Multi layer PCB Design

In this series of tutorials till now, we have worked on single side and two sided Printed circuit boards. Now let’s learn about multi layer PCBs.

 

 

For small and medium scale circuits, only two layer PCBs are enough. But in case of complex and big circuits, wiring occupies more space on the board. So,

 

• to save the space,  

• to improve assembly density and 

• to increase the area available for the wiring, 

 

Instead of increasing  the board  dimensions (length & width), we arrange  alternating layers of conductive material (copper) with insulated material (Prepreg) in between that is called multi layer design.

 

 

Multi layer PCB is a PCB with more than two layers. Here, layer means the conductive material which is buried in the center of the insulated material. Multi layer PCB contains at least three layers and above for example: 4, 6, 8, 16…..64- layers. Multi layer PCBs are especially used in case of the over loaded weight and volume. Apart from that these are mainly used in professional electronic equipment circuit designing and in complex circuits like military equipment, computers, mobiles, etc. 

 

 

In multilayer PCBs, alternating insulated and conductive layers are laminated together under high temperature and pressure.  This process ensures that air isn’t trapped between layers and conductors are completely encapsulated by resin. The range of the material combination is extensive from basic epoxy glass to exotic ceramic or Teflon materials. Multilayer PCB manufacturing needs a specialized hydraulic press with heated plates.

 

In PCB’s manufacturing process, different materials are used on the designer suggestions and in the concern of electrical, temperature and other related properties.  For example: Base or foundation of the PCB is the rigid fiber glass laminate, standard material for base is FR4, and for tracks or core 1 oz copper. The standard thickness would be changed as per the requirements and layers, but standard total thickness is 0.62”.  

 

 

 

 

 

 

 

 

 

1. Increased in flexibility.

2. Considerable saving of space.

3. High assembly density.

4. Reduces overall weight.

5. Reduces interconnection wiring harnesses. 

 

 

• Where cross-talk levels are critical, multilayer PCBs are very useful.

• Where there is a limitation on weight and space multilayer PCBs are especially valuable.

• In Aerospace PCBs.

• Computer mother board and mobile motherboards manufacturing.

• Satellite systems.

• X-ray equipment.

• Medical electronics. 

• Etc so many examples we may give

 

 

 First, the designer has to design a multi layer PCB design and then manufacturer proceeds with the manufacturing process with help of respective Gerber files. 

 

Designer may use any software like ORCAD, ENGLE CAD, KICAD, etc. Here in this tutorial, I am designing multiple layered PCB by using KiCad software. 

 

Before designing a multi layer PCB, we must know a few basic concepts like via and their types. I have already explained about via and types of via in the 1st tutorial of this series. Please refer to that for more information.

 

Via: It is a hole with copper inside, it can provide electrical connection between trace on one layer to trace on another layer.

Blind via: Via used between top layer and internal layer.

Buried via: Via between internal layers.

Through hole via: Via between top and bottom layers.

 

 

• In a two-layer PCB, designing through hole via plays a vital role. But in multi layer PCB designing, the wiring is done using blind and buried via. 

• If we have many Ground and power connections, assigning separate layers for Ground and power is better. It will reduce confusion and efforts. 

• If we need more inter connections to reduce the space of the board then selecting more than four layers is a better idea.  

• All the components are placed on either top and bottom layers. And layers in between top and bottom layers are used for wiring (like jumpers). 

• Usually, top and bottom layers are used as signal layers and 2nd and 3rd layers are used as Ground and power layers. If more layers are required then we can select 6, 8, or more layers even. These layers are usually used as inter connections.  But it is not fixed; we can assign them ground or power plane  as well.

 

 

 As per the requirement, select how many layers PCB we need. To select Layers click on “Design Rules” tab. We have two options, one is Design rules and another one is Layers setup. Click on layers setup option.

Design Rules >> Layers Setup.

 

 

 

 Then a window will appear on the screen with the name of “Layer setup”. In this window, we can select as to how many layers PCB we want to create and also we can assign here which layer is signal and which layer  is power or ground.

 

 

 

 After selecting a number of layers (4 or 6 or etc.), continue the PCB editing as explained in “Editing PCB tutorial”. Follow the same steps to edit. 

 

 In editing, to select working layer, use “Visibles” Tab at the right side of the window. 

 

 To set any type of via like blind or buried, right click and select via and up to which layer. Please watch the video to learn this better. 

 

 As per the requirement, we can create via as through hole, blind or buried. But try to create as less vias as possible.

 

 On ground and power layers, there is no need to interconnect all pins using track (wire). They will be connected to the copper layer.

 

 When creating via, take care of other connections. We cannot connect via on any track. 

 

 

 

 

 After this, please follow the same procedure as explained in previous tutorials to create Gerber files.

 

Note: In this tutorial video, only 4 and 6 layer PCB designing is explained. To understand it better, please watch the tutorial video.