Stackup

Introduction

A PCB (printed circuit board) is a multilayered board with the simplest being a single layer. The stack-up is the description of the arrangement of this multi-layered structure. This description is essential to the PCB manufacturer in order to make the PCB.

Background

A PCB is made by laminating each PCB piece from the center to the outer layers. Each inner piece contains two copper layers sandwiched between a core layer. After copper etching (removing unwanted copper to form traces and vias), each inner piece is laminated together by adhesive materials called prepreg. Via drilling comes after laminating.

At the outer layers, there will be additional finishing copper plating for the vias, adding to the thickness of the outer layers.

Definitions

• Via: A hole that connects PCB layers together.

• Trace: The copper wiring that connects IC pins, connectors, and electronic circuits together.

• Substrate layer: The core layer, often made of fiberglass.

• Adhesive layer: The prepreg layer, which is made of fiberglass mixed with half-cured resin material.

• Conductive layer: The copper layer.

Overview

The stack-up is summarized in a table as follows:

• Number of layers

• Description of each layer

• Materials of each layer

• Thickness of each layer

• Via arrangement for each layer

Practical Design

A high-density interconnector (HDI) PCB is a type of PCB that is designed to be thin and lightweight, with smaller trace widths and spacings. It also uses stacked laser-drilled microvias and vias in pads to allow for easier signal wiring fanout from finer pitch and dense pin areas under an IC package.

A typical 10-layered HDI PCB might have the following layer definition:

• Layer 1: Mixed component and signal

• Layer 2: Ground

• Layer 3: Signal (1/2 oz copper)

• Layer 4: Ground

• Layer 5: Power (1/2 oz copper)

• Layer 6: Power (1/2 oz copper)

• Layer 7: Ground

• Layer 8: Signal (1/2 oz copper)

• Layer 9: Ground (1/2 oz copper)

• Layer 10: Mixed component and signal

The total thickness of the PCB would be determined by the thickness of each layer and the via stackup.

Summary and Conclusion

The stack-up is a critical part of PCB design. It determines the electrical and mechanical properties of the PCB, as well as the manufacturing process. The stack-up should be carefully designed to meet the specific requirements of the application.

HDI PCB technology is a modern PCB technology that allows for high-density component design. It uses thin layers, small trace widths and spacings, and stacked laser-drilled microvias to achieve high density.

Prepreg is the glue layer that holds the PCB layers together. Copper is the conducting layer that carries the electrical signals. Core is the substrate layer that provides mechanical support for the PCB.

Due to the increasing miniaturization of IC chips and form factors, it is common to see HDI multi-layer boards with 8, 10, or 12 layers. These boards are used in a variety of applications, such as laptops, mobile phones, and high-end audio equipment.

References

• "Tips for PCB Stack Design", https://www.tempoautomation.com/blog/top-4-tips-for-pcb-stackup-design/

• "PCB Stackup", http://www.stormcircuit.com/pcb-stackup.html