Part of the rigid-flex circuit board resource.
Fine-pitch components, dense escape routing, and compact rigid islands can justify high-density interconnect features. Begin with the component fanout and the available routing channels. A smaller via is not automatically the best choice if a simpler construction meets the electrical and packaging requirements.
Describe the via structure explicitly
Identify blind, buried, and through connections by start and stop layers. Specify whether microvias are staggered or stacked and how they interface with other plated structures. Do not describe an entire board as “HDI” and assume the fabricator knows the intended lamination sequence.
Keep the flex requirement visible
HDI requirements in a rigid island do not eliminate the mechanical limitations of the flexible span. Copper distribution, the flex substack, and transition details still need separate review. Avoid forcing added copper or plated features into the active bend simply to preserve a rigid-region routing habit.
Review process and reliability implications
Sequential lamination, laser processing, filling, and additional registration steps can affect cost and yield. Request the partner’s proposed build sequence and the acceptance evidence relevant to the via construction. Reliability must be assessed for the actual stack, material system, and operating conditions.
Quote the same architecture across suppliers
Include the via map, regional stackup, material requirements, feature sizes, and test plan. Ask suppliers to identify construction changes in their responses. A quotation based on staggered microvias may not be comparable with one based on stacked structures, even when both are described as HDI.
Next steps for your project
Connect this review with the regional stackup, material choices, and design checklist. Use the design workbench to organize initial parameters, then send the project for a quotation review.