High level circuit boards are generally defined as high-level multi-layer circuit boards with 10 ~ 20 layers or more, which are more difficult to process than traditional multi-layer circuit boards and have high quality 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-rise boards in application communication, base station, aviation, military and other fields is still strong. With the rapid development of China’s telecom equipment market, the market prospect of high-rise boards is promising.



At present, PCB manufacturers that can mass produce high-rise circuit boards in China mainly come from foreign-funded enterprises or a few domestic enterprises. The production of high-rise circuit boards requires not only higher technology and equipment investment, but also the experience accumulation of technicians and production personnel. At the same time, the introduction of high-rise circuit boards requires strict and cumbersome customer certification procedures. Therefore, the threshold for high-rise circuit boards to enter the enterprise is high and the industrialization production cycle is long. The average number of PCB layers has become an important technical index to measure the technical level and product structure of PCB enterprises. This paper briefly describes the main processing difficulties encountered in the production of high-rise circuit boards, and introduces the key control points of the key production processes of high-rise circuit boards for reference and reference.



1、 Main manufacturing difficulties



Compared with the characteristics of conventional circuit boards, high-rise circuit boards have the characteristics of thicker boards, more layers, denser lines and vias, larger unit sizes, thinner dielectric layers, and stricter requirements for inner space, inter layer alignment, impedance control and reliability.



1.1 difficulties in interlayer alignment



Due to the large number of layers of high-rise boards, the customer’s design end has increasingly strict requirements on the alignment of PCB layers. Generally, the alignment tolerance between layers is controlled by ± 75 μ m. Considering the large unit size design of high-rise panels, the ambient temperature and humidity of the graphic transfer workshop, the dislocation superposition caused by the inconsistent expansion and contraction of different core layers, the interlayer positioning mode and other factors, it is more difficult to control the interlayer alignment of high-rise panels.



1.2 manufacturing difficulties of inner circuit



The high-rise 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 fabrication of inner circuit and the control of graphic size, such as the integrity of impedance signal transmission, which increases the difficulty of making inner circuit. The line width and line spacing are small, the number of open and short circuits increases, the number of micro short circuits increases, and the qualification rate is low; There are many signal layers of fine lines, and the probability of missed detection of inner AOI increases; The inner core plate is thin, easy to fold, resulting in poor exposure, and easy to roll after etching; Most of the high-rise boards are system boards with large unit size, and the cost of scrapping the finished products is relatively high.



1.3 pressing manufacturing difficulties



When multiple inner core plates and semi cured sheets are superimposed, it is easy to produce defects such as sliding plate, delamination, resin cavity and bubble residue. When designing the laminated structure, it is necessary to fully consider the heat resistance, voltage resistance, glue filling amount and medium thickness of the material, and set a reasonable high-rise plate pressing program. There are many layers, so the control of expansion and contraction and the compensation of size coefficient cannot be consistent; The thin interlayer insulation layer is easy to cause the failure of interlayer reliability test. Fig. 1 is a diagram showing the defect of delamination after thermal stress test.




1.4 drilling difficulties



The use of high Tg, high-speed, high-frequency, thick copper special plates increases the difficulty of drilling roughness, drilling burr and drilling dirt removal. There are many layers, the total copper thickness and plate thickness are accumulated, and the drilling tool is easy to break; Caf failure caused by many dense BGAs and narrow hole wall spacing; Due to the thickness of the plate, it is easy to cause the problem of oblique drilling.



2、 Key production process control



2.1 material selection



With the development of electronic components in the direction of high-performance and multi-function, it also brings high-frequency and high-speed signal transmission. Therefore, it is required that the dielectric constant and dielectric loss of electronic circuit materials 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-rise boards. Common plate suppliers mainly include a series, B series, C series and D series. See Table 1 for the comparison of the main characteristics of these four inner substrates. For the high-rise thick copper circuit board, the semi cured sheet with high resin content is selected. The glue flow of the inter layer semi cured sheet is enough to fill the inner layer graphics. If the insulating medium layer is too thick, the finished board is likely to be too thick. On the contrary, if the insulating medium layer is too thin, it is easy to cause quality problems such as media layering and high-voltage test failure. Therefore, the selection of insulating medium materials is extremely important.




2.2 design of laminated structure



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



(1) The manufacturer of semi cured sheet and core board must be consistent. In order to ensure the reliability of PCB, single 1080 or 106 semi cured sheets shall not be used for all layers of semi cured sheets (unless the customer has special requirements). When the customer has no requirements for the thickness of media, the thickness of media between layers must be guaranteed to be ≥ 0.09mm according to ipc-a-600g.



(2) When customers require high Tg board, the core board and semi cured sheet shall use corresponding high Tg materials.



(3) For the inner substrate 3oz or above, select the semi cured sheet with high resin content, such as 1080r/c65%, 1080hr/c 68%, 106r/c 73%, 106hr/c76%; However, the structural design of 106 high adhesive semi cured sheets shall be avoided as far as possible to prevent the superposition of multiple 106 semi cured sheets. Because the glass fiber yarn is too thin, the glass fiber yarn collapses in the large substrate area, which will affect the dimensional stability and the delamination.



(4) If the customer has no special requirements, the thickness tolerance of interlayer dielectric layer is generally controlled as +/-10%. For impedance plate, the thickness tolerance of dielectric layer is controlled as per ipc-4101 c/m tolerance. If the impedance influencing factor is related to the thickness of substrate, the plate tolerance must also be controlled as per ipc-4101 c/m tolerance.



