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APRIL 2024 I DESIGN007 MAGAZINE 23 of conductors that can be routed between each channel (conductors per channel). When the surface channels are not wide enough to facili- tate the conductor routing, the designer will need to consider reducing the conductor width and spacing on the circuit board surface or resort to sub-surface circuit routing to achieve interconnect. Note: Whenever possible, to reduce bow and twist and to increase dimensional stability, conductor routing density should be uniformly distributed throughout the printed circuit board's layer structure, and conductor density should be balanced within individual layers. Supplier Assessment and Process Refinement Know your PCB supplier and their capa- bilities; you don't want to end up design- ing a board that can't be manufactured. In addition to the design guidelines furnished in IPC-2221 and IPC-2222 for rigid circuit boards, suppliers will oen publish their own design rules. To ensure a successful outcome for the circuit board design, it will be impor- tant that the designer recognize the manu- facturing process complexities and become familiar with the circuit fabrication suppli- er's technical capability. But before commit- ting to adopting HDI or UHDI technology, the designer should confirm that the PCB supplier(s) selected can furnish the preferred complexity level at the anticipated produc- tion quantity. Many OEM companies have already estab- lished a business relationship with their pri- mary circuit board suppliers and have qualified their level of expertise and capability. Review your PCB suppliers' material offerings: • Discuss any unique base material or copper foil needs • Clarify layer stacking alternatives with the supplier • Define power and ground distribution objectives • Specify controlled impedance requirements When estimating the circuit board's manu- facturing cost, the designer must consider materials, circuit density, the number of cir- cuit layers, and fabrication process complexity. While mechanically drilled and plated via holes are a mature technology for multilayer circuit board fabrication, the buried via and laser- ablated blind microvia hole forming requires more sophisticated systems. DESIGN007 Vern Solberg is an indepen- dent technical consultant, specializing in SMT and microelectronics design and manufacturing technol- ogy. To read past columns, click here. Table 1: Establishing HDI complexity levels (Source: IPC-2226)