How do you balance HDI design decisions with manufacturability and cost?

Today's Discussion
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1

HDI designs provide greater routing density and flexibility, but choices such as layer count, sequential laminations, via structures, drilling methods, and trace/space can directly affect manufacturing complexity and cost.

I wanted to start a discussion on how designers balance these factors early in the design process.

3-N-3 HDI stackup with staggered microvias
3-N-3 HDI stackup with staggered microvias475×291 49 KB

2

Stack-up complexity is an important factor in HDI cost and manufacturability:

  • Lamination cycles: Moving from 1-N-1 to 2-N-2 or 3-N-3 adds sequential lamination steps, increasing process time and cost. Furthermore, it may also increase the risk of registration.

  • Material selection: More complex builds place additional demands on the material. For example, a 3-N-3 structure can involve four lamination cycles before assembly, making material robustness an important consideration. Materials must be chosen not just for Dk/Df, but for thermal robustness (Tg, Td, and z-axis CTE).

  • Eliminating unnecessary drills: Some designs may not need a final mechanical drill if the required connections can be achieved using laser drills and buried mechanical vias. This can eliminate an additional drill cycle and registration step.

3

The choice between stacked and staggered microvias can also affect process complexity:

  • Stacked microvias: Filling or plating microvias from the bottom-up to support the next layer’s landing pad adds time and cost. Stacking on top of buried mechanical vias is also known to create thermal stress concentration points and micro-cracking risks.
  • Staggered microvias: Staggering can eliminate the need to fill the first laser-drilled microvia because the second drill does not land directly on it. The required spacing between the laser drills still needs to meet the manufacturer’s capabilities.
  • Via-in-pad: Via-in-pad can require additional drilling, plating, and filling steps. Where the design permits, offset laser-drilled microvias can provide an alternative with fewer process steps.
4

Drill selection directly affects manufacturing cost and the available routing space:

  • Mechanical drilling: Smaller mechanical drills, for example, 6-mil drills, can significantly increase drilling costs compared to a 10-mil drill size.
  • Microvia aspect ratio: A laser-drilled microvia needs to maintain an appropriate aspect ratio of 0.8:1 for reliable plating.

For a deeper breakdown of HDI lamination classes, drill comparisons, and cost drivers, check out this article: 9 HDI Considerations for Manufacturability and Cost.