Issue link: https://iconnect007.uberflip.com/i/1537388
28 DESIGN007 MAGAZINE I JULY 2025 • Establishing a PDN requires strategic plan- ning. This is like utilities in a city and requires similar planning strategies. Power traces are like highways: they need to be wider to han- dle more traffic (i.e., current carrying capacity). Via Rules Here we have the "layer-to-layer interconnect" rules: • The intent of a via is to traverse/transition a routed signal from one layer to another throughout the PCB stackup. They may also be used to connect one layer plane to another plane. • Know the via technology that is to be implemented and/or utilized (i.e., standard through-hole, HDI, or UHDI). » Standard through-holes are standard via structures and are typically mechan- ically drilled holes that are plated, span the entire PCB stackup, and are used for reliably inter-connecting different layers within the stack-up. » HDI/UHDI are typically laser drilled advanced via structures such as: • Microvias are particularly important in HDI, as they are extremely small holes (less than 150 microns in diameter) that connect different layers of the PCB, allowing for denser wiring and smaller board sizes. • Blind vias connect an outer layer to an inner layer without going through the entire board. • Buried vias connect two or more inner layers without reaching the outer layers. • Vias in pads are placed directly within the solder pads of a component, maxi- mizing space utilization. • Note that UHDI pushes HDI even further, fea- turing even finer traces and spaces (typically sub-25 microns) and smaller microvia diam- eters (below 75 microns). This enables even greater component density and miniatur- ization. Increased cost and limited supplier capabilities are factors that must be consid- ered when choosing which via technology to implement. • Don't make them too small and design them at a minimum according to the performance class of your design. • When possible, minimize the number of hole sizes used. Also, don't go crazy with the aspect ratios and annular rings. It's important to know your fabricator's capabilities and lim- itations when it comes to vias. • Keep them far enough apart. Appropriate spacing is key. Manufacturing Details Now we have the "DFM/MDD" rules: • Remember, someone has to actually fabri- cate and assemble this board. • Don't over-constrain your design, making it impossible to manufacture. • Don't specify your engineering intent beyond your supplier's capabilities. ▼ F i g u re 3 : E xa m p l e of p hys i c a l r u l e s : c l e a ra n c e s w i t h t h e c o n st ra i nt s m a n a g e r.