16‑Layer Any‑Layer HDI for Implantable Medical Device Miniaturization

1. Background — When Conventional HDI Is No Longer Enough

In this project, the customer was developing an implantable medical device with strict size constraints and high integration requirements.

The initial design used a conventional HDI structure (sequential lamination with limited layer interconnection).

Electrically, the design was feasible, but physically it reached a bottleneck:

• Routing congestion in core layers
• Long signal paths due to limited via transitions
• Inability to further reduce board size without redesign

At this stage, the issue was not layout skill. It was the limitation of the interconnect architecture.

2. Why Any‑Layer HDI Was Required

To break this limitation, the structure had to change.

A 16‑layer Any‑Layer HDI design was introduced, allowing:

• Microvias between any two adjacent layers
• Stacked via structures across multiple layers
• Signal transitions without being restricted to specific build‑up layers

This fundamentally changed routing freedom.

Instead of routing around constraints, the layout could follow signal and functional requirements directly.

3. What Makes 16‑Layer Any‑Layer HDI Difficult

This is not a standard HDI board.

The technical difficulty is not only in design, but in manufacturing control.

1. Microvia Density and Stacking 
Laser microvia diameter: ~0.1 mm 
Multiple stacked microvia layers (via‑in‑pad structures) 
High via density under fine‑pitch components

Challenges:

• Alignment accuracy across multiple lamination cycles
• Reliable copper filling without voids
• Preventing microvia cracking under stress

2. Fine Line Routing Capability 
To support miniaturization:

• Line/space reduced to fine pitch range (e.g., ≤3/3 mil level)
• High routing density under BGA and IC areas

Challenges:

• Imaging accuracy (LDI requirement)
• Etching control to maintain impedance and uniformity
• Yield stability at high density

3. Sequential Lamination Complexity 
16‑layer Any‑Layer HDI typically requires multiple lamination cycles.

Challenges include:

• Registration control across each lamination stage
• Resin flow control to avoid voids
• Stack‑up stability after repeated thermal processes

Each lamination cycle increases cumulative error risk.

4. Via‑in‑Pad and Planarity 
To maximize space usage:

• Via‑in‑pad structures were used under key components
• Requires flat, filled, and reliable via surfaces

Challenges:

• Copper filling consistency
• Surface planarity for assembly reliability
• Avoiding voids or sink marks affecting soldering

5. Material and Reliability Constraints 
In medical applications:

• Materials must maintain long‑term stability
• Thermal and mechanical reliability must be ensured
• Structure must withstand repeated environmental stress

Miniaturization increases stress concentration, which directly impacts microvia reliability.

4. What Changed in This Project

By implementing the 16‑layer Any‑Layer HDI structure:

• Routing paths became significantly shorter
• Signal layers were used more efficiently
• Component density increased without adding board size

More importantly:

👉 Layout constraints were removed

👉 Interconnect became flexible instead of restrictive

5. Result

The final design achieved:

• ~30% reduction in board space
• Improved routing efficiency
• Stable electrical performance across all layers
• No microvia‑related reliability issues during validation

The structure passed all required verification without introducing new risks.

6. What This Case Shows

Miniaturization at this level is not achieved by making things smaller.

It is achieved by changing how the PCB is built.

When interconnection is no longer constrained by layer hierarchy:

• Routing becomes direct
• Space is used more efficiently
• System integration becomes feasible

7. Final Thought

Any‑Layer HDI is not just a higher‑end PCB option.

It is used when conventional structures can no longer support system requirements.

ULTRONIU Electronics 
When structure becomes the limitation, we change the structure.