Abstract: This article analyzes the main manufacturing difficulties of high multilayer circuit boards, such as the degree of alignment between layers, the production of inner layers, the production of pressing, and the production of drilling. In view of the main production difficulties, the production control points of the key processes such as the control of layer alignment, the design of the laminated structure, the inner line process, the pressing process, and the drilling process are introduced.

High multi-layer circuit boards are generally defined as 10 to 20 layers or more, which are more difficult to process than traditional multi-layer circuit boards and require high quality and reliability. They are mainly used in communication equipment, high-end servers, medical electronics, aviation, and industrial , Military and other fields. In recent years, the market demand for high multilayer circuit boards in applications such as communications, base stations, aviation, and military has remained strong. With the rapid development of China's telecommunications equipment market, the market prospects of high multilayer circuit boards are promising.

At present, domestic PCB manufacturers that can mass produce high multilayer circuit boards mainly come from foreign-funded enterprises or a few domestic-funded enterprises. The production of high multilayer circuit board 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 import of high multilayer circuit board customer certification procedures are strict and cumbersome, so the entry barrier for high multilayer circuit board is higher. , To achieve a long industrial production cycle. 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 multilayer circuit boards, and introduces the control points of the key production processes of high multilayer circuit boards.

1. Main production difficulties

Compared with the characteristics of conventional circuit board products, high multilayer circuit board has the characteristics of thicker board, more layers, denser lines and vias, larger unit size, thinner dielectric layer, etc., inner layer space and interlayer alignment. , Impedance control and reliability requirements are more stringent.

1.1 Difficulties in alignment between layers

Due to the large number of layers of high multilayer circuit board, 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 larger unit size design of the high multilayer circuit board and the graphics transfer workshop Ambient temperature and humidity, as well as factors such as misalignment and superposition caused by inconsistencies in the expansion and contraction of different core layers, and interlayer positioning methods, make it more difficult to control the alignment of high multilayer circuit boards.

1.2 Difficulties in making inner circuit

The high multilayer circuit board adopts special materials such as high TG, high speed, high frequency, thick copper, thin dielectric layer, etc., and puts forward high requirements on the inner layer circuit production and pattern size control, such as the integrity of the impedance signal transmission, which increases the inner layer circuit production Difficulty. 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 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 multilayer circuit 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 production

When multiple inner core boards and prepregs are superimposed, defects such as sliding plates, delamination, resin cavities and air bubbles are likely to occur 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 dielectric, and set a reasonable high multilayer circuit board pressing program. With a large number of layers, the amount of expansion and contraction control and the compensation of the size factor cannot be consistent; the thin interlayer insulation layer can easily lead to the failure of the interlayer reliability test. Picture 1 is a defect diagram showing the delamination of the plate after thermal stress test.

1.4 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 removing drilling dirt. 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, it also brings high-frequency and high-speed signal transmission. Therefore, 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 multilayer circuit board. For high-layer 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, 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.

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 core board manufacturers must be consistent. 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 media between each layer must be guaranteed ≥0.09mm in accordance with IPC-A-600G.

(2) When customers request 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-glue 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.

(4) If the customer has no special requirements, the thickness tolerance of the interlayer dielectric layer is generally controlled by +/-10%. For the impedance board, the dielectric thickness tolerance is controlled by IPC-4101 C/M tolerance. If the impedance affects the factor and the thickness of the substrate If relevant, the sheet tolerance must also be in accordance with IPC-4101 C/M tolerance.

2.3 Interlayer 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 production to accurately compensate the size of each layer of the high multilayer circuit board to ensure that each layer of the core board Consistency of expansion and contraction. 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 appropriate pressing process and routine maintenance of the press is the key to ensuring the quality of the pressing, controlling the pressing flow and cooling effect, and reducing the problem of interlayer misalignment. Interlayer alignment control needs to be comprehensively considered from factors such as inner layer compensation value, pressing positioning method, pressing process parameters, and material characteristics.

