One article to understand the PCB multilayer board


What is a multilayer board and what are the characteristics of a multilayer board

Answer: PCB multilayer board refers to the multilayer circuit board used in electrical products. The multilayer board uses more single-panel or double-panel wiring boards. With a double-sided as the inner layer, two single-sided as the outer layer or two double-sided as the inner layer, two single-sided as the outer layer of the printed circuit board, through the positioning system and insulating adhesive material alternately together and conductive graphics according to the design requirements of the printed circuit board for interconnection becomes a four-layer, six-layer printed circuit board, also known as multilayer printed circuit board.

    

With the continuous development of SMT (surface mount technology) and the continuous introduction of a new generation of SMD (surface mount devices), such as QFP, QFN, CSP, BGA (especially MBGA), electronic products are more intelligent and miniaturized, thus promoting the major reform and progress of PCB industry technology. Since IBM first successfully developed high-density multi-layer board (SLC) in 1991, major groups in various countries have also developed a variety of high-density interconnect (HDI) microplates. The rapid development of these processing technologies has prompted the PCB design to gradually develop in the direction of multi-layer, high-density wiring. Multi-layer printed circuit board with its flexible design, stable and reliable electrical performance and superior economic performance, has been widely used in the manufacture of electronic products.

    

The biggest difference between PCB multilayer board and single-sided board and double-sided board is that the internal power supply layer (maintaining the inner power layer) and ground layer are added. The power supply and ground network are mainly wired on the power supply layer. However, the multilayer board wiring is mainly based on the top and bottom layers, supplemented by the intermediate wiring layer. Therefore, the design of the multilayer board is basically the same as the design method of the double-sided board. The key is how to optimize the wiring of the inner electrical layer, so that the wiring of the circuit board is more reasonable and the electromagnetic compatibility is better.

    

    02

multilayer board is how to laminate?

Answer: Lamination, as the name suggests, is the process of bonding each layer of circuit sheet into a whole. The whole process, including kiss pressure, full pressure, cold pressure. In the kissing stage, the resin infiltrates the bonding surface and fills the voids in the line, and then enters the full pressure to bond all the voids. The so-called cold pressing is to make the circuit board cool quickly and keep the size stable.

    

Matters needing attention in the lamination process. First of all, in the design, the inner core board that must meet the lamination requirements, mainly the thickness, external dimensions, positioning holes, etc., needs to be designed according to specific requirements. Generally, the inner core board requires no opening, short, open circuit, no oxidation, and no residual film.

Secondly, when the multilayer board is laminated, the inner core board needs to be treated, and the treatment process includes black oxidation treatment and browning treatment. The oxidation treatment is to form a black oxide film on the inner layer of copper foil, and the browning treatment process is to form an organic film on the inner layer of copper foil.

    

Finally, when laminating, you need to pay attention to the three major issues of temperature, pressure, and time. Temperature, mainly pay attention to the melting temperature and curing temperature of the resin, the set temperature of the hot plate, the actual temperature of the material and the speed of heating, etc., these parameters need to be paid attention. As for the pressure, the basic principle is to fill the interlayer cavity with resin and exhaust the interlayer gas and volatiles. Time parameters are mainly the control of the timing of pressurization, the control of the timing of heating, and the gel time.

    

    03

What are the basic principles of impedance and lamination design considerations for multilayer boards?

Answer: When designing impedance and stacking, the main basis is PCB thickness, number of layers, impedance value requirements, current size, signal integrity, power integrity, etc. The general reference principles are as follows:

    

l The stack has symmetry;

l Impedance has continuity;

    

L the reference layer below the component surface is as complete as possible to the ground or power supply (usually the second layer or the penultimate layer);

L power plane and ground plane tightly coupled;

    

L signal layer as close as possible to the reference plane layer;

L try to pull the spacing between two adjacent signal layers. The alignment is orthogonal;

    

L The upper and lower reference layers of the signal are ground and power supply, and the distance between the signal layer and the formation is shortened as much as possible;

L differential signal spacing ≤ 2 times the line width;

    

l Semi-cured sheets between plies ≤ 3;

l The secondary outer layer has at least one 7628 or 2116 or 3313;

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