Multilayer PCB Design Tips

Before the multilayer PCB design, the designer needs to first determine the PCB structure to be used according to the scale of the circuit, the size of the PCB board and the requirements of electromagnetic compatibility (EMC), that is, to decide whether to use 4 layers, 6 layers, or more layers of PCB boards. After determining the number of layers, determine where to place the inner electrical layers and how to distribute different signals on these layers. This is the selection problem of the stacked structure of the multilayer PCB design.

The laminated structure is an important factor affecting the EMC performance of the PCB board, and it is also an important means of suppressing electromagnetic interference. The following is the relevant content of the multilayer PCB design stack structure.

1. The superposition principle of multilayer PCB design

More factors need to be considered to determine the stacked structure of multilayer PCB design. From the perspective of wiring, the more layers, the better the wiring, but the cost and difficulty of board making will also increase. For multilayer PCB design manufacturers, whether the laminated structure is symmetrical or not is the focus of PCB board manufacturing, so the selection of the number of layers needs to consider the needs of various aspects to achieve the best balance.

For experienced multilayer PCB designers, after completing the pre-layout of components, they will focus on analyzing the wiring of the PCB. The multilayer PCB design will combine other EDA tools to analyze the wiring density of the circuit board; then determine the number and types of signal lines with special wiring requirements such as differential lines, sensitive signal lines, etc. to determine the number of layers of the signal layer; and then according to the type of power supply, isolation and anti-interference requirements to determine the number of inner electrical layers. In this way, the number of layers of the entire circuit board is basically determined.

After determining the number of layers of the circuit board, the next job of multilayer PCB design is to arrange the placement order of the circuits of each layer reasonably. In this step, the factors that need to be considered in multilayer PCB design mainly include the following two points.

(1) Distribution of special signal layers.

(2) Distribution of power supply layer and formation.

If there are more layers in the multilayer PCB design, there will be more types of arrangement and combination of special signal layers, ground layers and power layers, and it will be more difficult to determine which combination is the best, but the general principles are as follows.

(1) When a multilayer PCB design, the signal layer should be adjacent to an inner electrical layer (internal power supply/ground layer), and use the large copper film of the inner electrical layer to provide shielding for the signal layer.

(2) The internal power layer and the ground layer should be tightly coupled, that is to say, the dielectric thickness between the internal power layer and the ground layer of the multilayer PCB design should take a smaller value to improve the capacitance between the power layer and the ground layer. large resonant frequency. The dielectric thickness between the internal power layer and the ground layer of the multilayer PCB design can be set in Protel's Layer Stack Manager (layer stack manager). If the potential difference between the power supply and the ground wire is not large, a smaller insulation layer thickness can be used in multilayer PCB design, such as 5mil (0.127mm).

(3) The high-speed signal transmission layer of the multilayer PCB design circuit should be the signal middle layer and sandwiched between the two internal electrical layers. In this way, the copper films of the two inner electrical layers can provide electromagnetic shielding for high-speed signal transmission, and at the same time, can effectively limit the radiation of high-speed signals between the two inner electrical layers, so as not to cause external interference.

(4) Multilayer PCB design should avoid two signal layers directly adjacent to each other. It is easy to introduce crosstalk between adjacent signal layers, resulting in circuit function failure. Adding a ground plane between two signals can effectively avoid crosstalk.

(5) Multilayer PCB design with multiple grounded internal layers can effectively reduce the grounding impedance. For example, the A signal layer and the B signal layer use separate ground planes, which can effectively reduce common-mode interference.

(6) The multilayer PCB design should take into account the symmetry of the layer structure.

2. The stacked structure of multilayer PCB design.

The following is an example of a 4-layer multilayer PCB design to illustrate how to optimize the arrangement and combination of various stacked structures.

For the commonly used 4-layer multilayer PCB design, there are several stacking methods (from top layer to bottom layer).

(1) Siganl 1 (Top), GND (Inner_1), POWER (Inner. 2), Sigan! _2 (Bottom).

(2) Siganl_ 1 (Top), POWER (Inner_ 1) , GND (Inner 2) , Siganl 2 (Bottom).

