PCB layering or stacking is simply a way to obtain multiple printed circuit boards in the same device by stacking them together while ensuring that there is a predefined interconnection between them. These multilayer PCBs can add speed and functionality to a device and consist of at least three conductive layers, with the bottom layer synthesized with an insulating board.
What is stack-up?
Stack-up generally refers to an arrangement of the copper and insulation layers that make up a PCB before the board layout design. Although various PCB layers can use stack-up to get more circuits on a board, the structure of the PCB stack-up design will also give other advantages to the board layout design.
- PCB lamination can minimize the impact of circuitry on external noise and minimize radiation, which can reduce impedance and crosstalk problems on the high-speed PCB layout.
- Good PCB lamination can balance the need for cost reduction, efficient manufacturing methods, and attention to signal integrity issues.
- The correct PCB lamination can improve the board layout design and electromagnetic compatibility.
So in printed circuit board-based applications, stack-up PCB configuration is often beneficial. For multilayer PCB design, the available number of layers includes the ground plane (GND plane), internal signal layer, and power plane (PWR plane).
The development of modern electronics technology in driving the PCB to lightweight, high-speed, and miniaturization, better functionality, better reliability, and more extended service life requirements of developing multilayer PCBs to promote the popularity further. The combination of a pre-preg semi-solid adhesive allows two or more single-sided / double-sided PCBs to be stack-up together and reliably interconnected to produce multilayer PCBs.
A multilayer PCB will typically have three or more conductive layers, two of which will be on the outside and one synthesized in an insulating board. As PCB complexity and density increase, problems such as noise, crosstalk, and stray capacitance may occur when the layer arrangement gets inefficiently designed.
One of the essential factors in determining the product's electromagnetic compatibility (EMC) performance is the need to plan the best multilayer stack-up; a well-designed layer stack can minimize radiation and prevent the circuit from external noise sources. Stack up suitable PCB substrate to reduce signal crosstalk and impedance mismatch problems. But here is a problem: poor quality stack-up may allow EMI (electromagnetic interference) radiation to rise because the system, due to impedance mismatch and the reflection and ringing, reduces product performance and reliability.
Multilayer PCB Stack-up Planning
In order to properly design the multilayer printed circuit board, one should carefully consider the design of the layer stack-up. The need for planning the layer stack-up lies, on the one hand, in the technological capabilities of the production, and on another hand, in the requirements for electrical properties of the printed circuit board, which includes impedance control, signal integrity, noise immunity and electromagnetic compatibility. Another important thing is the optimization of the layers stack-up and interconnections from the point of view of costs. The structure of printed circuit boards can differ greatly, so the production costs will vary significantly.
One needs to remember that the multilayer PCB consists of cores, prepregs and copper foils. Thus, we have a huge amount of materials in stock for your choice.
A good stack-up can be very effective in reducing radiation from the loops on the PCB as well as the cables attached to it.
Four factors are important with respect to board stack-up:
1. The number of layers,
2. The number and types of the plane (power and/or ground) used
3. The order or sequence of the layers
4. The spacing between the layers.
Usually, not much consideration is given except for the number of layers. In many cases, the other three factors are of equal importance.
For the settlement of the number of layers, the following should be considered:
1. The number of signals to be routed and costs
3. Class A or Class B emission requirements
Usually, only the first item is considered. In reality, all the items are of critical importance and should be considered equally. If an optimum design is to be achieved within a minimum amount of time and at the lowest cost, the last item can be especially important and should not be ignored.
The above paragraph should not be construed to mean that you can’t do a good EMC design on a four-layer or six-layer board and the truth is that you can. It only indicates that all the objectives cannot be met simultaneously and some compromises are inevitable. Since all the desired EMC objectives can be met with an eight-layer board, there is no need to adopt boards with more than eight layers if you don't need to accommodate additional signal routing layers.
The standard pooling thickness for multilayer PCBs is 1.55mm.
Here are some examples of multilayer PCB stack up.