![\"How](\"\/wp-content\/uploads\/2022\/11\/2-1669468259.png\" "\\"How")
<\/p>\nWhen designing multilayer PCBs, everyone needs to set up a layer stack-up scheme. The more layers a board has, the more layer stack-up options there are. Many people do not fully understand the layer stack-up of multilayer PCBs; typically, a good layer stack-up scheme can reduce interference generated by the board. Our layer stack-up structure is a key factor affecting the EMC performance of a PCB. Below, we\u2019ll use four-layer and six-layer boards as examples to introduce their stacking schemes, so we can select the optimal stacking structure.<\/p>\n
The first type:<\/p>\n
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The second type:<\/p>\n
![\"How](\"\/wp-content\/uploads\/2022\/11\/1-1669468259.png\" "\\"How")
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The third type:<\/p>\n
![\"How](\"\/wp-content\/uploads\/2022\/11\/0-1669468259.png\" "\\"How")
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Let\u2019s first analyze the first and second stack-up configurations. The difference between these two is that the second layer is the opposite of the third layer; these are also the most commonly used stack-up schemes for four-layer boards. Both of these stack-up configurations are viable; the choice depends on the specific requirements of the board. Typically, when designing a four-layer board, we primarily consider whether components are placed on the top or bottom layers. The GND layer is typically placed close to the component layer, which provides a continuous ground plane for the components and helps shield against interference. Traces should also be routed as close as possible to the bottom layer, ensuring signal stability and preventing signal splitting. This is the key difference between these two stack-up configurations.<\/p>\n
Next, let\u2019s look at the third stacking scheme, which distributes layers and power planes across the first and fourth layers. For a two-layer board, we would need to route signal lines, ground, and power planes on the top and bottom layers; however, since this is a four-layer board, stacking it this way would negate the advantages of a four-layer design and result in poor signal quality. Layers 2 and 3 are routing layers. First, we need to understand that our components must be placed on the top and bottom layers. If we assign the top and bottom layers to ground and power, respectively, we cannot guarantee the integrity of the ground plane\u2014and the same applies to the power plane. As a result, the traces on layers 2 and 3 are likely to lack a complete reference plane. This stack-up scheme also offers poor interference shielding. Typically, after designing a board, we need to cover the top and bottom layers with full-plate ground copper. Using the first layer as a ground plane in this stack-up scheme is somewhat redundant, so this stack-up would not be selected for standard four-layer PCB design.<\/p>\n
The above shows common lamination schemes for four-layer boards. We can choose between the first and second lamination schemes based on the actual conditions of the board. Next, we\u2019ll introduce common lamination schemes for six-layer boards.<\/p>\n
The first type:<\/p>\n
![\"How](\"\/wp-content\/uploads\/2022\/11\/2-1669468259-1.png\" "\\"How")
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The second type:<\/p>\n
![\"How](\"\/wp-content\/uploads\/2022\/11\/5-1669468260.png\" "\\"How")
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The third type:<\/p>\n
![\"How](\"\/wp-content\/uploads\/2022\/11\/10-1669468260.png\" "\\"How")
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The first stacking scheme includes two routing layers, which can reduce the difficulty of routing. However, since the third and fourth layers are adjacent routing layers, this can easily cause crosstalk. Therefore, when using this stacking scheme, we must take special care to ensure that adjacent layers use perpendicular routing to avoid parallel traces (as shown in the figure below).<\/p>\n
![\"How](\"\/wp-content\/uploads\/2022\/11\/10-1669468260-1.png\" "\\"How")
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![\"How](\"\/wp-content\/uploads\/2022\/11\/5-1669468261.png\" "\\"How")
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The third layer is the signal layer. Since its adjacent layers are both GND, it can be effectively isolated from other layers and is less prone to crosstalk. However, having only one internal routing layer may increase the difficulty of our routing. so it is essential to carefully plan the routing channels and paths during the routing process to ensure that all signal lines can be accommodated. Furthermore, since the first three layers have only one inner signal layer while the last three layers have two, this asymmetry in the inner signal layers creates a risk of warpage.<\/p>\n
Compared to the second stacking scheme, the third stacking scheme simply swaps the fourth and fifth layers. This stacking arrangement can also provide effective isolation for our signals; however, it also involves a mismatch between the internal electrical layers, which can lead to warpage. With this stackup, having two adjacent ground layers improves signal integrity. During the design process, many customers require two reference planes for traces that need impedance control; in more stringent cases, two ground planes are required. This necessitates ensuring the integrity of the reference planes. Generally, ground planes are relatively intact, Power planes typically carry different power signals; if two adjacent planes need to be referenced, there is a risk of crossing the split.<\/p>\n