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Multilayer PCBs are widely used in the fields of communication, medical, industrial control, security, automobile, electric power, aviation, military, and computer peripherals. The more the corresponding number of layers, the higher the cost of multi-layer PCB Prototyp. So what are the difficulties in making multilayer circuit boards?

1. Difficulties in making inner circuit

Multi-layer PCBs have various special requirements, such as high speed, thick copper, high frequency, and high Tg values, which all cause the requirements in inner layer layout and pattern size control getting higher and higher. For example, the ARM development board, there are plenty of impedance signal lines in the inner layer. And in order to ensure the integrity of the impedance which are definitely going to increase the difficulty of producing the inner layer circuits.

Furthermore, if there are a lot inner layer signal lines, whose width and spacing of the lines are basically about 4mil or less; The production of multi-core and thin-layer boards is easy to wrinkle, and these factors will obviously increase the production of the inner layer.

Advice:Design the line width and line spacing above 3.5 / 3.5mil (In this case, there will be no difficulty for most PCB factories in production).

For example, six-layer board, it is recommended to use a similar eight-layer structure design, which can meet the requirements of line width 4-6mil and impedance  50ohm, 90ohm, 100ohm.

2. Difficulties in alignment requirement between inner layers

As the number of board layers increases, the alignment requirements of the inner layer are also getting higher and higher. The film will expand and contract under the influence of the temperature and humidity of the workshop environmen as well as the board, which makes the alignment accuracy between the inner layers more difficult to control.

3. Difficulties in lamination process

The stacking of multiple boards and PP is prone to problems such as delamination, skateboard and drum residue during lamination. Therefore, in the structure design process of the inner layer, factors such as the dielectric thickness between layers, flow of glue and heat resistance of the plate should be considered, then designing the corresponding laminated structure reasonably.

Advice: keep the inner layer copper evenly, and balance copper in the same area without PAD.

4. Difficulties in drilling production

Multi-layer boards use high Tg or other special plates. The roughness of the drilled holes of different materials is various, which increases the difficulty of removing rubber residue in the holes. High-density multi-layer boards have high hole density, low production efficiency, and easy to break the blade of drilling machine. That’s to say, if the hole edges of different networks are too close, the CAF effect will be caused. (CAF:Conductive Anodic Filamentation )

Advice: The hole edge spacing of different networks should be ≥0.3mm.

PCB design layout rules

1.Generally, all components should be arranged on the same side of the circuit board. Only the top layer components are too dense can some components with limited height and low heat generation, such as SMD resistors, SMD capacitors and SMD IC are goign to be placed on the lower layer.

2. On the premise of ensuring electrical performance, components should be placed on a grid and arranged parallel or perpendicular to each other consideringr neat and charming appearance. What’s more, components are not allowed to overlap; Components should be arranged compactly, and whose distribution should be even.

3. The minimum distance between the adjacent pad of different components on the circuit board should be over 1MM.

4. Not less than 2MM from the edge of the circuit board. The optimal shape of the circuit board is rectangular, and the aspect ratio is 3: 2 or 4: 3. However, if the surface area of the circuit board is larger than 200MM by 150MM, we should consider whether the circuit board can bear Mechanical strength or not.

PCB design layout tips

In the layout design of the PCB, the units of the circuit board should be analyzed, and the layout design should be performed according to the functions. When laying out all the components of the circuit, the following principles must be met:

1. Arrange the position of each functional circuit unit according to the flow of the circuit, make the layout convenient for signal circulation, and keep the signal in the same direction as possible.

2. Regard the core components of each functional unit as the center, then PCB layout around them. Furthermore, the components should be evenly, integrally and compactly arranged on the PCB to minimize and shorten the leads and connections between the components.

3. As for those circuits which are operating at high frequencies, the distribution parameters between components must be considered. In a word, the components should be arranged in parallel as much as possible, which looks not only gorgeous, but also be easy to install and mass production.

PCB design layout check

1. Whether the size of the circuit board and the drawing are in accordance with the processing size.

2. Whether the layout of the components is balanced, arranged neatly, or

all layout.

3. Whether there are conflicts at each layers. For example, are all the components, the frame, and the layer of silkscreen necessary?

4. Whether the commonly used components are convenient to use in practical. Such as switches, plug-in boards insertion device and components that will be replaced frequently.

5. Whether the distance between the thermal components and the heating components is reasonable.

6. Whether the heat dissipation is good.

7. Whether the problem of line interference need to be considered, etc.