PCB STACK-UP DESIGN

What Is Stack-up?

Stack-up refers to the arrangement of copper layers and insulating layers that make up a PCB prior to board layout design. While a layer stack-up allows you to get more circuitry on a single board through the various PCB board layers, the structure of PCB stack-up design confers many other advantages:

• A PCB layer stack can help you minimize your circuit's vulnerability to external noise as well as minimize radiation and reduce impedance and crosstalk concerns on high-speed PCB layouts.

• A good layer PCB stack-up can also help you balance your need for low-cost, efficient manufacturing methods with concerns about signal integrity issues

• The right PCB layer stack can enhance the Electromagnetic Compatibility of your design as well.

It will very often be to your benefit to pursue a stacked PCB configuration for your printed circuit board-based applications.

For multilayer PCBs, general layers include ground plane (GND plane), power plane (PWR plane), and inner signal layers.

Why stack-up?

The irreversible development of modern electronics has been increasingly pushing PCBs towards such demands as miniaturization, light weight, high speed, better functionality and reliability, and longer lifetime, which results in the popularity of Multilayer PCBs. Combined by a type of semi-solid adhesive which is called "prepreg", two or more single and/or double-sided PCBs are stacked together to generate multilayer PCBs through reliable predefined mutual connection between them. There are three or more conductive layers in one multilayer PCB with two layers outside and one layer synthesized in the insulation board. With the increase of PCB complexities and densities, it's possible for some issues to take place such as noise, stray capacitance and cross talk when layer arrangement gets inefficient design.

Planning optimal multilayer stack-up is one of the most important elements in determining the Electromagnetic Compatibility (EMC) performance of a product. A well-designed layer stack-up can both minimize the radiation and can stop circuit from being interfered by external noise sources. Well-stacked PCB substrates can also reduce signal cross talk and impedance mismatch issues. However, an inferior stack-up may get EMI (Electromagnetic Interference) radiation rising, because reflections and ringing in the system as a result of impedance mismatch can dramatically lower products' performance and reliability. This article then focuses on layer stack up definition, its designing rules and essential considerations.

PCB Stack-Up Principle

After the number of circuit board layers determined, the following job is to reasonably arrange the placement order of the circuit of each layer. In this part, there are two main factors to be considered:

(1) The distribution of special signal layers

(2) The distribution of power layer and ground layer

The more layers of circuit boards, the more varieties of arrangement of special signal layers, ground layers and power layers, thus it is more difficult to choose the best combination method, but the general principles are as follows.

(1) The signal layer should be next to an internal power layer (internal power/ground layer), shielded by the copper film of internal power layer.

(2) The internal power layer should be integrated with ground layer tightly, which means the thickness of medium between internal power layer and ground layer should take the smaller value, in order to improve the power supply capacitor between the internal power layer and ground layer, and increase the resonant frequency. If the electric potential difference between internal power layer and ground layer is not significant, a smaller insulation thickness can be used, like 5mil (0.127mm).

(3) To avoid the two signal layers directly adjacent. It is easy to introduce crosstalk between adjacent signal layers, leading to the fail of the circuit. To place a ground layer between two signal layers can avoid cross talk efficiently.

(4) Multiple grounded internal power layers can reduce the ground impedance effectively. For example, A signal layer and B signal layer use ground plane respectively can reduce common-mode interference effectively.

(5) The symmetry of layer structure.

Here is our standard stack-up for 4/6/8 layer board,

4 layer standard stack up is below

Top---------------------------------- 1oz

                PP  7628(0.17mm)

      L2---------------------------------- 1oz

              core  1.2mm

      L3---------------------------------- 1oz

               PP  7628(0.17mm)

Bottom--------------------------------1oz

1.61mm+/-10%

6 layers standard stack up is below

   Top---------------------------------- 1oz

                PP  2116(0.10mm)

      L2---------------------------------- 1oz

              core  0.6mm

      L3---------------------------------- 1oz

                PP  2116(0.10mm)

      L4---------------------------------- 1oz

               core  0.6mm

      L5---------------------------------- 1oz

               PP  2116(0.10mm)

Bottom--------------------------------1oz

1.6mm+/-10%

8 layers standard stack up is below

Top---------------------------------- 1oz

          PP  1080x2(0.14mm)

  L2---------------------------------- 1oz

          core  0.35mm (including the copper thickness)

  L3---------------------------------- 1oz

          PP  2116(0.10mm)

   L4---------------------------------- 1oz

          core  0.35mm (including the copper thickness)

   L5---------------------------------- 1oz

           PP  2116(0.10mm)

   L6---------------------------------- 1oz

           core  0.35mm (including the copper thickness)

   L7---------------------------------- 1oz

           PP  1080x2(0.14mm)

Bottom--------------------------------1oz

1.6mm +/-10%