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OCTOBER 2024 I DESIGN007 MAGAZINE 23 fabricator will likely utilize base materials pre- pared for a semi-additive copper plating pro- cess. While process proficiency for fabricating HDI or UHDI circuits will vary somewhat from one supplier to another, the examples shown in Table 1 compare the general process targets for subtractive, semi-additive, and fully-additive copper deposition and the process differences in microvia hole-forming methods. A key contributor to enabling higher-density circuits is in the advances made in imaging technology. Laser direct imaging (LDI) and diode imaging systems have become main- stream technology for a wide segment of the PCB fabrication industry. While semi-additive circuit fabrication has enabled significantly finer circuit geometry, an alternative process is available from a relatively specialized sup- plier base that can furnish fully additive copper deposition to furnish even finer copper con- ductor geometry. Supplier Communication Developing the HDI or UHDI board is not a trivial endeavor. Materials and process com- plexities play a significant role in terms of man- ufacturability and the direct cost of fabricating multilayer circuit boards. e challenge here is to select the most suitable base materials for the application while meeting the end-prod- uct's electrical and operating environment requirements. e best advice for the circuit board designer is to establish a dialogue with the circuit board fabrication specialist early on for each new design. Although a great deal of guidance for both high-density and sequential lamination circuit board designs are furnished in publications and IPC standards documents, the board supplier will ultimately be the designer's most meaningful source for ensur- ing end-product satisfaction. In regard to fabrication process efficiency and meeting the cost target for the circuit board, fabricators encourage the designer to evalu- ate the overall physical aspects of the circuit in order to simplify the design. For example, reducing conductor width and spacing will, as noted, significantly contribute to reducing layer count. As far as implementing SBU tech- nology, the most expensive stackup configura- Table 1: HDI and UHDI circuit board process variations. (Source: IPC-2226)