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70 PCB007 MAGAZINE I JULY 2023 As seen in Figure 2, the embedded traces do not interfere with components assembled over them. Processes e transfer process starts with a polished metal substrate that has been coated with a thin conductive release agent. Next, the car- rier is coated with a photoresist and exposed to UV light through a mask (or DD imaging) to define the circuit. Once the photoresist is developed, the carrier is cleaned, and metals are plated into the exposed image. Since the carrier will be removed later, the first metal plated (usually gold or silver) will be exposed aer fabrication, followed by a plated barrier metal of nickel, and finally copper for conduc- tivity. Following plating, a planarization step provides a flat surface for attachment to a per- manent dielectric; then the steps are repeated until the entire circuit is connected. is pro- cess is shown in Figure 3. The Laser Scribing Process As I described earlier, laser drilling is an integral part of HDI fabrication. Several OEMs have developed processes that use the laser Figure 2: Embedded circuits can provide finer circuitry and flexibility in assembly. Figure 3: Transfer process using a carrier and photoresist with plating.