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Depending on the design features of the board, electrical connections in a multilayer structure can be made either through vias or using vias.

• Outer layers. In complex boards, these layers are called assembly layers and are often used only for installing components. In simple boards, in most cases, the outer layers play not only the role of mounting but also signal (conductive) layers.
• Signal layers. These are the PCB Layer that bears the brunt of creating electrical circuits between electronic components.
• Power layers. These layers are made with solid polygons (or “mesh”) with minimal ohmic resistances. In addition to supplying voltage to the circuit, these layers act as electrical “shields” between the signal layers.
• Heat dissipating layers. Excess heat is removed and distributed over the entire plane of the board from circuit components during product operation.

Further, the board can be covered with a protective solder mask. It prevents the solder from spreading out of the contact pads and protects the conductors from external influences.

The final operation is the application of a finishing coating to protect the copper from the influence of the external environment and improve solderability.

According to the materials used in PCB Layer, the boards are divided into:

• Microwave (high frequency)
• Flexible boards
• Flexible-rigid printed circuit boards
• Boards on a metal base
• Conventional rigid printed circuit boards

The design and basic technology of manufacturing double-sided microwave printed circuit boards do not differ from ordinary ones.

However, the design of multilayer microwave boards is somewhat different from the design of conventional MPPs.

Only pairs of outer PCB Layers are made from high-frequency laminates, while the inner layers are made of ordinary glass-epoxy materials or vice versa.

In this way, a clear separation of the microwave and low-frequency parts of the MPP is achieved in the inner layers of the board, together with the grounding and power layers.

Rigid-Flex PCB

Such a design of multilayer microwave boards presupposes the use of the method of MPP pairwise pressing or layer-by-layer build-up.

There is no protective solder mask on the entire microwave board or on a significant part of it, while immersion tin and immersion silver are often used as finishing coatings.

A flexible printed circuit board (FPC) is a board that uses a thin and flexible dielectric as its base material.

The main base material in the manufacture of PPG is a polyimide film. These boards can bend, bend, and compact.

The main difference between flexible printed circuit boards and rigid ones is the possibility of mounting in three-dimensional space.

And the ability to bend around the contours of blocks inside the housing of an electronic device.

It reduces the dimensions of the final product. A type of flexible printed circuit board – flexible printed cable (FPC) – has a thin insulating base up to several meters long with printed conductors located parallel to each other.

Flexible PCB

The thickness of the CHP can be between 0.06 mm and 0.3 mm. Flexible printed cables are usually made with one or two layers of printed conductors.

Rigid-flex printed circuit boards (FPCBs) are the most complex interconnecting structures in electronic equipment.

Typically, a simple GZPP has one rigid and one flexible dielectric layer in its structure, and complex flexible-rigid boards can contain dozens of layers of flexible and rigid cores (cores) assembled into one structure in almost any order.

The design of flexible and flex-rigid printed circuit boards is not much different from that of conventional rigid printed circuit boards.

The main difference is in the materials used. It makes it possible to reliably connect the dielectric and conductive PCB Layer.

Another feature is the use of specialized equipment and technological modes of manufacturing boards of this type.

For products that operate in severe thermal modes of operation, printed circuit boards are made on a metal base.

Such structures have an electrically insulated metal core in the form of a heat-conducting plate.

• Single-layer and double side-CEM-1 and CEM -3FR2
• Multilayer printed circuit boards –FR4, FR5, FR408, FR408HR, IS410, FR406, 370HR, IT180A;
• Microwave printed circuit boards – Isola, Rogers, Taconic, Arlon;
• Combined “composite” printed circuit boards – made of microwave materials with FR4 materials;
• Flexible and flexible-rigid printed circuit boards -made of polyimide and FR4 materials(Shengyi, Panasonic, DuPont, Taiflex).

The number of layers is referred to as the number of separate conductor patterns.

It is usually even and includes the two outer layers. Most mainboards have between 4 and 8 layers, but PCBs with almost 40 layers can be made.

2 layers PCB has more layers than a 1 layer PCB but fewer than a multilayer PCB. 2 layers PCB can mount conductive copper and components on both sides of the printed circuit board so that the traces can cross over each other.

Double Flexible PCB

So it leads to a higher density of boards without the need for point-to-point soldering.

The 4-layer PCB stack-up, on the other hand, is quite elaborate.

Two inner layers, inner layers 1 and 2, are sandwiched between the top and bottom layers.

A typical 6 layer PCB stack-up features the following six layers: two internal layers, two external layers, and two internal planes— one for the power and the other for the ground.

6 layer Rigid-Flex PCB

This design improves EMI and offers better routing for low and high-speed signals.

Typical 8-layer PCB stack-ups feature the power and ground layers in the center, offering good capacitance between layers as well as a separation between the second and third signal planes to protect signal integrity.

8 layer Rigid-Flex PCB

This results in a common 8-layer PCB stack-up that looks like the following:

• Signal layer 1
• Prepreg
• Ground plane
• Core
• Signal layer 2
• Prepreg
• Power plane
• Core
• Ground plane
• Prepreg
• Signal layer 3
• Core
• Power plane
• Prepreg
• Signal layer 4

A 10 layer board should be used when six routing layers are required. 10 layer boards, therefore, usually have six signal layers and four planes.

Having more than six signal layers on a 10 layer board is not recommended.

Ten layers are also the largest number of layers that can usually be conveniently fabricated in a 0.062″ thick board.

It is 1.57 mm for the standard 2 Layer PCB thickness.

Many contract manufacturers may say that the standard PCB thickness is 1.57 mm, or approximately 0.062 in.

This is true to an extent. this was the size that bakelite sheets were manufactured in. during the earlier days of board fabrication and the size has stuck.

1. Route high-speed on minimum thickness microstrips.
2. Place signal layers next to internal power layers for tight coupling.
3. Power and ground layers should have minimal spacing between them.
4. Avoid having two signal layers adjacent to each other.

There are two key elements to consider when determining optimal PCB copper thickness.

The first is the current capacity of the barrel for acceptable heat rise.

The second is the mechanical strength determined by the copper thickness, hole size, and whether or not there are any support vias.

Copper Weight or Copper Thickness in a Printed Circuit Board represents the thickness of 1 ounce of copper rolled out over an area of 1 square foot.

For example, a PCB that has a copper weight of 1 oz will have a layer of copper that is 1.4 mils in thickness.

Prepreg is a dielectric material that is sandwiched between two cores or between a core and a copper foil in a PCB, to provide the required insulation.

It either binds two cores or a core and a copper foil.