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Key production process control of high-density interconnect circuit boards
PCB News
Key production process control of high-density interconnect circuit boards

Key production process control of high-density interconnect circuit boards

2021-11-11
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Author:Kavie

    High-density interconnect PCB circuit boards are generally HDI high multi-layer circuit boards with 10 to 20 layers or more, which are more difficult to process than traditional multi-layer 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 market for high-level boards has been promising.


PCB

   At present, the domestic PCB manufacturers that can mass produce high-level circuit boards are mainly foreign-funded enterprises or a few domestic-funded enterprises. 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 defects such as slippage, delamination, resin voids and bubble residues are likely to occur during lamination 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. 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 sufficient 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 issues 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 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.




(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 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




   Since the resolution capability of the traditional exposure machine is about 50μm, for the production of high-level boards, a laser direct imaging machine (LDI) can be introduced to improve the resolution of graphics, and the resolution can reach about 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-level 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, but also to make a more detailed design for the compensation amount of the special pattern, such as the return 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 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.




  2.5 pressing process




   The current 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-level board, the four-slot positioning method (Pin LAM) or the fusion + riveting method is used. The positioning hole is punched out by the OPE punching machine, and the punching accuracy is controlled at ±25μm. When fusing, adjust the machine to make the first board to use 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 alignment accuracy and reliability of the high-level board.




According to the laminated structure of the high-rise board and the materials used, study the appropriate pressing procedure, set the best heating rate and curve, and appropriately reduce the heating rate of the pressed sheet material and extend the high temperature in the conventional multilayer circuit board pressing procedure. The curing time allows the resin to flow and cure fully, while avoiding 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, and the special materials that have never been used need to verify the process parameters.




  2.6 Drilling process




   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, adopts step-drilling and positive and negative drilling methods; to control the drilling front and hole thickness, high-rise boards should be drilled with a new drill or a one-grind drill as far as possible, and the hole thickness should be controlled within 25um. In order to improve the drilling burr problem of high-rise 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.




For high-level boards for high-frequency, high-speed, and massive data transmission, back-drilling technology is an effective way 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 the back drilling function), or the drilling machine with the back drilling function must be purchased. The back-drilling technology used from industry-related literature and mature mass production applications 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.


   Three, reliability test




   High-layer boards are generally system boards, which are thicker, heavier, and larger in unit size than conventional multilayer boards. The corresponding heat capacity is also larger. During welding, more heat is required and the welding high temperature time is longer. It takes 50 seconds to 90 seconds at 217°C (melting point of tin-silver-copper solder). At the same time, the cooling rate of the high-layer board is relatively slow, so the time for the reflow soldering test is extended, and in accordance with IPC-6012C, IPC-TM-650 standards and industry requirements, The main reliability test of the high-layer board, as described in Table 2.



Fourth, the conclusion




   There are relatively few research literatures on high-level circuit board 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.