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As electronic devices develop towards higher performance and smaller sizes, the precision and performance requirements for PCBs are also increasing.
High multilayer PCBs can provide more routing layers, allowing for more complex and dense circuit designs, thus meeting the demands for high-frequency and high-speed transmission. Additionally, high multilayer PCBs can achieve better signal integrity and electromagnetic compatibility. This is particularly important for high-end application fields such as 5G communication, high-performance computing, and automotive electronics. Therefore, high multilayer PCBs have become one of the important trends in the future development of the PCB industry. For PCB design engineers or electronic hardware design engineers, it is also necessary to understand the manufacturing processes related to high multilayer PCBs.
High multilayer PCBs are not just about increasing the number of layers; their manufacturing difficulty also increases exponentially. Compared to single-sided and double-sided boards, the production of high multilayer PCBs requires attention to inter-layer connections, inter-layer stacking and alignment, as well as precise lamination control. In design, it is also necessary to consider signal integrity, electromagnetic interference, and thermal management issues to fully utilize the performance advantages of high multilayer PCBs.
From process, equipment, design capability to quality control and collaboration ability, high multilayer boards require higher manufacturing process levels from PCB manufacturers. In this article, I plan to introduce some key process steps related to high multilayer PCB manufacturing.
1. Submit Manufacturing Information
As the start of PCB manufacturing, we first need to submit relevant manufacturing information to the PCB manufacturer. The information and common data formats required for PCB manufacturing include the following:
Gerber Files (RS274X format)
Gerber RS274X is the current mainstream format, and the output Gerber files include all circuit layers, solder mask layers, paste layers, silkscreen (character) layers, board frames, drilling diagrams, and manufacturing requirements (such as multilayer stacking structure diagrams, inter-layer dielectric thickness, impedance control requirements, via requirements, etc.). At the same time, Gerber files should also allow the process engineers at the PCB manufacturer to identify the layer information corresponding to each Gerber file, so it is recommended to name Gerber files according to certain naming conventions. For example, the naming convention provided by JLCPCB is a good reference:
Drilling Files
Drilling files contain all drilling coordinates and diameter data, and the commonly used file format is Excellon format.
Netlist Data
IPC defines the compatible format IPC-356, which provides all the information necessary for generating netlists and electrical performance testing materials. Compared to single-sided or double-sided boards, complete PCB documentation is very important for multilayer PCB manufacturing, and the most important information in the manufacturing documentation is:
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Complete layer structure
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Accurate information about the substrate
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For high-frequency and high-speed boards, the substrate manufacturer and product name must also be provided
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Impedance control requirements
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Special process instructions (such as via requirements)
2. Review of Manufacturing Information
The purpose of the PCB manufacturer’s review of the manufacturing information is to determine the approximate manufacturing cost and prepare for manufacturing. Appropriate preliminary analysis can save time and materials before product manufacturing or processing. The responsibility of the PCB manufacturer is to determine whether its process capability can meet the given product.
The PCB manufacturer will adjust the wiring information of the PCB design based on its manufacturing process, such as via drilling diameter compensation or trace etching compensation, with the aim of improving PCB manufacturability. Some critical modifications will also be communicated with the PCB Layout for confirmation. Of course, the ideal situation is to consider DFM manufacturability during the PCB design process and optimize the design, which will save a lot of time in later communication and confirmation with the PCB manufacturer.
If it is produced by JLCPCB, they also offer a personalized service called “Confirm Production Draft”, which can be selected. As long as we carefully check and confirm, we can find problems in our design, and of course, we can also discover some errors in the processing by JLCPCB engineers. If it is JLCPCB’s problem, don’t forget to ask the engineering personnel to refund the confirmation production draft fee.
3. Material Preparation
Manufacturing single-sided and double-sided circuit boards directly uses copper-clad laminates that meet the final product thickness requirements, while multilayer circuit boards are different. Multilayer circuit boards have multiple copper layers in their structure, so special substrates are needed for manufacturing. Creating multilayer circuit boards requires the use of prepreg (PP) and relatively thin copper-clad laminates (core boards) to form the final thickness after combined lamination and curing. The laminate structure depends on electrical parameters, which should be determined in consultation between the PCB designer and the circuit board manufacturer, and should be planned in advance before the PCB layout to meet the specific impedance requirements for trace width/spacing.
