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PCB007-Jan2020

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80 PCB007 MAGAZINE I JANUARY 2020 OEMs specify buried vias today, and more ex- pect to do so by 2023 [1] . As Happy Holden has pointed out in one of his many articles, designs with both multilay- er blind vias and buried vias allow designers to reduce layer counts while increasing wiring density [2] . Originally, the board shown in Fig- ure 1 was a 26-layer through-hole-only board. By redesigning the structure to meet design rules and satisfy routing density, the 26-layer board was reduced to the design shown in Fig- ure 1. This is significant both from the stand- point of cost reduction and routing density. Of course, such a drastic change in design spurs changes as well as opportunities in pro- cesses. One such opportunity arises with the use of direct metalization as an alternative to conventional electroless copper. Graphite-based Direct Metallization Colloidal graphite direct metallization pro- cesses have proven their usefulness as a re- placement for electroless copper. This is es- pecially the case in high technology and quick-turn applications. The ability of the col- loidal graphite to successfully enable the direct electroplating of difficult-to-metalize materi- als—such as polyimide, PTFE, ceramic-filled resins, PPO, and PPE—is well documented. Aiding this has been the improvement of the consistency of colloidal graphite direct met- allization processes. This is accomplished through a better understanding of the in- fluence of conditioning agents, the stability of the dispersion, and other process refinements that will be detailed in this column. The im- plementation of polyelectrolyte surface- active agents that enhance the adsorption of the graphite to the non-conductive surfac- es without causing excessive film thickness is quite critical. And the introduction of post- process technology further enhances the uniformity and conductivity of the graph- ite coating. These improvements are neces- sary as the industry migrates to more complex designs. The key takeaway here is quite simple: Di- rect metallization is an ideal process for HDI applications. Application of Graphite Metallization for Flexible Circuits and HDI While graphite-based direct metallization is a production-proven process for rigid PCBs, the Figure 1: A 16-layer design incorporating blind and buried vias reduced from 26 layers.

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