(3) POWER (Top), Siganl 1 (Inner. 1) , GND (Inner. 2) , Siganl 2 (Bottom).

Obviously, the multilayer PCB design of Scheme 3 lacks effective coupling between the power layer and the ground layer and should not be used.

So how should plan 1 and plan 2 be chosen? Under normal circumstances, multilayer PCB designers will choose plan 1 as the structure of 4-layer multilayer PCB design. The reason is not that option 2 can be adopted, but that the general multilayer PCB design only places components on the top layer, so it is more appropriate to adopt option 1.

When the multilayer PCB design needs to place components on both the top layer and the bottom layer, and the dielectric thickness between the internal power layer and the ground layer is large, and the coupling is not good, it is necessary to consider which layer has fewer signal lines. For option 1, there are fewer signal lines on the bottom layer, and a large area of copper film can be used to couple with the POWER layer; on the contrary, if the components of the multilayer PCB design are mainly arranged on the bottom layer, then option 2 should be used to make the board.

3. Requirements for multilayer PCB design

Under normal circumstances, multilayer PCB design and wiring are carried out according to circuit functions. When wiring on the outer layer, more wiring is required on the soldering surface and less wiring on the component surface, which is conducive to the maintenance and troubleshooting of printed boards. Thin, dense wires and signal lines that are susceptible to interference are usually arranged in the inner layer.

The large-area copper foil should be evenly distributed in the inner and outer layers, which will help reduce the warpage of the board and also enable a more uniform coating on the surface during electroplating. In order to prevent the shape processing from damaging the printed wires and causing interlayer short circuit during mechanical processing, the distance between the conductive pattern in the inner and outer layer wiring area and the edge of the board should be greater than 50mil.

(1) Wire direction and line width requirements

When a multilayer PCB design, the wiring should separate the power layer, ground layer, and signal layer to reduce interference between power, ground, and signals. The lines of two adjacent layers of printed boards should try to be perpendicular to each other or follow oblique lines or curves instead of parallel lines. The multilayer PCB design is designed to reduce the interlayer coupling and interference of the substrate. And the wire should be as short as possible, especially for small signal circuits. The shorter the wire, the smaller the resistance and the smaller the interference. For signal lines on the same layer, sharp corners should be avoided when changing directions. The width of the wire, the multilayer PCB design should be determined according to the current and impedance requirements of the circuit, the power input line should be larger, and the signal line can be relatively smaller.

Multilayer PCB design should also pay attention to the width of the lines to be as consistent as possible to avoid sudden thickening and thinning of the wires, which is conducive to impedance matching.

(2) Drilling size and pad requirements

The size of the drilled holes in the multilayer PCB design is related to the pin size of the selected components. If the drilled holes are too small, it will affect the installation and tinning of the device; if the drilled holes are too large, the solder joints will not be full enough during soldering. Generally speaking, the calculation method of component hole diameter and pad size is:

Component hole diameter = component pin diameter (or diagonal) + (10~30mil)

Component pad diameter ≥ component hole diameter + 18mil

(3) Requirements for power supply layer, stratum division and flower hole

For multilayer PCB design, there is at least one power layer and one ground layer. Since all the voltages on the printed board are connected to the same power layer, the multilayer PCB design must isolate the power layer by partition. The size of the partition line is generally 20-80mil line width. The thicker the line.

(4) Requirements for safety distance

The setting of safe spacing in multilayer PCB design should meet the requirements of electrical safety. Generally speaking, the minimum spacing between the outer layer conductors should not be less than 4mil, and the minimum spacing between the inner layer conductors should not be less than 4mil. In the multilayer PCB design, when the wiring can be arranged, the spacing should be as large as possible to improve the yield of the board and reduce the hidden danger of the failure of the finished board.

(5) Requirements for improving the anti-interference ability of the whole board

Multilayer PCB design must also pay attention to the anti-interference ability of the whole board. The general methods are:

a. Add a filter capacitor near the power supply and ground of each IC, the capacity is generally 473 or 104.

b. For sensitive signals on the printed board, accompanying shielding lines should be added separately, and the wiring near the signal source should be as little as possible.

c. Multilayer PCB design should choose a reasonable grounding point.

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