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84 PCB007 MAGAZINE I SEPTEMBER 2023 Direct metallization (DM), in particular, is applicable to horizontal processing, although vertical systems are also used. ese processes typically involve the deposition of a conduc- tive coating (palladium, conductive polymer, graphite, carbon black). is step is followed by electrolytic copper; thus, the actual electro- less copper step is eliminated. ese processes have been presented and thoroughly discussed elsewhere 1 . While direct metallization processes may reach certain limi- tations for use with very high aspect ratio rigid circuit boards, these processes are very effi- cient and effective for HDI. Direct metalliza- tion systems primarily function by coating the substrate, as opposed to a true chemical reac- tion that is inherent in conventional electro- less copper plating processes, such as electro- less copper. Contrarians of direct metalliza- tion point to sheet resistance measurements of the direct metallization coatings vs. electroless copper. Yet, while the DM films are somewhat less conductive, most of the direct metalliza- tion processes have resistances in the neigh- borhood of 5–25 ohms square. is is more than sufficient to promote electroplating prop- agation in blind vias and mid- to high-aspect ratio through-holes. Another advantage that DM processes have over conventional electroless copper is the ability of these DM films to render higher per- formance materials conductive without overly aggressive desmear tactics. It is well known that electroless copper requires sufficient roughening of the resin to promote palladium adsorption and to insure adhesion of the sub- sequent copper deposit. However, most direct metallization processes require only minimal resin roughening to promote adhesion. is is because the more popular systems com- mercially available today rely on coating tech- nology. And with the use of special polymers, these DM materials bond and adhere to a wide variety of resin materials with relative ease 2 . It is understood that higher performance resin materials—with the characteristics of higher Tg, lower CTE, and higher Td (temper- ature of decomposition)—are more chemically resistant, making desmear more difficult. With less resin removal and minimal restructur- ing of the resin surface aer desmear, one can surmise that electroless copper would be less effective on these types of materials. e car- bon- and graphite-based direct metallization systems do not require a palladium catalyst to first adsorb onto the glass and resin in order to catalyze the deposition of the copper. e electroless copper deposition process is shown below. e presence of palladium is required to effect the reaction as shown below. e pro- cess is essentially two half-cell reactions (see Figure 1). A byproduct of electroless copper plating is hydrogen gas (H 2 ). e ability of these fine gas bubbles to lodge in blind vias and high aspect ratio through-holes is well understood. Even with the use of vibration and modified plat- ing solution agitation systems, hydrogen gas remains an issue. However, the direct plating processes, as stated earlier, are coating pro- cesses, not chemical reactions. us, hydrogen gas evolution is not present. Overall Reaction: Cu(EDTA) 2 - + 2HCHO + 4OH - → Cu + H 2 + H 2 O + 2CHOO - + EDTA 4+ Sources of formate: secondary reducing agent 1. Cu(EDTA) 2 - + 2HCHO + 4OH - → Cu + H 2 + H 2 O + 2CHOO - + EDTA 4+ 2. 2HCHO + OH - ±° CH 3 OH + HCOO - Figure 1.