PCB Tech - Key production process control of high-level circuit boards

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 have high quality and reliability requirements. They are 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 high-level board market has a promising future.

At present, 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 requires 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, so high-level circuit boards have a higher threshold to enter the enterprise and realize 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.

1. Main production difficulties

Compared with the characteristics of conventional circuit boards, high-level circuit boards have the characteristics of thicker boards, more layers, denser lines and vias, larger cell sizes, and thinner dielectric layers. The inner layer space, the degree of alignment between layers, Impedance control and reliability requirements are more stringent.

1.1 Difficulties in alignment between layers

Due to 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 by ±75μm. Considering the large-scale design of the high-level board unit and the ambient temperature and humidity of the graphics transfer workshop, As well as factors such as misalignment and superposition caused by inconsistency of expansion and contraction of different core layers, interlayer positioning methods, etc., it is more difficult to control the degree of alignment between layers of high-rise boards.

1.2 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 on the production of the inner circuit and the control of the pattern size, such as the integrity of the impedance signal transmission, which increases the difficulty of the production of the inner circuit. 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 increases; the inner core board is thinner, 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, and the unit size is relatively large, and the cost of scrapping the finished product is relatively high.

1.3 Difficulties in pressing

Multiple inner core boards and prepregs are superimposed, and it is easy to produce defects such as sliding plate, delamination, resin cavity and air bubble residue during pressing and production. When designing the laminated structure, it is necessary to fully consider the heat resistance of the material, the 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 coefficient 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.

1.4 Difficulties in drilling drilling

Using high-TG, high-speed, high-frequency, thick copper special plates, increasing 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.

2. Key production process control

2.1 Material selection

With the development of high-performance and multi-functional electronic components, high-frequency, high-speed development of signal transmission is brought about, so the dielectric constant and dielectric loss of electronic circuit materials are required to be relatively low, as well as low CTE and low water absorption. Rate and better high-performance copper clad laminate materials to meet the processing and reliability requirements of high-level boards.

2.2 Design of laminated laminated structure

The main factors considered in the design of the laminated structure are the heat resistance of the material, the withstand voltage, the amount of filler, and the thickness of the dielectric layer. The following main principles should be followed.

(1) The prepreg and the core board manufacturer must be consistent. In order to ensure PCB reliability, avoid using a single 1080 or 106 prepreg for all layers of prepreg (except for special requirements of customers). When the customer has no media thickness requirements, the thickness of the interlayer media must be guaranteed ≥0.09mm in accordance with IPC-A-600G.

(2) When customers require high TG sheets, the core board and prepreg must use corresponding high TG materials.

(3) For the inner substrate 3OZ or above, use prepregs with high resin content, such as 1080R/C65%, 1080HR/C 68%, 106R/C 73%, 106HR/C76%; but try to avoid using all 106 high-adhesive prepregs The structure is designed to prevent the overlapping of multiple 106 prepregs. Because the glass fiber yarn is too thin, the glass fiber yarn collapses in the large substrate area, which affects the dimensional stability and the delamination of the plate.

2.3 Layer alignment control

The accuracy of the inner core board size compensation and production size control requires a certain period of time to collect data and historical data experience in the production to accurately compensate the size of each layer of the high-layer board to ensure that the core board of each layer expands and shrinks. consistency. Choose a high-precision, high-reliability interlayer positioning method before pressing, such as four-slot positioning (Pin LAM), hot melt and rivet combination. Setting the proper pressing process and routine maintenance of the press is the key to ensuring the quality of the pressing, controlling the glue flow and cooling effect of the pressing, and reducing the problem of interlayer misalignment. The layer-to-layer alignment control needs to comprehensively consider factors such as the inner layer compensation value, the pressing positioning method, the pressing process parameters, and the material characteristics.

2.4 Inner circuit technology

In order to improve the etching ability of the circuit, it is necessary to give proper compensation to the width of the circuit and the pad (or solder ring) in the PCB engineering design, but also to make more detailed compensation for special graphics, such as return lines, independent lines, etc. Design considerations. Confirm whether the design compensation of inner line width, line distance, isolation ring size, independent line, and hole-to-line distance is reasonable, otherwise change the engineering design. There are impedance and inductive reactance design requirements. Pay attention to whether the design compensation of independent line and impedance line is sufficient, control the parameters during etching, and mass production can be done after the first piece is confirmed to be qualified. In order to reduce the etching side corrosion, it is necessary to control the composition of each group of the etching solution within the optimal range. The traditional etching line equipment has insufficient etching capability, and it is possible to carry out technical transformation of the equipment or introduce high-precision etching line equipment to improve etching uniformity and reduce etching burrs and unclean etching.

Fourth, conclusion

There are relatively few research literatures on high-level PCB processing technology in the industry. This article introduces the key production process process control points such as material selection, laminated structure design, interlayer alignment, inner layer line production, pressing process, drilling process, etc., in order to provide peer reference and understanding, and hope that more peers will participate Technical research and communication of high-level circuit boards.