PCB007 Magazine

PCB-Jun2017

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76 The PCB Magazine • June 2017 as a manufacturing tool to assist the etchant abilities of wet chemical processes. Examples include enhancement to the surface treatment of chemically inert materials by applying 40 kHz ultrasound waves within the permanganate desmear process used in the electroless copper plating [6] and for the cleaning of silicon wafers for the adhesion of photoresist dryfilm [7] . An- other application is the enhancement to the streaming of fluidic currents within copper plat- ing baths, which has a use in electrodeposition processes as an assistance to the circulation of electrolyte plating solution in difficult-to-plate regions on the PCB. The resulting use of ultra- sonic (US) and megasonic (MS) assisted agita- tion, leads to increases in the throwing power of the electrolyte bath—its ability to plate into low current density areas with the same thick- ness as higher current density areas. The introduction of MS agitations within a copper suphate plating bath has been demon- strated in laboratory trials at HWU to increase the throwing power of the bath [8] . The advan- tages outlined in those laboratory-scale tri- als indicate that the microfeatures present in PCB interconnects, such as a through-hole via (THV)—a drilled feature which provides electri- cal connection between a board's two outer sur- faces and inner layers—can be uniformly filled with copper with a feature size of 0.2 mm diam- eter and diameter-to-depth aspect ratio (ar) 8:1. Also, the uniform filling of a blind via (BV)—a drilled feature smaller than a THV connecting an outer layer with the underlying one, two or three innerlayers—observed on features of di- ameter 0.1 mm and ar 3:1 [3] . The increased throwing power of the bath due to the MS-assisted agitation has demonstrat- ed a potential to manufacture via interconnects with a uniform fill of copper, which is desir- able for enhancing thermal transport on a PCB whilst enabling HD interconnection [9] . Increas- ing the throwing power enables an increased plating performance and via ar, as shown by the 3:1 ar BV which is typically manufactured with a maximum ar of 1.2:1. The outcome of this in- creased connectivity is illustrated in Figure 1. The figure shows a PCB schematic of a 26-layer MEGASONIC ACOUSTIC SURFACE TREATMENT PROCESS Figure 1: PCB build comparison of current technology and megasonic processing.

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