High-level circuit boards are generally defined as high-layer circuit boards with 10 to 20 layers or more, which are more difficult to process than traditional multilayer circuit boards, and their quality and reliability requirements High, mainly used in communication equipment, high-end servers, medical electronics, aviation, industrial control, military and other fields. In recent years, the market demand for high-level boards in the fields of application communications, base stations, aviation, and military has remained strong. With the rapid development of China's telecommunications equipment market, the market for high-level boards has been promising.
Currently, domestic PCB manufacturers that can mass produce high-level circuit boards are mainly foreign-funded companies or a few domestic-funded companies. The production of high-level circuit boards not only requires high technology and equipment investment, but also the accumulation of experience of technicians and production personnel. At the same time, the introduction of high-level board customer certification procedures is strict and cumbersome. Therefore, the threshold for high-level circuit boards to enter the enterprise is relatively high to achieve industrialization. The production cycle is longer. The average number of PCB layers has become an important technical indicator to measure the technical level and product structure of PCB companies. This article briefly describes the main processing difficulties encountered in the production of high-level circuit boards, and introduces the control points of the key production processes of high-level circuit boards for reference and reference by the peers.
First, the main production difficulties
Compared with the features of conventional circuit boards, high-level circuit boards have thicker boards, more layers, denser lines and vias, larger cell sizes, and more dielectric layers. Thinness and other characteristics, inner layer space, interlayer alignment, impedance control and reliability requirements are more stringent.
Difficulties in alignment between layers
Because of the large number of high-level boards, the customer design side has more and more stringent requirements for the alignment of each layer of the PCB. Usually the alignment tolerance between layers is controlled ±75μm Considering the large unit size design of the high-rise board, the temperature and humidity of the graphics transfer workshop environment, the misalignment superposition caused by the inconsistency of the expansion and contraction of different core board layers, and the inter-layer positioning method, which make the control of the alignment of the high-rise board difficult Bigger.
Difficulties in making inner circuit
The high-level board adopts special materials such as high TG, high speed, high frequency, thick copper, thin dielectric layer, etc., which puts forward high requirements for the inner layer circuit production and pattern size control, such as impedance The integrity of signal transmission increases the difficulty of making inner-layer circuits. Line width and line spacing are small, open and short circuits increase, short circuit increases, and pass rate is low; there are more fine circuit signal layers, and the probability of missing AOI detection in the inner layer is increased; the inner core board has a thinner thickness, which is easy to wrinkle and cause poor exposure and etching It is easy to roll the board when it passes the machine; most of the high-level boards are system boards, the unit size is relatively large, and the cost of scrapping the finished product is relatively high.
Difficulties in pressing and making
Multiple inner core boards and prepregs are superimposed, and it is easy to produce defects such as slippage, delamination, resin voids and air bubbles during production. When designing the laminated structure, it is necessary to fully consider the heat resistance of the material, withstand voltage, the amount of glue and the thickness of the medium, and set a reasonable high-level board pressing program. There are many layers, and the amount of expansion and contraction control and the compensation of the size factor cannot be kept consistent; the thin interlayer insulation layer can easily lead to the failure of the interlayer reliability test. Figure 1 is a defect diagram of the delamination of the plate after the thermal stress test.
Difficulties in drilling drilling
The use of high-TG, high-speed, high-frequency, and thick copper special plates increases the difficulty of drilling roughness, drilling burrs and de-drilling. There are many layers, the cumulative total copper thickness and the plate thickness, the drilling is easy to break the knife; the dense BGA is many, the CAF failure problem caused by the narrow hole wall spacing; the plate thickness is easy to cause the inclined drilling problem.
Key production process control
With the development of high-performance and multi-functional electronic components, it also brings high-frequency and high-speed signal transmission, so the dielectric of electronic circuit materials is required The constant and dielectric loss are relatively low, as well as low CTE, low water absorption and better high-performance copper clad laminate materials to meet the processing and reliability requirements of high-level boards. Commonly used board suppliers mainly include A series, B series, C series, and D series. The main characteristics of these four inner substrates are compared, see Table 1. For high-rise thick copper circuit boards, use prepregs with high resin content. The amount of glue flowing between the interlayer prepregs is enough to fill the inner layer pattern. If the insulating dielectric layer is too thick, the finished board may be too thick. On the contrary, if the insulating dielectric layer is too thin, it is easy to cause Quality problems such as dielectric delamination and high-voltage test failure, so the selection of insulating dielectric materials is extremely important.