14 Layer 2 Step HDI RO4350B PCB

Designing and manufacturing a 14-layer, 2-step High-Density Interconnect (HDI) PCB using RO4350B material is a complex process that requires careful planning and execution. This guide will walk you through the key considerations and steps involved in creating such a sophisticated printed circuit board.

Material Selection: RO4350B

RO4350B is a high-frequency laminate and prepreg material from Rogers Corporation. It’s ideal for high-layer count multilayer boards due to its excellent electrical and mechanical properties:

• Low dielectric constant (3.48 @ 10 GHz)
• Low loss tangent (0.0037 @ 10 GHz)
• Tight thickness tolerance
• Good thermal stability
• Compatible with lead-free processes

These properties make RO4350B suitable for high-speed digital and RF applications, which are common use cases for complex HDI designs.

HDI Technology Overview

High-Density Interconnect (HDI) technology allows for higher circuitry density and improved electrical performance. The 2-step HDI process involves:

1. Creating a core with buried vias
2. Adding build-up layers on both sides with microvias

This approach allows for more routing channels and shorter signal paths, which is crucial for high-speed designs.

Layer Stack-up Design

For a 14-layer 2-step HDI PCB, a typical stack-up might look like this:

1. Top layer (Signal)
2. Prepreg
3. Signal layer
4. Core
5. Signal layer
6. Prepreg
7. Signal layer
8. Core
9. Signal layer
10. Prepreg
11. Signal layer
12. Core
13. Signal layer
14. Bottom layer (Signal)

The core layers would have buried vias, while the outer build-up layers would use microvias for connections.

Design Considerations

1. Impedance Control: Calculate and maintain consistent impedance for critical traces. Use field solvers and work closely with your PCB manufacturer to achieve the desired impedance.

2. Signal Integrity: Use proper routing techniques, such as length matching for differential pairs and avoiding sharp corners.

3. Power Integrity: Implement a robust power distribution network (PDN) with sufficient decoupling capacitors and power planes.

4. EMI/EMC: Consider electromagnetic interference and compatibility by using proper shielding techniques and adhering to best practices for high-speed design.

5. Thermal Management: Plan for heat dissipation, especially for high-power components. Consider using thermal vias and copper pours.

6. DFM (Design for Manufacturability): Follow your manufacturer’s design rules for minimum trace widths, spacings, and via sizes.

Manufacturing Process

1. Inner Layer Processing:

   • Create artwork for each inner layer
   • Clean and prepare the core material
   • Apply photoresist and expose to UV light
   • Develop the resist and etch the copper
   • Strip the remaining resist
   • Perform automated optical inspection (AOI)

2. Lamination:

   • Stack the etched inner layers with prepreg between them
   • Apply heat and pressure to bond the layers

3. Drilling:

   • Drill holes for through-hole vias and component leads
   • Use laser drilling for microvias in the build-up layers

4. Plating and Filling:

   • Electroless copper plating to coat the hole walls
   • Electrolytic copper plating to build up copper thickness
   • Fill larger vias with epoxy or copper

5. Outer Layer Processing:

   • Similar to inner layer processing, but for the outer layers

6. Solder Mask and Surface Finish:

   • Apply solder mask to protect traces
   • Add surface finish (e.g., ENIG, immersion tin) to exposed pads

7. Silkscreen:

   • Print component designators and other markings

8. Electrical Testing:

   • Perform flying probe or bed-of-nails testing to ensure connectivity

9. Final Inspection and Packaging:

   • Visual inspection
   • X-ray inspection for hidden features
   • Packaging for shipping

Challenges and Solutions

1. Registration: Accurate alignment of layers is crucial. Use fiducial marks and advanced imaging systems during manufacturing.

2. Aspect Ratio: Maintain appropriate aspect ratios for vias to ensure reliable plating. Work with your manufacturer to determine the minimum viable drill size.

3. Z-axis Expansion: RO4350B has a lower coefficient of thermal expansion (CTE) in the Z-axis compared to FR-4. This can lead to reliability issues with plated through-holes. Use proper via design and consider adding flexibility to the stack-up with appropriate prepreg selections.

4. Cost: HDI designs with specialized materials like RO4350B can be expensive. Optimize the design to use HDI features only where necessary.

Conclusion

Designing and manufacturing a 14-layer 2-step HDI PCB with RO4350B material requires a deep understanding of high-speed PCB design principles, material properties, and advanced manufacturing techniques. Close collaboration with your PCB manufacturer is essential to ensure that the design meets all electrical and mechanical requirements while remaining manufacturable. By carefully considering signal integrity, power integrity, thermal management, and following proper HDI design guidelines, you can create a high-performance PCB suitable for demanding applications in telecommunications, aerospace, and other high-tech industries.