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68 PCB007 MAGAZINE I JULY 2023 Introduction In his Tech Talk series, Karl Dietz wrote sev- eral times about embedded circuits 1 , partic- ularly detailing flush circuit technology and recessed circuit processes. In his column, Karl outlined two processes that have been used to produce these "flush" circuits: 1. Imprint technology, based on the work of George Gregoire at Dimensional Imprint Technology, Inc. 2. Circuit transfer process, based on Sam- sung Electro-Mechanics. e transfer pro- cess, using polished stainless steel as a con- ductive carrier, is not new. I was intro- duced to this plate-post process in 1971, when PACTEL in Westlake Village, Califor- nia, demonstrated its 50-micron microvias, produced using photoresist and the plated- post transfer process. But as laser technol- ogy improved for microvia blind drilling, new applications were developed by Amkor and Siemens for using these lasers to create trenches for circuits and SMT lands. Flush-Embedded Circuits e flush/embedded circuit (Figure 1) is compared to the conventional subtractive- etched PCB circuit. It has several advantages as the geometries shrink to under 75 microns: • Reliability and performance are advanced as adhesion is from three sides. • In many cases, solder mask is not required because lead-free solder paste will not flow or bridge tight geometries. • Signal integrity improves at very high speeds. • For some fab processes, photolithography is eliminated (organo-metallic solder pastes). • An increased number of insulation materials can now be employed. Happy's Tech Talk #21 by Happy Holden, I-CONNECT007 Embedded (Flush) Circuits Figure 1: a) An embedded conductor compared to the conventional Cu-foil etched conductor; b) A laser-scribed conductor/land/via path ready for metallization. a b

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