PCBs are the basis of all electronic devices, without which that device cannot function. Essentially PCBs, also known as Printed Circuit Boards, consist of electric components and conductive traces that are built onto a board to help connect the components and to direct electricity. PCBs can be divided into distinctive types; one of them is Multilayer PCBs.

What are Multilayer PCBs?

Multilayer PCBs, as the name suggest are a combination of different layers. Single sided PCBs and double sided PCBs are both combined to give this complex designed PCB. Multilayer PCBs have increased number of layers and therefore have increased area available for wiring. The boards are made by combining double layered PCBs and separating them with insulating material such as dielectric.

The Number of Layers

The minimum number of conductive layers between the insulating materials is three, and we can incorporate as much as a hundred layers according to our need. We consider Multilayer PCBs as Rigid PCBs because it is difficult to manufacture flexible multilayer PCBs. Majority of our common Multilayer PCBs consist of 4 to 8 layers. Smartphones can incorporate as many as 12 layers since they fit the complexity of the application. Manufacturers prefer even number of layers over odd as laminating odd number of layers can make the circuit too complex and has issues, high cost is also a factor to consider.

Instead of building normal viases into some layers, which may waste space, buried or blind viases are used. They penetrate the necessary layers only. The different layers can also be categorized as different planes, such as ground plane, power plane, signal plane etc.

Building a Multilayer PCB

Multilayer PCBs are built by joining all the number of layers and materials at high temperature and pressure so as to remove any trapped air between the layers. Resin and adhesive material is used to stick the components and different layers together. You can choose from a number of materials such as exotic ceramic, epoxy glass, or Teflon etc. to build your PCB. The different prepreg and core layers are combined and go through the lamination process taking place at high temperature and pressure that helps to melt the layers together. After that the PCB is cooled down to form a hard and solid board. 

Factors to keep in mind while Manufacturing Multilayer PCBS

There are a number of factors you have to keep in mind while building your Multilayer PCBs. For example, signal integrity and power integrity etc. Copper traces on the PCB allow current to flow through but also has resistance which can, in some cases, alter the current or quality of current which will affect the overall performance of the board. This is why controlled impedance traces are required. The copper thickness should be increased than normal value because these traces can exceed in temperature. In order to control the temperature, the width of the traces is increased accordingly. 

Magnetic field is generated from individual components when current passes through them. In multilayer PCBs return path or ground place is used as a solution by acting as a shield and helping to provide a return path for the unnecessary signals. 90 degree traces should be avoided so there is no EMI (electromagnetic induction) issue.

Identifying the Different Layers in a Multilayer PCB

In order to identify the individual layers on a Multilayer PCB you should keep a few things in mind. For example, using a bright light to see the copper planes will let you see the traces more clearly and will help you to analyze the inner layers. You can easily identify the inner layers when there are no lines or paths crossing your line of vision. In some PCBs the total number of layers is imprinted onto the edges.

Drawbacks of Multilayer PCBs

However useful multilayer PCBs can be, they will have their drawbacks and limitations. The cost for multilayer PCBs is a factor to consider, you should consider the cost for your project before you choose multilayered PCBs. Multilayered PCBs also require more time to manufacture and are not suitable for projects you need on short notice. The repairing for multilayered PCBs also is more complex as compared to normal PCB designs.

Advantages of Multilayer PCBs

Multilayer PCBs have a few advantages over single or double layered PCBs. They have higher density which allows greater functionality, capacity and speed. Electromagnetic shielding is easier if power and ground layers are placed correctly. There is no need of extensive wiring and hence, makes them low weight. They reduce size and help to save space. They have increased flexibility than other types of PCBs.

Applications of Multilayer PCBs

There are numerous applications of multilayered PCBs. These include:

· Computers

· Fiber Optic Receptors

· Data storage

· Signal transmission

· Cell phone transmission

· Cell phone repeaters

· GPS technology

· Industrial controls

· Satellite systems

· Hand held devices

· Test equipment

· X-ray equipment

· Heart monitors

· Cat scan technology

· Atomic accelerators

· Central fire alarm systems

· Fiber optic receptors

· Nuclear detection systems

· Space probe equipment

· Weather analysis

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