2.3 interlayer alignment control



For the accuracy of inner core board size compensation and production size control, it is necessary to accurately compensate the graphic size of each layer of high-rise board through the data and historical data collected in production for a certain period of time to ensure the consistency of each layer of core board expansion and contraction. Select high-precision and reliable interlayer positioning mode before pressing, such as pin Lam, hot-melt and rivet combination. Setting appropriate pressing process procedures and daily maintenance of the press are the key to ensure the pressing quality, control the laminating glue and cooling effect, and reduce the interlayer dislocation. The control of interlayer alignment should be comprehensively considered from the internal compensation value, pressing positioning mode, pressing process parameters, material characteristics and other factors.



2.4 inner line process



Because the resolution capability of the traditional exposure machine is 50 μ M. for the production of high-rise panels, laser direct imager (LDI) can be introduced to improve the graphic analysis ability, which can reach 20 μ M or so. The alignment accuracy of traditional exposure machine is ± 25 μ m. Inter layer alignment accuracy is greater than 50 μ m。 With high-precision alignment exposure machine, the graphics alignment accuracy can be increased to 15 μ M or so, the interlayer alignment accuracy is controlled by 30 μ M, which reduces the alignment deviation of traditional equipment and improves the interlayer alignment accuracy of high-rise slabs.



In order to improve the etching ability of the line, it is necessary to give appropriate compensation for the width of the line and the pad (or ring) in the engineering design, and to make more detailed design consideration for the compensation amount of special graphics, such as return lines and independent lines. Confirm whether the design compensation of inner line width, line distance, isolation ring size, independent line and hole to line distance is reasonable, or 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. Batch production can be carried out only after the first piece is confirmed to be qualified. In order to reduce etching side corrosion, it is necessary to control the composition of each group of solution in the best range. The traditional etching line equipment has insufficient etching capacity. The equipment can be technically transformed or high-precision etching line equipment can be introduced to improve the etching uniformity and reduce the problems such as rough edges and unclean etching.



2.5 pressing process



At present, the interlayer positioning methods before pressing mainly include: pin Lam, hot melt, rivet, hot melt and rivet combination. Different product structures adopt different positioning methods. For the high-rise slab, the four groove positioning method (pin Lam) or the fusion + riveting method is used. The ope punching machine punches the positioning hole, and the punching accuracy is controlled within ± 25 μ m。 During fusion, X-ray shall be used to check the layer deviation of the first plate made by the adjustment machine. Batch production can be carried out only after the layer deviation is qualified. During batch production, it is necessary to check whether each plate is fused into the unit to prevent subsequent layering. The pressing equipment adopts a high-performance matching press to meet the interlayer alignment accuracy and reliability of high-rise plates.



According to the laminated structure of high-rise boards and the materials used, study the appropriate pressing procedure, set the best temperature rise rate and curve, properly reduce the temperature rise rate of the laminated boards in the conventional multi-layer circuit board pressing procedure, extend the high-temperature curing time, make the resin fully flow and solidify, and avoid the problems such as sliding plate and interlayer dislocation in the pressing process. Plates with different TG values shall not be the same as grate plates; Plates with common parameters shall not be mixed with plates with special parameters; To ensure the rationality of the expansion and contraction coefficient, different plates and semi cured sheets have different properties, so the corresponding plate semi cured sheet parameters need to be used for pressing, and the process parameters need to be verified for special materials that have never been used.



2.6 drilling process



Due to the over thickness of the plate and copper layer caused by the superposition of each layer, the drill bit is severely worn, and the drill bit is easy to be broken. The number of holes, falling speed and rotating speed shall be properly reduced. Accurately measure the expansion and contraction of the plate to provide accurate coefficient; The number of layers ≥ 14, the hole diameter ≤ 0.2mm or the distance from hole to line ≤ 0.175mm, and the drilling rig with hole location accuracy ≤ 0.025mm shall be used for production; diameter φ The hole diameter above 4.0mm shall be drilled step by step, and the thickness diameter ratio of 12:1 shall be drilled step by step, and the positive and negative drilling method shall be used for production; Control the burr and hole diameter of the drill hole. For high-rise slabs, try to use a new drill or grind a drill. The hole diameter shall be controlled within 25um. In order to improve the drilling burr problem of high-rise thick copper plates, through batch verification, high-density backing plates are used, the number of laminated plates is one, and the grinding times of the drill bit are controlled within 3 times, which can effectively improve the drilling burr, as shown in Fig. 2 and Fig. 3.




Back drilling technology is an effective method to improve signal integrity for high-frequency, high-speed and massive data transmission. Back drilling mainly controls the length of residual stub, the hole position consistency of the two boreholes and the copper wire in the hole. Not all drill equipment has back drilling function, so it is necessary to upgrade the drill equipment (with back drilling function) or purchase a drill with back drilling function. The back drilling technology applied from the relevant industry literature and mature mass production mainly includes: traditional depth control back drilling method, back drilling with signal feedback layer in the inner layer, and calculation of depth back drilling according to the proportion of plate thickness, which will not be repeated here.



3、 Reliability test



The high-rise plate is generally a system plate, which is thicker, heavier, larger in unit size and larger in heat capacity than the conventional multilayer plate. During welding, more heat is required and the welding high temperature time is long. At 217 ℃ (melting point of tin silver copper solder), it takes 50 seconds to 90 seconds. Meanwhile, the cooling speed of the high-rise plate is relatively slow, so the time for reflow test is extended. In combination with ipc-6012c, IPC-TM-650 standards and industry requirements, the main reliability tests of the high-rise plate are described in Table 2.




4、 Conclusion



The research literature on high-level circuit board processing technology is relatively few in the industry. This paper introduces the key process control points of key production processes such as material selection, laminated structure design, interlayer alignment, inner circuit fabrication, pressing process, drilling process, etc., in order to provide peer reference and understanding, and hope that more peers will participate in the technical research and communication of high-rise circuit boards.