Issue link: https://iconnect007.uberflip.com/i/1527613
48 DESIGN007 MAGAZINE I OCTOBER 2024 Shaughnessy: When does this construction make sense? Partial HDI construction makes sense when: • Higher routing density is needed only in certain sections of the board (e.g., around a BGA component with high pin count) • Cost optimization is a pri- ority, but some benefits of HDI (like increased I/O or better signal integrity) are still needed • ere is a need to integrate complex chips with finer pitches on certain layers, while the rest of the board can be built using standard methods • For cost-conscious designs that require a balance between HDI and conventional PCB technology, making the board easier and cheaper to manufacture while still supporting advanced components Shaughnessy: What are some of the other pros and cons, and trade-offs that designers need to consider? Pros: • Cost savings compared to full HDI boards • Less complexity in manufacturing (no need for sequential lamination across all layers) • Improved thermal management in non-HDI layers, as fewer layers are involved in the complex HDI process Reduced signal loss in layers that don't use HDI technology due to simpler layer stackups Cons: • Limited routing flexibility compared to full HDI. If complex routing is needed in multi- ple layers, partial HDI may not be sufficient • Design constraints as only certain layers can take advantage of microvias and high-density interconnects, potentially complicating the overall layout • More difficult via transitions, as transition- ing signals from HDI layers to non-HDI layers may require more care in routing • Compromises in perfor- mance, especially in applica- tions needing more intricate interconnectivity across mul- tiple layers Shaughnessy: What sort of cost increase will we see with partial HDI? Partial HDI boards incur a mod- erate cost increase compared to conventional boards but will still be cheaper than full HDI boards. e cost depends on factors such as: • e number of HDI layers (fewer layers = lower cost) • e density of microvias and other HDI features • e complexity of the design, especially the need for specialized materials or manufacturing processes (e.g., laser drilling for microvias) • Depending on the design's specifics, you can expect a 10–30% cost increase compared to a standard PCB. Shaughnessy: Does this process require a very thin layer of copper and dielectric? Does it cause DFM issues and make the fabrication process more complicated? Yes, the HDI layers in partial HDI boards typi- cally use thinner copper and dielectric layers, which can lead to: • Design for manufacturing (DFM) chal- lenges, like more stringent tolerances on aspect ratios, hole sizes, and trace widths • Fabrication complications due to the need for laser-drilled microvias, which require precise control over material thickness • Higher potential for defects—especially if the manufacturer is not experienced with HDI processes—leading to issues like delamination, misalignment, or reliability concerns with vias Stephen Chavez