Due to different laminate structures, there are many thicknesses of prepreg to meet different transmission line and power plane combination requirements. Each prepreg consists of a specific type of fiberglass weave and is numbered, such as 1080, 2116, 3313, or 7628. The following image shows the identification of this type:
The second component of multilayer circuit boards is relatively thinner copper-clad laminates (relative to the copper-clad laminates used for manufacturing single-sided and double-sided PCBs), also known as core boards. It is a fully cured substrate, coated with copper foil on one or both sides. Of course, there are also those without copper, known as blank boards.
Core boards are actually also made of prepreg and copper foil bonded together, completed by substrate suppliers, who will select different woven styles of fiberglass cloth and resin content prepreg according to IPC-4101 specifications combined with market demand, paired with copper foil of specified specifications, and laminated to generate different specifications of copper-clad laminates.
Multilayer circuit board manufacturing is completed by PCB manufacturers, but the substrates are provided by substrate manufacturers. It is important to note that there are many specifications for substrates, and the substrate reserves of each PCB manufacturer vary. If the PCB stacking design requires the use of special specifications of prepreg and core boards, it is best to communicate with the PCB manufacturer in advance to understand the supply cycle of the substrates.
Choosing good raw materials is essential to produce high-performance PCBs. The materials play a key role in the PCB manufacturing process, significantly affecting the performance and reliability of PCBs, including electrical performance, thermal performance, mechanical strength, processing performance, and environmental adaptability.
In terms of materials, JLCPCB uses raw materials from large manufacturers. For 4-layer and 6-layer boards, JLCPCB uses KB and Taiwan’s Nanya materials, which are of high quality and guaranteed. KB materials use high-quality fiberglass reinforced epoxy resin (FR-4) as the substrate, with high-purity copper foil as the conductive layer, and undergo strict processing, thus possessing high quality and good performance, widely used in the electronics industry.
Similarly, Taiwan’s Nanya is also well-known in the market, and the materials they provide not only have good electrical performance, high strength, and rigidity but also high temperature resistance and chemical resistance, improving product reliability and lifespan.
For 8-layer boards and higher, JLCPCB uses Taiwan’s Nanya and Shengyi materials. Among them, Shengyi, as a well-known supplier of copper-clad laminates in China, is characterized by high standards, high quality, high performance, and high reliability, with high industry recognition and widely used in industrial control, medical instruments, consumer electronics, automotive, and other electronic products.
4. Manufacturing Process of Multilayer Boards
The manufacturing process of multilayer boards, as shown in the above diagram, has an additional inner layer process compared to the manufacturing of single-sided and double-sided PCBs, with the critical step being the control of the inner layer lamination process, which is crucial for the electrical performance of controlled impedance transmission lines. After completing the inner layer process, it proceeds to the same manufacturing process flow as single-sided and double-sided boards, until the final inspection process.
The production process of multilayer boards, if detailed, typically requires about 200 different processing steps. Therefore, for PCB designers, it is very important to be familiar with the different types and performance of substrates, the manufacturing processes of multilayer boards, and the soldering processes. By combining different specifications of prepreg and copper-clad laminates (core boards), all required thicknesses can be achieved. For the stacking structure of multilayer boards, it is important to note that each layer structure must be symmetrical and have the same layer thickness. The copper in the inner layers should be evenly distributed across these symmetrical layers. If the distribution is uneven, thermal stress during heating may cause warping of the circuit board.
