Currently, the ability to mass-produce high-level circuit boards is predominantly held by foreign-invested companies or a few advanced domestic manufacturers. The production of these boards requires not only cutting-edge technology and substantial equipment investment but also the expertise and accumulated experience of engineers and production personnel. Additionally, the process of obtaining customer certifications for high-level boards is rigorous and often cumbersome, contributing to a higher entry barrier for companies attempting to industrialize their production. The production cycle for these boards is also relatively long, adding to the complexity. As a result, the average number of layers in a PCB has become a key technical indicator for evaluating the capabilities and product structure of PCB manufacturers.
This article aims to outline the primary challenges encountered during the production of high-level circuit boards, as well as to highlight the critical control points in key production processes. This information serves as a reference for industry peers looking to enhance their understanding and improve their production capabilities in this advanced segment of the PCB market.
**1. Main Production Challenges of High-Density PCBs**
High-density PCBs (Printed Circuit Boards) differ significantly from traditional circuit boards in terms of their material properties, design complexity, and production requirements. These include thicker boards, more layers, denser traces and vias, larger cell sizes, and thinner dielectric layers. Additionally, the inner-layer space, layer alignment, impedance control, and overall reliability expectations are much more stringent.
### 1.1 Alignment Challenges Between Layers
The alignment between layers is crucial for high-density boards. Customers have increasingly stringent requirements for layer-to-layer alignment, often requiring tolerances of ±75μm. Factors such as the large scale of high-density designs, temperature and humidity fluctuations in the graphics transfer workshop, and expansion-contraction differences between various core layers complicate this task. Misalignment and layer offset can occur due to inconsistent interlayer positioning methods, which make it more difficult to control alignment accuracy during production.
### 1.2 Challenges in Inner Circuit Fabrication
The use of special materials, such as high-TG (glass transition temperature) laminates, high-speed, high-frequency materials, and thick copper layers, imposes greater demands on inner circuit fabrication. These materials require precise control over pattern sizes, impedance integrity, and signal transmission quality. Fine line widths and small spacings are common in high-density boards, increasing the likelihood of open circuits, shorts, and other defects. Additionally, the thinner inner core boards are prone to warping, poor exposure, and etching issues. This can lead to inconsistent production yields and higher scrap rates, especially since high-density PCBs are often large system boards.
### 1.3 Pressing Process Difficulties
The pressing process for multi-layer boards involves stacking multiple inner core boards and prepregs. This process is prone to defects such as delamination, resin cavities, and air bubbles. The key to successful pressing is the careful design of the lamination structure, considering factors like material heat resistance, voltage withstand capability, adhesive volume, and dielectric layer thickness. During the pressing phase, it is essential to manage expansion and contraction uniformly across layers. Any inconsistency in the material’s thermal behavior or interlayer insulation can lead to failures in interlayer reliability testing.
### 1.4 Drilling Challenges
Drilling in high-density boards, especially those made from high-TG, high-frequency, thick copper materials, poses several challenges. The difficulty arises from issues such as rough drilling, burr formation, and drill breakage. With numerous layers and thicker copper, the risk of tool breakage is higher. The small via diameters and narrow hole wall spacing can also exacerbate issues like CAF (Conductive Anodic Filament) failure. Additionally, drilling through thick boards increases the likelihood of angled holes or inconsistent hole depths, which impacts the overall integrity of the PCB.
**2. Key Production Process Controls**
To address the challenges mentioned above, several key production processes must be carefully controlled and optimized. These processes include material selection, laminate structure design, layer alignment, inner circuit technology, pressing, and drilling.
### 2.1 Material Selection
As electronic components evolve to demand higher performance and multifunctionality, the signal transmission requirements for PCBs have become more stringent. This requires materials with low dielectric constants, low dielectric loss, low CTE (Coefficient of Thermal Expansion), low water absorption, and high performance. Selecting high-quality copper-clad laminates and prepregs is critical to meet the requirements for high-frequency, high-speed signal transmission and overall PCB reliability.
### 2.2 Lamination Structure Design
The lamination structure must consider several factors, including heat resistance, voltage withstand capabilities, filler amounts, and dielectric layer thickness. The design should follow these key principles:
– **Prepreg and core board consistency**: To ensure PCB reliability, prepregs and core boards should come from the same manufacturer. Avoid using a single prepreg type for all layers (unless specified by the customer). The interlayer dielectric thickness should meet the minimum requirement of 0.09mm per IPC-A-600G standards.
– **High TG materials**: When the customer requests high TG materials, both the core boards and prepregs should be chosen accordingly.
– **High resin content prepregs**: For inner substrates with 3OZ copper or greater, use high-resin-content prepregs like 1080R/C65%, 1080HR/C68%, 106R/C73%, and 106HR/C76%. Avoid excessive use of 106 prepregs as the glass fibers are too thin, potentially causing dimensional instability and delamination during lamination.
### 2.3 Layer Alignment Control
Accurate layer alignment requires compensating for the expansion and shrinkage of each layer. This can be achieved through historical data analysis and experience. Precise interlayer positioning methods should be chosen, such as four-slot positioning (Pin LAM), hot melt, or rivet combinations, to ensure consistency. Additionally, setting an appropriate pressing process and performing regular maintenance of pressing machines are crucial for controlling glue flow, ensuring proper cooling, and minimizing interlayer misalignment. Comprehensive consideration of inner layer compensation, interlayer positioning methods, and pressing parameters is essential for layer alignment.
### 2.4 Inner Circuit Technology
For optimal inner circuit fabrication, the design should consider proper compensation for line widths, pads, and solder rings. Special compensation is required for specific features, such as return lines and isolated tracks. Impedance and inductive reactance should be carefully controlled, especially for high-speed signals. Ensuring that the design compensations for these parameters are correct can prevent issues during production. During the etching process, it’s important to manage the composition of etching solutions to maintain uniformity and minimize sidewall corrosion. Upgrading equipment or introducing high-precision etching lines can further improve etching consistency and reduce defects such as burrs and incomplete etching.
**3. Conclusion**
The research literature on high-density PCB processing technology remains relatively scarce, making this article a valuable reference for the industry. By addressing the key production challenges, such as alignment, inner circuit fabrication, pressing, and drilling, and by focusing on critical process controls, we can improve the manufacturing process of high-density PCBs. Continued technical research and communication among industry peers are essential for advancing the capabilities of high-density PCB technology.
