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74 The PCB Magazine • February 2016 by George Milad UYEMURA InTERnATIonAl CoRPoRATIon Blind vias that connect layer 1 to layer 2 or layer 3 are an enabling technology for HDI-type boards. Via fill makes for a robust connection (Figure 1) with no chance of any voids during assembly. Vias with 1:1 aspect ratio are com- mon. Successful via-fill plating requires a specific electrolyte; the copper concentration is high at 50–60 g/L copper with low sulfuric acid at 30–60 g/L. This is combined with a unique organic ad- ditive combination with a prominent leveling component. The leveling component acts pre- dominantly on the surface and suppresses the surface plating allowing the brightener and car- rier combination to plate up from the bottom of the via. Ideally the solution movement must be vigorous and parallel to the surface (laminar flow); this ensures adequate leveler replenish- ment. The mechanism of via filling is different from through-hole plating. A good understand- ing of the following influencing factors is para- mount to the success of the process. The Electrolyte The electrolyte is composed of inorganic and organic components. The inorganic component for via fill is primarily a high-copper low-acid system. This ensures that there is no shortage or depletion of copper ions anywhere on the plat- ing surface. The organic additives are composed of three components, namely the carrier or sup- pressor, the brightener or accelerator, and the leveler which is a selective suppressor. Carriers increase the polarization resistance and are current suppressors. The suppression is a result of the carrier being adsorbed to the sur- How to Set Up a Successful Blind Via Hole Fill DC Plating Process Figure 1: Stacked vias as an enabling HDI technology. artiCle

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