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28 PCB007 MAGAZINE I MARCH 2020 processes became widely used on polyimide film in flexible circuits and on high-perfor- mance and exotic materials like PTFE. Car- bon- and graphite-based direct metallization technology is approved for space and military avionics applications under the requirements of IPC 6012D, Board Evolution Leading direct metallization processes have continued to evolve throughout the years with the demands of PCB designs. As the drive for miniaturization resulted in the change from leaded to surface-mounted components, PCB designs evolved to accommodate smaller parts with higher pin counts. This then led to PCBs with higher layer counts, thicker panels, and smaller diameter through-holes. To meet the challenges of high-aspect-ratio holes, line spec- ifications encompassed improvements for so- lution transfer in small holes. Upgrades, such as the use of ultrasonics to quickly wet holes and remove air bubbles, were implemented, along with in-line air knives and dryers spe- cially modified to dry the small holes on the thicker panels. After this, PCB designers reached the next stage: via starvation, the point where the pin count and grid density exceeded the available real estate to drill through-holes and route nets. As the industry moved from BGAs with 1.27–1.00 mm grid to CSPs with 0.80–0.64 mm grid, the microvia became the enabler for designers to meet the challenge of HDI tech- nology. In 1997, the feature phone began using a 1+N_1 design in mass production. This is a one buildup layer with microvias over a multi- layer core. As mobile phone production grew, microvias were formed by conformal etch and CO 2 lasers, and later by UV, UV YAG, and com- bo UV CO2 lasers. Microvias allowed design- ers to route lines under vias so larger pin grids could be redistributed without increasing layer count. HDI is widely used today in three plat- forms: miniaturization, advanced packaging, and high performance. The miniaturization seen in mobile phone designs is the current highest volume contributor. Direct Metallization to the Rescue Direct metallization systems like Blackhole had to overcome technical hurdles to meet the challenge of metallizing the blind vias and small diameter features of HDI. The small mi- crovia size presented trouble in the removal of the carbon black from the via target pad, which is essential to ensuring clean copper to copper bonds. From a chemical perspective, cleaner and microetch product developments were implemented to improve the lifting of carbon off the copper. From an equipment perspective, the mi- croetch spray modules of the process were completely reconfigured. The combination of spray–flood–spray bar configurations proved to be the most efficient design. The distance between the nozzle tip and panel surface was reduced, and the pitch of the fan nozzles was narrowed to increase spray impact force on the panel. This design proved beneficial for high aspect holes and blind vias. With the next generation of smartphones, makers moved to buildup anylayer designs using stacked vias and no through-holes. This initiated a trend wherein starting copper foil thickness on panels has steadily reduced from 18 µm to 12 µm to 9 µm, as line and spacing has decreased from 60 µm to 40 µm. Each buildup layer in these processes requires a metallization and electrolytic plating cycle with more wet processing capacity. Smartphones were also major users of flex and rigid-flex circuits. Adoption of direct metallization grew significantly in anylayer, FPC, and R/F board production due to the lower cost, less water usage, and less waste generation of the process compared to traditional electroless copper processes (Figure 1). Copper Budget: The New Metric for mSAP Process Performance Fast forward to today, and the newest gener- ation of smartphones and advanced packaging are utilizing a fabrication technique called the modified semi-additive process (mSAP). mSAP utilizes ultra-thin foils of 3 µm to reach line and spacing of 30/30 µm. The ultra-thin foils

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