2.4 Inner circuit process

Since the resolution capability of the traditional exposure machine is around 50μm, for the production of high multilayer circuit boards, a laser direct imaging machine (LDI) can be introduced to improve the graphics resolution capability, and the resolution can reach around 20μm. The alignment accuracy of the traditional exposure machine is ±25μm, and the alignment accuracy between layers is greater than 50μm. Using a high-precision alignment exposure machine, the graphic alignment accuracy can be increased to about 15μm, and the interlayer alignment accuracy can be controlled within 30μm, which reduces the alignment deviation of traditional equipment and improves the interlayer alignment accuracy of the high multilayer circuit board.

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 engineering design, and it is also necessary to make a more detailed design for the compensation of special graphics, such as the return type circuit and the independent circuit. consider. 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 technical transformation of the equipment or introduction of high-precision etching line equipment can be carried out to improve the etching uniformity and reduce the problems of etching burrs and unclean etching.

2.5 Pressing process

At present, the positioning methods between layers before pressing mainly include: four-slot positioning (Pin LAM), hot melt, rivet, hot melt and rivet combination, and different product structures adopt different positioning methods. For the high multilayer circuit board, the four-slot positioning method (Pin LAM) or the fusion + riveting method is used. The OPE punching machine punches the positioning holes, and the punching accuracy is controlled within ±25μm. When fusing, adjust the machine to make the first board using X-RAY to check the layer deviation, and the layer deviation can be produced in batches. During mass production, it is necessary to check whether each plate is fused into the unit to prevent subsequent delamination. The pressing equipment adopts high-performance supporting equipment. The press meets the interlayer alignment accuracy and reliability of the high multilayer pcb circuit board.

According to the laminated structure of the high multilayer circuit board and the materials used, study the appropriate pressing procedure and set the best heating rate and curve. In the conventional multilayer circuit board pressing procedure, appropriately reduce the heating rate of the laminated board. Extend the high temperature curing time to make the resin flow and cure fully, and avoid the problems of sliding plate and interlayer dislocation during the pressing process. Plates with different material TG values cannot be the same as grate plates; plates with common parameters cannot be mixed with plates with special parameters; to ensure the rationality of the expansion and shrinkage coefficients given, the properties of different plates and prepregs are different, and the corresponding plates must be used The prepreg parameters are pressed together. Special materials that have never been used need to verify the process parameters.

2.6 Drilling technology

Due to the superposition of each layer, the plate and copper layer are too thick, which will cause serious wear on the drill bit and easily break the drill bit. The number of holes, falling speed and rotation speed are appropriately reduced. Accurately measure the expansion and contraction of the board to provide accurate coefficients; the number of layers is ≥14, the hole diameter is ≤0.2mm or the hole-to-line distance is ≤0.175mm, and the hole position accuracy is ≤0.025mm. The hole diameter is larger than φ4.0mm. Step drilling, with a thickness-to-diameter ratio of 12:1, use step drilling and positive and negative drilling methods; control the drilling tip and hole thickness, and try to drill with a new drill or a grinding drill for the high multilayer pcb circuit board. The hole thickness is controlled Within 25um. In order to improve the drilling burr problem of high-multilayer thick copper plates, after batch verification, the use of high-density backing plates, the number of stacked plates is one, the drill bit grinding times are controlled within 3 times, which can effectively improve the drilling burrs, as shown in Picture2，Picture 3 shown.

For high multilayer circuit boards used for high frequency, high speed, and mass data transmission, back drilling technology is an effective method to improve signal integrity. The back drill mainly controls the length of the remaining stub, the consistency of the hole position of the two holes, and the copper wire in the hole. Not all drilling machine equipment has the back drilling function, the drilling machine equipment must be technically upgraded (with back drilling function), or a drilling machine with back drilling function must be purchased. The back-drilling technology applied from industry-related literature and mature mass production mainly includes: traditional depth-controlled back-drilling method, inner layer is back-drilling with signal feedback layer, depth back-drilling is calculated according to the plate thickness ratio, which will not be repeated here.

3. Reliability test

The high multilayer circuit board is generally a system board, which is thicker, heavier, and larger in unit size than conventional multilayer boards. The corresponding heat capacity is also larger. During soldering, more heat is required and the soldering high temperature time is longer. . It takes 50 to 90 seconds at 217°C (melting point of tin-silver-copper solder), and the cooling speed of the high multilayer pcb board is relatively slow, so it takes longer to pass the reflow test.

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