One of the factors that greatly affects the quality of multilayer board structure is the precise adjustment between the layers. These layers must overlap precisely; otherwise, after drilling connections, there may be short circuits or open circuits between the layers. Precise adjustment is achieved through mechanical alignment holes, and positioning pins are used during lamination to adjust the stacking. To ensure good bonding between the inner layers and the prepreg, the copper surface must undergo chemical roughening treatment, known as browning. Checking the internal circuit layers before laminating multilayer PCBs is crucial for ensuring quality. At this stage, if defects in connections or others are found, they can still be repaired. Inspections are usually performed automatically using AOI (Automated Optical Inspection), where the AOI system directly visually compares the etched circuit patterns with CAD data.
The above image shows the manufacturing diagram of a 6-layer rigid multilayer board, where A1, A2, and A3 are prepreg, L2-L3 and L4-L5 are double-sided copper-clad laminates with completed inner layer patterns, and B1, B2 are copper foils for outer layer circuits.
The conventional principle of laminating rigid multilayer boards is to combine a certain number of double-sided copper-clad boards (with inner layer patterns already completed and browned for enhanced bonding strength), separated by prepreg, which acts as an insulating material to prevent short circuits between copper layers. At the same time, after heating, the resin in the prepreg will melt again to bond the copper-clad laminates together. Finally, the laminated layers are connected through metallized holes. Currently, JLCPCB’s multilayer board manufacturing process can manufacture multilayer boards of up to 32 layers, sufficient to cover most application scenarios.
Precise control of lamination is crucial for the characteristic impedance of controlled impedance transmission lines. During the lamination process, as the temperature rises, the epoxy resin in the prepreg melts again, filling the gaps between the conductors and bonding the inner layers together. The flow characteristics of the resin will affect the distance between the signal layers and the reference layers, and the variation in distance between the signal layer and its reference layer has the greatest impact on impedance variation.
As shown in the above image, the PCB design draft is ultimately spliced onto a large working panel for production. For characteristic impedance control, the uniformity of resin flow during the lamination of the entire large panel also cannot be ignored. The performance of the lamination equipment used at this time will be crucial.
Equipment is one of the important factors affecting the quality of high multilayer boards. Therefore, JLCPCB uses top-notch equipment to produce high multilayer boards, ensuring product quality.
Laminating machine
JLCPCB uses the latest generation of fully automatic laminating machines from Taiwan Vigor, which is more stable and has better lamination quality. As a professional PCB equipment supplier, Vigor laminating machines feature high precision, high reliability, and advanced control systems, capable of meeting the stacking and lamination of high multilayer PCBs.
After lamination is completed, it proceeds to the drilling process, followed by the same process flow as single-sided and double-sided boards, but there are also slight differences. For example, JLCPCB has launched a free service to improve PCB quality specifically for high multilayer boards.
One is the enhanced gold immersion process, JLCPCB uses gold immersion process for all 6-32 layer circuit boards, and the thickness of gold immersion is upgraded to 2u” for free. Gold immersion is a relatively expensive surface treatment method in the industry, which provides good electrical connections, corrosion resistance, and soldering performance. The gold layer can provide a smooth and even metal surface, helping to maintain good signal transmission and impedance control. Moreover, it ensures the stability and durability of the metal layer during the soldering process, providing excellent corrosion resistance and prolonging the lifespan of the PCB.
In addition to the gold immersion process, JLCPCB offers free blind hole technology for all 6-32 layer boards (resin-filled vias + plated caps). For PCB quality, vias are very important as they play a crucial role in supporting the realization and reliability of complex circuits. Due to various factors, vias can slowly corrode, leading to connection failures, signal attenuation, short circuits, leakage, and reliability issues, while blind hole technology effectively solves these problems.
PCBs produced using gold immersion and blind hole technology
In summary, the manufacturing of multilayer boards is not simply an additional inner layer process compared to single-sided and double-sided boards. It is also not just about handing over the PCB production draft directly to the PCB manufacturer. At least in the PCB design stage, we should understand the manufacturing capabilities of the PCB manufacturer. During the design, we should introduce DFM (Design for Manufacturability) principles, and before implementing the routing, we need to communicate with the PCB manufacturer to determine the required materials and stacking structure to meet the performance requirements of specific transmission line structures and to ensure reasonable costs and manufacturing times.
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