Multilayer Rigid PCB

As we can observe, the electronics used in our daily lives are becoming increasingly compact without compromising their functionality. This is achieved through the utilization of SMT and advancements in manufacturing techniques, particularly in the field of multi-layer circuit structures. In this article, FS Technology will introduce the multilayer rigid PCB, which is a printed circuit board that combines high circuit density with robustness.

rigid multilayer pcb

What is a multilayer rigid PCB

Printed circuit boards can be classified into four main types: rigid, flexible, semi-flexible, and rigid-flexible. The key factor determining the type of PCB is the substrate material used. Among these types, rigid boards are the most common and straightforward to manufacture with multiple layers. When a rigid board consists of four or more conductive layers, it is referred to as a multilayer rigid PCB. These layers are separated by an insulating material, forming a single integrated unit.

Compared to single or double-sided, multilayer structure offer the advantage of accommodating a higher number of layers within a single unit. The layers in a multilayer rigid board are connected using vias, which allow for the integration of more components and complex circuits.

There are three main types of vias used to establish connections between the layers in a multilayer rigid PCB: buried vias, blind vias, and through-hole vias. This structure typically consists of external layers located at the top and bottom, which function as single-sided boards. The internal layers, on the other hand, act as double-sided boards and are interconnected. Some of these layers may serve specific purposes such as power planes or ground planes. Depending on the requirements of industrial projects and design specifications, rigid multilayer PCB can have a large number of layers, ranging from a few to over a hundred. However, it is more common to find multilayer PCB with four to eight layers in various devices, such as smartphones, where the circuitry is often configured on 12-layer boards. In these applications, the inner core of multilayer rigid printed circuit boards is composed of core layer, prepreg, intermediate layer and inner layer. The following is the explanation of these layers:

  • Core: It is the central part and base of the board that is made of rigid material FR4 and maintains the stability of the board with provides the mechanical support
  • Prepreg: It is resin-impregnated fiberglass fabric it bonded the board layers together. it is the main component of a multilayer board that provides insulation between board layers so we can say it is an insulator
  • Midlayer: It is the internal layer of the board that exists between the core layer and the prepreg. it normally has copper traces, vias, and components used to make a connection between different layers of board
  • Internal plane:  An internal plane, also called a power plane or ground plane is the layer in the board used for voltage stability and grounding of the board. Internal planes are created with copper and provide low-impedance paths to distribute power.

Rigid Multilayer PCB Via Technology

The via process plays a crucial role in the manufacturing of multilayer rigid PCBs. It involves the creation of small holes that establish connections between different layers of the board. These vias are essential for handling complex circuit designs effectively. The material used for vias needs to exhibit good electrical conductivity and mechanical strength. Copper is commonly used for vias, but nickel and tin can also be utilized. Tin provides excellent soldering properties, while nickel offers corrosion resistance. The shape and structure of the vias are determined based on the specific requirements of the board design. They can come in round, square, or rectangular shapes, with diameters typically ranging from 0.010 to 0.030 inches and thicknesses of 0.002 to 0.004 inches.

  • Plated Through-Hole (PTH) Vias: These vias are widely used in multilayer rigid circuit structure and extend through the entire thickness of the board, with copper lining the vias to establish connections between layers.
  • Blind Vias: Drilling from the outer layers of the board to the inner layers makes connections between specific layers and does not extend across the entire board.
  • Buried Vias: Being only on the inner layer of a rigid PCB without connecting to the outer layer provides routing flexibility and helps to design a compact structure by effectively utilizing the inner layers.
  • Microvias: A type of miniaturized vias that have a smaller diameter than conventional vias, enabling high-density interconnects for projects requiring fine-pitch components and high-speed signal transmission.

Rigid Multi-Layer PCB Features

  • Reduced Form Factor: The use of conductive and insulation layers in rigid multilayer PCB allows for compact density and a smaller board size, resulting in a decreased form factor. This advantage makes these boards suitable for applications that require space-saving designs, such as compact electronic devices.
  • Manufacturing Techniques: Including precise layer stacking, accurate alignment, and controlled lamination processes, enable the production of accurate and reliable boards for complex circuits and high-performance operations.
  • Lightweight Construction: The materials used in rigid multilayer PCB is lightweight compared to commonly used fiberglass. This characteristic contributes to the creation of lightweight boards, making them ideal for weight-sensitive applications like the aviation industry, mobile devices, and automobiles.
  • Multifunctional Board Design: Boards can accommodate numerous components and complex designs on a single board. This capability allows for the integration of multiple functionalities onto a single board, reducing the need for additional boards and interconnections. This results in a compact and efficient system for electronic devices.
  • Enhanced Quality: Rigorous quality control measures, such as AOI and X-ray inspection, are implemented during the manufacturing process of these boards. These measures ensure high quality and decrease failure rates.
  • Durability and Flexibility: Circuit offers greater durability and flexibility compared to other boards. Their reinforced and multilayer structure enhances mechanical strength, making them resistant to bending, warping, and thermal stresses. These features enable them to perform reliably even in harsh environmental conditions.
  • Single Connection Point: These boards simplify the management of multiple connections by consolidating them into single connection points. This simplification reduces the complexity of the system, minimizes the risk of faulty connections, and improves overall reliability. Additionally, it facilitates easier board maintenance.
  • Higher Production Costs: Rigid multilayer PCB production is more expensive compared to single-layer or double-layer. The cost is attributed to the expensive materials used and the complexity of the manufacturing process.
  • Complex Design Process: Multilayer Designing involves additional complexity compared to other boards. Considerations such as signal routing between layers, trace impedance, and thermal operation of the PCB need to be carefully addressed, requiring advanced design techniques.
  • Limited Availability: Most manufacturers can make rigid multilayer PCBs with less than ten layers, but very few when the number of layers rises to 30 layers. Obviously FS Technology has this capability, for rigid boards, we can manufacture up to 56 layers!
  • Longer Lead Times: Due to the increased number of layers and complexity in the manufacturing process, rigid multilayer boards typically have longer lead times compared to other boards.