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24 The PCB Magazine • May 2015 tions, including on-line additive analysis, void- free, highly planar through-hole filling and ex- cellent bottom-up blind microvia filling with low surface copper deposition thickness were demonstrated. Introduction Portability of consumer electronics has be- come the primary driver for the ever-increasing circuit density of today's printed circuit designs. Based on the small dimensions of these devices, through-hole and blind via diameters are typi- cally in the 75–150 μm range. Performance im- provement and process cost reduction make through-hole filling technology with copper an excellent approach, rather than the conven- tional plated through-hole. Prior to the development of electrolytic copper through-hole filling, substrates for such applications were electroplated with a conven- tional through-hole process, then plugged with an epoxy material. Following these steps, addi- tional planarization, re-metallization and elec- trolytic copper capping processes were required before the build-up process steps could begin. Use of electrolytic copper through-hole fill- ing eliminates several of these manufacturing steps and offers a number of additional advan- tages over the conventional build-up process by enhancing the thermal and electrical conduc- tivity of the interconnections, and by reducing overall costs. Inorganic Components The vast majority of through-hole fill elec- troplating baths are based on acid sulfate elec- trolytes, containing copper sulfate (the primary source of cupric ions), sulfuric acid (for solution conductivity) and chloride ion (as a co-sup- pressor). Of these components, sulfuric acid, typically at concentrations below 60 g/L, has the most significant effect on achieving good through-hole filling. A plating solution with elevated acid concentration leads to conformal plating causing poor filling performance. Organic Components Acid copper sulfate system operated with- out additives typically yield deposits of poor physical properties. Organic additives, typical- ly consisting of materials described as suppres- sors, brighteners, and levelers, are therefore used to further refine deposit characteristics. Via fill plating systems were not designed for through-hole filling applications. The organic components and their respective concentra- tions used to achieve bottom-up filling in blind microvias are less effective for initiating fill in the very different geometry of a through-hole. Therefore, a novel organic package was devel- oped specifically to favor the fill mode essen- tial to completely plug the through-hole with copper. While an accelerated bottom-up fill mechanism is the critical aspect of blind mi- crovia filling, through-hole filling requires ini- tial creation of a "butterfly" structure, formed by rapid copper deposition at the center of the holes. This change in geometry is the critical first step in achieving good copper through- hole filling. ELECTrOPLATING THrOUGH-HOLES wITH DIFFErENT GEOMETry continues FEaturE Figure 1: Through-hole plug: conventional vs. current technology. Figure 2: Conformal plating of a through-hole.