HDI and via types
Explain blind, buried and microvias. Why use HDI in a phone or wearable?
| Level | ●● onsite technical |
| Track | General board |
| Really testing | Fabrication literacy. |
Going deeper
Extra explanation that is not in the book: worked numbers, where people go wrong, and how to say it in the room.
Worked example — why 0.4 mm pitch forces HDI. BGA pads are about 0.25 mm, leaving about 0.15 mm between them. A standard through via (0.2 mm drill, ~0.45 mm pad) doesn't fit between the balls, so a dog-bone fan-out is impossible. A laser microvia (≈ 0.1 mm) placed in the pad is the way out.
Aspect ratios to quote. Through vias: up to about 8–10:1 (depth:drill) on standard processes. Microvias: about 1:1 or less — which is why each one spans only one dielectric layer.
How stack-ups are named. "1+N+1" = one microvia layer on each side of a standard core; "2+N+2" = two. Each "+1" adds a lamination cycle — cost and lead time.
Where people go wrong
- Stacking microvias three high without asking the fab about reliability — staggered is safer.
- Via-in-pad without fill and cap: solder wicks into the via and the joint starves.
Errata
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