Introduction
In modern days the PCBs are getting smaller and the stack up take an important part in it. Before get into designing, the designers and respective manufacturers must consider the layer stack up. By choosing better stack up can improves the signal integrity and avoid manufacturing issues
What is Stack up?
Stack up is an arrangement of copper layers and insulating layers that make a PCB board. The most commonly used insulated or dielectric material is flame retardant-4 (FR-4) in the form of core and prepreg (pre-impregnated)
Figure 1 : Stackup structure
Copper :
Copper is a material used in PCB to carry the current throughout the board. There is lots of substance that can carry the electricity but copper is a highly conductive material. The reason for high level conductivity is that the electrons can freely move from one atom to another with low resistance. It can easily transmit the signals without current loss and also it is cost effective so, copper is a standard material used in PCB. If the current is high, the copper thickness also varied with respect to the amps used in PCB. The unit of copper is ounce/ft2
Most commercial PCBs which manufactured for low power application are made up of copper weights from ½oz/ft2 to 4oz/ft2. A heavy copper circuits are manufactured with copper weights between 4oz/ft2 to 20oz/ft2. Copper weights above 20oz/ft2 to 200oz/ft2 is also possible and are called as extreme copper. These extreme coppers are used for military applications. 1 ounce copper is equal to 1.4 mils
Core :
Flame retardant (FR-4) is the dielectric core material used in PCB. It is made up of fiber glass that laminated with epoxy resin. The copper foils are bonded on both sides of core. There is also have alternative materials like Teflon, polyimide, GETEK etc. Boards made of FR-4 are strong, water resistant and provide good insulation between copper layers so, FR-4 is better for PCB construction. However in high frequencies, FR-4 material is not preferable. At high frequencies cannot maintain a constant impedance and reflection that will impact signal integrity. FR-4 operative temperature is 135 degree Celsius. Dielectric constant (Ԑr) for the FR-4 is from 3.8 to 4.6. The standard thickness of FR-4 material is 1.5 mm
Prepreg :
Prepreg is thin sheets of fiberglass with epoxy resin which hardens when heated and pressed during the PCB manufacturing process. Prepreg also produce an insulation between two copper layers like core. The operative temperature for prepreg is from 60°C to 180°C. Once the prepreg is heated, it will start to melt and get hardened with the two copper layers due to the pressure. Prepreg take most important role in multilayer stack up. To achieve the gap between two coppers can use multiple sheets together
Figure 11 : FR4 Material Characteristics
Guidelines for better stack up :
- Every signal layer must need a reference plane layer for signal return path
- The reference plane should be GROUND or POWER plane and it is tightly coupled with adjacent signal layer (<10mils)
- High speed signals are routed in buried inner layer and if routed in between two plane layers is better
- Avoid two adjacent signal layers
- Multiple ground planes are better that it reduces the ground impedance and radiation
Different types of stack up in PCB :
There are different types of stack-up can implement in PCB are
- Single and double layer stack up
- Multilayer stack up
Single and double layer stack up:
Single layer stack-up are made with one copper layer and a substrate. It only have one conductive layers and the components are placed in one end. The standard PCB thickness for single layer stack-up is 0.785mm
Figure 2 : Single layer stack up
Double layer stack-up are had two copper layers that bonded on both sides of the substrate or core. The components can place in both side of the board and can connect the traces by using via. Maximum frequency range for two layer board is 25MHz. The standard PCB thickness for double layer board is 1.57mm
Figure 3 : double layer stack up
Multi layer Stack up :
Multilayer stack-up had more than two copper layers like 4,8,10 and so on that attached multiple coppers by using prepreg. Multiple core can also use in multilayer stack-up. Multilayer PCBs frequency are above 10MHz. It produce 15 dB less radiation than two layer board. Here are some example of multilayer stack-up
4 Layer Stack up :
Figure 3 : Four layer stack up
Above image clearly indicates that ground net is used as a reference plane layer. But this stack up is not constant for all the manufacturers. Some PCBs are prefer one of the plane layer as a power plane. If we use power net as a plane layer, the return signal cannot find the source quickly and it leads to more EMI and impedance mismatch. So, ground net is better used as a plane layer. Because of ground net has zero resistance, the return signal can find the source as quick as possible
6 Layer Stack up :
Figure 3 : Six layer stack up
In six layer and other multilayer stack up can use multiple core to arrange the layers. The inner signal layer prefer second ground layer as a reference. If use the inner power layer as another signal layer, the crosstalk may occur. If two inner signal layer is mandatory, then discuss with manufacturer and try to increase the dielectric layer between two inner signal layers. And should route one of the signal layer in horizontal and another signal layer in vertical to avoid crosstalk and EMI issues