The Ultimate Guide to High-Multilayer PCB Manufacturing for Engineers

With the development of electronic devices 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, meeting the needs for high-frequency, high-speed transmission. Moreover, high-multilayer PCBs can achieve better signal integrity and electromagnetic compatibility. This is particularly important for high-end applications such as 5G communications, high-performance computing, and automotive electronics. Therefore, high-multilayer PCBs have become one of the important trends for the future development of the PCB industry. For PCB design engineers or electronic hardware design engineers, understanding the manufacturing process related to high-multilayer PCBs is also essential.

High-multilayer PCBs are not just about increasing the number of layers; the manufacturing difficulty also increases exponentially. Compared to single- and double-layer boards, the manufacturing of high-multilayer PCBs requires attention to inter-layer connection, inter-layer stacking and alignment, as well as precise lamination control. During the design process, factors such as signal integrity, electromagnetic interference, and thermal management need to be considered to fully leverage the performance advantages of high-multilayer PCBs.

From process, equipment, design capability, to quality control and collaboration capability, high-multilayer boards require higher manufacturing process standards from PCB manufacturers. In this article,we can learn some key process steps in high-multilayer PCB manufacturing.

1.Submitting Manufacturing Information  

As the start of PCB manufacturing, we first need to submit the relevant manufacturing information to the PCB manufacturer. The information and commonly used data formats required for PCB manufacturing include the following:

Gerber Files (RS274X Format)  

Gerber RS274X is the mainstream format. The output Gerber file includes all circuit layers, solder mask layers, paste layers, silkscreen layers, board outline, drill map, and manufacturing requirements (such as multilayer stack-up structure diagram, interlayer dielectric thickness, impedance control requirements, via fill requirements, etc.). The Gerber file should also enable the PCB manufacturer’s process engineers to identify the layer information of each Gerber file easily. It is recommended to name the Gerber files according to a naming convention, and JLC provides a good reference for this.

Drill File  

The drill file contains all drill coordinates and diameter data, with Excellon format being the most commonly used.

Netlist Data  

IPC defines a compatible format IPC-356, providing all the necessary information to generate netlists and electrical performance test data. Compared to single- or double-layer boards, comprehensive PCB documentation is crucial for multilayer PCB manufacturing. The most important information in the manufacturing documentation includes:

- Complete layer structure  

- Precise information about the substrate  

- For high-frequency high-speed boards, information on the substrate manufacturer and product name  

- Impedance control requirements  

- Special process instructions (such as via fill requirements)

2. Manufacturing Information Review  

The purpose of reviewing manufacturing information at the PCB manufacturer is to estimate the approximate manufacturing cost and prepare for production. Preliminary analysis before product manufacturing can save time and materials. The PCB manufacturer’s responsibility is to determine whether its process capabilities meet the requirements for the given product.

The PCB manufacturer may adjust the PCB design’s routing information based on its manufacturing process, such as compensating via hole diameters or etching lines, with the goal of improving PCB manufacturability. Some critical adjustments are communicated with the PCB Layout team for confirmation. Ideally, manufacturability considerations (DFM) are included during the PCB design process to optimize design, saving considerable time for later communication with the PCB manufacturer.

If you are ordering a board at JLCPCB, they offer a “production draft confirmation” personalized service option. By carefully reviewing and confirming the draft, you can identify any issues in your design, as well as any errors in JLC’s processing.

3. Material Preparation  

For manufacturing single- and double-sided boards, copper-clad laminates that meet the final product thickness requirements are directly used. Multilayer boards, however, are different. In multilayer boards, multiple copper layers are included within the board structure, requiring special substrates. To create multilayer boards, prepregs (PP) and relatively thin copper-clad laminates (core boards) are combined and laminated to form the final thickness. The laminate structure is determined by electrical parameters, agreed upon by the PCB designer and the board manufacturer, and planned before PCB Layout to meet specific impedance requirements for line width/spacing.

Due to the differences in laminate structure, prepreg thickness varies to meet different requirements for transmission lines and power plane combinations. Each type of prepreg is made from a specific glass fiber weave type, labeled with numbers like 1080, 2116, 3313, or 7628. The following image shows these identifiers:

The second component in multilayer boards is a relatively thinner copper-clad laminate (compared to copper-clad laminates used for single- and double-sided PCBs), also known as a core board. It is a fully cured substrate with copper foil on one or both sides. There are also bare boards without copper, known as blank boards.

Core boards are also made by laminating prepregs and copper foil, manufactured by substrate suppliers. These suppliers follow the IPC-4101 standard and market demand, using different glass fiber weave styles and resin content prepregs, combined with copper foil of specified thicknesses to produce various types of copper-clad laminates.

While multilayer board manufacturing is completed by the PCB manufacturer, the substrates are provided by substrate suppliers. It’s worth noting that there are many substrate specifications, and each PCB manufacturer has different substrate inventories. If the PCB stack-up design requires special types of prepregs and core boards, it is best to communicate with the PCB manufacturer in advance to understand the substrate’s supply cycle.

High-quality raw materials are necessary to produce high-performance PCBs. The substrate plays a crucial role in PCB manufacturing, impacting the PCB’s performance and reliability, including electrical properties, thermal performance, mechanical strength, processability, and environmental adaptability.

In terms of substrate, JLCPCB uses high-quality materials from leading manufacturers. For 4-layer and 6-layer boards, JLC uses KB and Taiwan Nanya materials, which are high-quality and reliable. KB materials use high-quality glass fiber reinforced epoxy resin (FR-4) as the base material, with high-purity copper foil as the conductive layer, processed through strict procedures, resulting in high-quality, high-performance characteristics, widely used in the electronics industry.

