The stackup is a crucial aspect of the PCB fabrication process, encompassing the selection of layers, their arrangement, and the materials used. The primary goal of stackup design is to achieve optimal signal performance and power integrity while keeping the PCB cost at a minimum.

Key Considerations in Stackup Design:

• Required Layers: The first consideration is determining the necessary number of layers for the PCB, depending on the complexity of the circuit and the required functionality.
• Layer Arrangement: The arrangement of layers plays a significant role in ensuring efficient signal flow and minimizing interference. Proper layer stacking helps manage signal integrity and power distribution.
• Layer Material: Selecting appropriate materials for each layer is vital for achieving desired electrical properties, thermal management, and mechanical strength.
• Routing and Vias: Efficient routing and via placement are critical for maintaining signal integrity and minimizing signal losses during transmission.

PCB stack structure

The PCB stackup is the backbone of any electronic design, and its importance cannot be overstated. An inadequately designed stackup or improper material selection can significantly impact the performance, manufacturability, and reliability of the entire circuit assembly.

Typically, a PCB stackup comprises alternating layers of core, prepreg, and copper foil materials that are bonded together through a hot pressing process.

The core material, composed of cured fiberglass woven material with epoxy resin, serves as a thin electrical insulator (dielectric) with copper foil bonded on both sides. This dielectric layer ensures insulation between the copper foils, while the internal copper forms the signal, power, and ground planes of the PCB. The prepreg acts as an insulator, bonding the core layers together.

The PCB fabrication process involves laminating the layer arrangement of core and prepreg layers with top and bottom copper layers. This bonding of various layers forms the complete PCB stack-up through the application of heat and pressure.

The number of layers required in the PCB stackup depends on critical factors such as power, ground, and signal layer requirements, the complexity of the board, component density, and the type of component packages used. In cases involving high channel count or high-density devices, blind vias may be necessary in channel routing to ensure optimal signal integrity and proper layer placement.

Hierarchical arrangement

In the arrangement of the layer stack, it is common practice to position power planes directly alongside ground planes. This layout ensures optimal signal performance and robustness in the circuit. Additionally, to minimize the overall number of layers in the PCB, it is beneficial to share power planes whenever possible.

When it comes to the spacing between adjacent signal layers, it is essential to increase it appropriately. However, one should be mindful that larger spacing may impact the overall thickness of the PCB, especially in cases where large pitches are involved.

For effective high-speed routing, careful planning of routing layers is crucial during the stack-up design phase. High-speed signals should be routed on microstrips with minimal thickness to maintain signal integrity and reduce interference.

Dielectric material

FR4 is widely used as the most popular dielectric material in the fabrication of PCBs. It is available in two main forms: core and prepreg. The core material consists of a thin layer of cured fiberglass epoxy with copper foil bonded on both sides. For instance, Isola offers a range of FR406 cores in various thicknesses, such as 5, 8, 9.5, 14, 18, 21, 28, 35, 39, 47, 59, and 93 MIL. The copper thickness typically ranges from ½ to 2 ounces (17 to 70 microns).

On the other hand, prepregs are thin fiberglass sheets impregnated with uncured epoxy resin. During PCB manufacturing, these sheets harden when subjected to heat and pressure. Isola's FR406 material also includes a variety of prepregs, such as 1.7, 2.3, 3.9, and 7.1 MIL, which can be combined to achieve the desired prepreg thickness.

The commonly used layup method, known as the "foil method," involves bonding a prepreg with copper foil to the outermost layers (top and bottom) and then alternating core material with prepreg throughout the substrate.

FR4's versatility, availability in various thicknesses, and ability to form the core and prepreg layers make it a popular choice for PCB fabrication, offering reliable and efficient performance for electronic applications.