Introduction: High-density HDI PCBs for optical interaction involve precise 3 mil/3 mil trace/space, Rogers or Panasonic M6 substrates, and electrolytic tricky gold finish for reputable 8-layer, 3–5 cm² https://thejillist.com/story12111710/variety-requirements-for-prime-density-hdi-pcb-boards-in-optical-interaction