Similarly, Taiwan Nanya has a good reputation in the market. Their materials offer excellent electrical properties, high strength, rigidity, and resistance to high temperatures and chemicals, enhancing product reliability and longevity.

For 8-layer and higher boards, JLC uses Taiwan Nanya and Shengyi materials. As a well-known domestic copper-clad laminate supplier, Shengyi materials are high-standard, high-quality, high-performance, and highly reliable, widely recognized and used in industrial control, medical instrumentation, consumer electronics, automotive, and other electronic products.

4. Manufacturing Process for Multilayer Boards  

As shown in the multilayer board manufacturing process diagram above, the process for multilayer boards includes an additional inner layer processing step compared to single- and double-sided PCBs. The key step is the inner layer stacking and lamination process control, which is crucial for the electrical performance of controlled impedance transmission lines. After completing the inner layer process, it proceeds with the same manufacturing process as single- and double-sided boards until the final inspection.

If the multilayer board production process is detailed, it typically involves around 200 different processing steps. For PCB designers, it is crucial to understand the various types and properties of substrates, the multilayer board manufacturing process, and soldering techniques. By combining different specifications of prepregs and copper-clad laminates (core boards), all required thicknesses can be achieved. For multilayer stack-up structures, it is essential to ensure that all layers are symmetrical with the same layer thickness. Copper on inner layers should be evenly distributed across these symmetrical layers. If distribution is uneven, thermal stress from heating could cause the PCB to warp.

One of the key factors impacting multilayer board structure quality is the precise alignment between each layer. These layers must be accurately aligned; otherwise, open or short circuits may occur between layers after drilling connections. Precise alignment is achieved using mechanical alignment holes and positioning pins during stacking. To ensure good adhesion between the inner layers and prepregs, the copper surface must undergo chemical roughening, known as browning. Inspecting the inner circuit layers before laminating multilayer PCBs is crucial for quality assurance. At this stage, any connectivity or other defects detected can still be repaired. Inspections are usually performed automatically using AOI (Automated Optical Inspection), which visually compares the etched circuit pattern with CAD data.

The image above shows the lamination process for a 6-layer rigid multilayer PCB, where A1, A2, and A3 are prepregs, L2-L3 and L4-L5 are double-sided copper-clad laminates with completed inner layer patterns, and B1 and B2 are copper foils for the outer layers.

The principle of lamination for conventional rigid multilayer PCBs is to stack a certain number of double-sided copper-clad boards (with completed inner layer patterns and browned for adhesion enhancement). These double-sided copper-clad boards are separated by prepregs, which act as insulating material to prevent short circuits between copper layers. When heated, the resin in the prepreg melts again, bonding each copper-clad laminate. The laminated layers are connected through metallized holes. JLC’s multilayer manufacturing process can produce multilayer boards with up to 32 layers, covering most application scenarios.

Precise control of lamination is critical for the characteristic impedance of controlled impedance transmission lines. During pressing, as the temperature increases, the epoxy resin in the prepreg melts and flows, filling the gaps between conductors and bonding the inner layers. The resin flow affects the distance between the signal layer and its reference layer, which has the greatest impact on impedance variations.

As shown in the image above, the PCB design file is eventually panelized into a large working panel for production. For characteristic impedance control, the uniformity of resin flow across the entire panel during lamination is also essential for impedance stability. In this case, the performance of the lamination equipment is critical.

Equipment is one of the factors affecting high-multilayer quality. JLC uses top-tier equipment in the industry to ensure high-multilayer board quality.

Lamination Machine

JLCPCB uses the latest generation of fully automatic laminating machines from Vigor in Taiwan, providing more stability and higher lamination quality. As a professional PCB equipment supplier, Vigor’s laminating machines offer high precision, high reliability, and advanced control systems to meet the stacking and lamination requirements of high-multilayer PCBs.

After lamination, the process proceeds to drilling, followed by the same steps as single- and double-sided boards. However, there are slight differences; for instance, JLC offers additional quality-enhancing services for high-multilayer board manufacturing, free of charge.

One of these enhancements is an upgraded immersion gold process. JLC uses immersion gold for all 6- to 32-layer PCBs, with a free thickness upgrade to 2u". Immersion gold is a relatively expensive surface treatment method in the industry, providing excellent electrical connection, corrosion resistance, and solderability. The immersion gold layer provides a smooth and uniform metal surface, supporting good signal transmission and impedance control. It also ensures stability and durability during soldering, offering excellent corrosion resistance and extending the PCB’s lifespan.

In addition to immersion gold, JLCPCB applies the resin-filled via technology for all 6- to 32-layer boards free of charge (resin-filled and plated cap). For PCB quality, vias are critical as they play a vital role in electronic devices, supporting complex circuits and reliability. Vias can gradually corrode due to various factors, leading to connection failure, signal attenuation, short circuits, leakage, and reliability issues. Resin-filled via technology effectively addresses these issues.

PCBs produced with immersion gold and via- in- pad  technology

In conclusion, the manufacturing of multilayer boards is more than just adding an inner layer process compared to single- and double-sided boards. It’s also not as simple as handing over the PCB production files to the PCB manufacturer. At least during the PCB design phase, we should understand the PCB manufacturer’s process capabilities and incorporate DFM (Design for Manufacturability) principles. Before actual routing, communicate with the PCB manufacturer to confirm the required materials and stack-up structure to meet specific transmission line performance requirements while achieving reasonable costs and timelines.