Multilayer circuit boards are generally defined as having 10 to 20 layers or more, making them more complex to process than traditional multilayer circuit boards. They require high quality and reliability, and are primarily used in communication equipment, high-end servers, medical electronics, aviation, industrial control, military, and other fields. In recent years, the demand for multilayer boards in communication, base station, aviation, military, and other fields has remained strong. With the rapid development of China’s telecom equipment market, the prospects for the multilayer board market are promising.
Currently, most domestic PCB manufacturers capable of mass-producing multilayer circuit boards are either foreign enterprises or a few domestic enterprises. The production of multilayer circuit boards not only requires advanced technology and equipment but also relies on the experience accumulated by technical and production personnel. Moreover, the strict and cumbersome customer certification procedures for multilayer boards increase the entry threshold for enterprises, leading to longer industrial production cycles. The average number of PCB layers has become an important technical index for measuring the technical level and product structure of PCB enterprises.
Main production difficulties:
Compared with conventional circuit board products, multilayer circuit boards have thicker parts, more layers, denser lines and through holes, larger unit sizes, and thinner dielectric layers. They also have stricter requirements for inner space, inter-layer alignment, impedance control, and reliability.
1.1 Difficulties in layer alignment:
Due to the large number of layers, end customers increasingly demand strict alignment tolerance control for each layer, usually within ±75 microns. Factors such as the multilayer unit size design, the environment temperature and humidity of the graphic transfer workshop, and the inconsistency in the increase of different core board layers and the superposition of dislocations between layers make it more difficult to control interlayer alignment in multilayer boards.
1.2 Difficulties in inner layer fabrication:
The use of special materials such as high TG, high speed, high frequency, thick copper, and thin dielectric layers in multilayer boards imposes high requirements on inner layer circuit production and graphic size control. For example, ensuring the integrity of impedance signal transmission increases the difficulty of inner layer circuit production. Small wire widths lead to increased short circuits, micro-short circuits, and lower pass rates. Additionally, with more signal layers in fine circuits, the probability of AOI missing detection in inner layers increases. The thinness of the inner core plate leads to folding issues, resulting in poor exposure and easy rolling of the etching machine. Since most multilayer boards are system boards with large unit sizes, the cost of scrap in finished products is relatively high.
1.3 Difficulties in lamination:
During lamination, multiple inner core plates and semi-cured sheets are stacked, leading to defects such as sliding plates, resin cavities, and bubble residue. The design of the laminated structure must fully consider the heat resistance of the material, voltage resistance, glue amount, medium thickness, and a reasonable multilayer pressing program must be set. However, ensuring consistency in the number of layers, expansion and shrinkage control, and size coefficient compensation can be challenging. Thin insulation layers can easily lead to interlayer reliability test failures.
1.4 Difficulties in drilling:
The use of special plates with high TG, high speed, high frequency, and thick copper increases the difficulty of drilling roughness, burrs, and dirt removal. The number of layers, total copper thickness, and thickness of the finished board make it easy to break drill bits. Narrow hole wall spacing and dense BGA can lead to CAF failures. The thickness of the plate can also cause problems with oblique drilling.
Reliability testing:
Multilayer boards are generally system boards that are thicker, heavier, and larger in unit size than conventional multilayer boards, with correspondingly larger heat capacities. More heat and longer welding times are required during soldering, typically taking 50 to 90 seconds at 217℃ (the melting point of tin, silver, and copper solder). Additionally, the slow cooling rate of multilayer PCBs extends the time of the over-reflow welding test. Following IPC-6012C, IPC-TM-650 standards, and industry requirements, the main reliability tests for multilayer PCBs are conducted.
Currently, most domestic PCB manufacturers capable of mass-producing multilayer circuit boards are either foreign enterprises or a few domestic enterprises. The production of multilayer circuit boards not only requires advanced technology and equipment but also relies on the experience accumulated by technical and production personnel. Moreover, the strict and cumbersome customer certification procedures for multilayer boards increase the entry threshold for enterprises, leading to longer industrial production cycles. The average number of PCB layers has become an important technical index for measuring the technical level and product structure of PCB enterprises.
Main production difficulties:
Compared with conventional circuit board products, multilayer circuit boards have thicker parts, more layers, denser lines and through holes, larger unit sizes, and thinner dielectric layers. They also have stricter requirements for inner space, inter-layer alignment, impedance control, and reliability.
1.1 Difficulties in layer alignment:
Due to the large number of layers, end customers increasingly demand strict alignment tolerance control for each layer, usually within ±75 microns. Factors such as the multilayer unit size design, the environment temperature and humidity of the graphic transfer workshop, and the inconsistency in the increase of different core board layers and the superposition of dislocations between layers make it more difficult to control interlayer alignment in multilayer boards.
1.2 Difficulties in inner layer fabrication:
The use of special materials such as high TG, high speed, high frequency, thick copper, and thin dielectric layers in multilayer boards imposes high requirements on inner layer circuit production and graphic size control. For example, ensuring the integrity of impedance signal transmission increases the difficulty of inner layer circuit production. Small wire widths lead to increased short circuits, micro-short circuits, and lower pass rates. Additionally, with more signal layers in fine circuits, the probability of AOI missing detection in inner layers increases. The thinness of the inner core plate leads to folding issues, resulting in poor exposure and easy rolling of the etching machine. Since most multilayer boards are system boards with large unit sizes, the cost of scrap in finished products is relatively high.
1.3 Difficulties in lamination:
During lamination, multiple inner core plates and semi-cured sheets are stacked, leading to defects such as sliding plates, resin cavities, and bubble residue. The design of the laminated structure must fully consider the heat resistance of the material, voltage resistance, glue amount, medium thickness, and a reasonable multilayer pressing program must be set. However, ensuring consistency in the number of layers, expansion and shrinkage control, and size coefficient compensation can be challenging. Thin insulation layers can easily lead to interlayer reliability test failures.
1.4 Difficulties in drilling:
The use of special plates with high TG, high speed, high frequency, and thick copper increases the difficulty of drilling roughness, burrs, and dirt removal. The number of layers, total copper thickness, and thickness of the finished board make it easy to break drill bits. Narrow hole wall spacing and dense BGA can lead to CAF failures. The thickness of the plate can also cause problems with oblique drilling.
Reliability testing:
Multilayer boards are generally system boards that are thicker, heavier, and larger in unit size than conventional multilayer boards, with correspondingly larger heat capacities. More heat and longer welding times are required during soldering, typically taking 50 to 90 seconds at 217℃ (the melting point of tin, silver, and copper solder). Additionally, the slow cooling rate of multilayer PCBs extends the time of the over-reflow welding test. Following IPC-6012C, IPC-TM-650 standards, and industry requirements, the main reliability tests for multilayer PCBs are conducted.
