 BV. In an attempt to reproduce the previous literature findings and to reduce the variation in the measurement of the average plated thickness, MS-assistance plating trials were performed to plating conditions outlined in Table 2.…”

“…The application of high frequency acoustics induces streaming mechanisms which have the ability to improve the transport of plating solution down microfeatures [5][6][7], improving the Cu plated thickness down vias over existing agitation techniques. A MS wave is used to deliver a uniform acoustic wave to the surface of the PCB, cathode, to investigate the influence of acoustic agitation within the electrolyte solution [8][9][10][11][12]. Increases in Cu deposition depth and improved uniformity of the Cu filling were obtained for high thickness-to-width aspect ratio (ar) vias, of the order of 8:1 and 3:1 for the THV and BV, respectively, as shown in Table 1.…”