In today’s fast-paced electronics industry, high-multilayer PCBs (Printed Circuit Boards) have become a critical component in the design and manufacturing of complex electronic devices. From smartphones to sophisticated aerospace systems, high-multilayer PCBs are indispensable for handling advanced functionalities while maintaining performance and compactness.
This guide is designed to give engineers a comprehensive understanding of some of the key process steps in High-Multilayer PCB manufacturing, helping engineers gain a deeper understanding of this technical field.
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.
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)
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.
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, KING SUN PCB uses high-quality materials from leading manufacturers. For 4-layer and 6-layer boards, KING SUN 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, KING SUN 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.
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. KING SUN’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.
Conclusion
By carefully considering these factors, engineers can ensure that high-multilayer PCBs meet the rigorous demands of modern electronic systems, while also maintaining cost-effectiveness and manufacturability.
This ultimate guide to high-multilayer PCB manufacturing provides a strong foundation for engineers looking to excel in designing and producing these advanced boards.