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20 The PCB Magazine • April 2014 Metalization After the desmear process, the task now is to insure a continuous, conductive and void-free deposit on the via walls and capture pad. Today, there are several processes that can be utilized to render vias conductive, including: • Conventional electroless copper • Palladium based direct metallization • Graphite • Carbon black • Conductive polymer These metalization processes (also known collectively as "making holes conductive" or MHC) are well developed for both plated through-hole and blind-via metallization. Di- rect metalization in particular is applicable to horizontal processing, although vertical sys- tems can also be used. These processes typically involve the deposition of a conductive coating (palladium, conductive polymer, graphite, car- bon black). This step is then followed by elec- trolytic copper. Thus, the actual electroless cop- per step is eliminated. These processes have been presented and thoroughly discussed elsewhere [7] , while direct metalization processes may reach certain limi- tations for use with very high-aspect ratio rigid circuit boards, these processes are highly effi- cient and effective for HDI. Direct metalization systems primarily function by coating the sub- strate, as opposed to a true chemical reaction type process such as electroless copper. Contrar- ians of direct metalization point to sheet resis- tance measurements of the direct metalization coatings versus electroless copper. Yet while the DM films are somewhat less conducitve, most of the DM processess have resistances in the neighborhood of 5–25 ohms square. This is more than sufficient to promote electroplat- ing propagation in blind vias and mid- to high- aspect ratio plated through-holes. Another advantage that DM processess have over conventional electroless copper is the ability of these DM films to render higher-per- formance material conductive without overly aggressive desmear tactics. It is well known that electroless copper requires sufficient roughen- ing of the resin to promote palladium adsorp- tion and to ensure adhesion of the subsequent copper deposit. However, most direct metal- ization processess require only minimal resin roughening to promote adhesion. This is be- cause the more popular systems commercially available today rely on coating technology. And with the use of special polymers, these DM ma- terials bond and adhere to a wide variety of res- in materials with relative ease [8] . Another critical advantage of direct metal- ization is the ability to conveyorize the system. Of course this is possible with conventional electroless copper. However, the capital outlay for horizontal electroelss copper equipement that can match the productivity of most direct metalization systems is considerably greater. This is particularly evident when a fabricator is processing HDI designs that require multiple sublamination steps. If each HDI layer must be metalized, direct metalization offers a much faster and more effective means to achiev ethe productivity required. Electrodeposition of Copper After the vias are rendered conductive, the subsequent acid copper depostion step is re- quired to further enhance the conductvity of the vias and to build reliability into the package. With respect to HDI and blind vias, the fabrica- tor has two plating options available. The first LEAD-FREE REFLOW FOR HIGH-LAYER-COUNT PCBS continues Figure 6: debris on capture pad preventing adhesion of plated copper.