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54 DESIGN007 MAGAZINE I JANUARY 2025 circuit routing becomes increasingly chal- lenging, oen forcing the designer to rely on additional circuit layers for subsurface circuit routing. Circuit board fabricators will caution the designer to show restraint with the circuit layer count as well as to implement overly complex layer-to-layer interconnect schemes. e cost can adversely affect the competitive position of the product. PCB Design Process With the fabricators' ground rules estab- lished, the next step is to confirm that the designer's CAD library includes all the parts data and process-proven land pattern geom- etry. Although the circuit board's design will continue to include a significant number of passive and single-function low to moder- ate I/O lead-frame and ball grid array semi- conductor packaging, the new generations of high-function semiconductor package families require more terminals and significantly nar- rower terminal pitch than their predecessors. e next step is establishing the circuit board's outline and calculating the basic area for component placement. When estimating the area for circuit interconnect, designers must contemplate the land pattern features for mounting the components, define ''keep out zones," and establish clearances reserved for assembly process evaluation (and, when neces- sary, post-assembly rework and repair). Fortu- nately, soware tools have been developed to assist the designer with component placement and include efficient auto-routing features to speed up the interconnect process. e final analysis will provide the designer with an esti- mated maximum surface area needed to com- plete all circuit interconnects. Interconnecting the high I/O semiconduc- tors can dramatically affect the procedures in circuit board design and assembly process- ing. While a significant number of semicon- ductor packages will have a moderate level of complexity (I/O and terminal pitch), oth- ers may have an excessively high I/O density that restricts conductor routing escape paths. "Channel width" is the term for the space between component attachment areas. e channel widths for routing array-configured semiconductors can be calculated using the terminal pitch (center-to-center distance) and the size of the land pattern. is provides the maximum number of conductors that can be routed between each channel (conductors per channel). When these routing channels are restricted further, the designer will need to consider sub-surface circuit routing to achieve interconnect. e most common solution to conductor routing roadblocks is to adopt blind via-in-land processing, transferring most of the interconnect responsibility between com- ponents to the circuit board's sub-surface lay- ers. Adopting blind and buried microvia holes and furnishing pre-defined routing channels will help the circuit board designer route these oen very fine-pitch and array terminal config- ured semiconductor packages. PCB Fabrication Circuit board fabrication specialists caution designers that the high-density interconnect circuit board manufacturing process will be more complex than the moderate-density cir- cuit board. To ensure a successful outcome for the HDI circuit board, the designer must con- sider the manufacturing process complexities and associated costs when implementing the more sophisticated fabrication procedures. e more technically competent PCB fabri- cation companies can produce conductors as narrow as 25 μm (~0.001"), but they rely on dielectric materials with a very thin copper foil to define the circuit pattern. When conductor lines and space widths must be reduced further, the fabricator will use base materials prepared for a semi-additive copper plating process. To help the designer establish copper con- ductor width and spacing for circuit routing, the IPC-2226 specification has defined three HDI circuit board complexity levels (Table 1) for both external and internal locations.