Multi-Layer PCB Production Process: From Start to Finish

Multi-Layer PCBs: An Overview

Multi-layer PCBs are designed to increase the available area for routing traces by adding more single- or double-sided boards. In simpler terms, a four-layer or six-layer PCB is made by alternating layers of inner and outer printed circuit boards, using a system of positioning and insulating adhesive materials to bond these layers together. For example, a four-layer PCB might use two single-sided layers for the outer layers, with a double-sided board serving as the inner layers. These boards are interconnected according to design requirements, making multi-layer PCBs a more complex but highly effective solution for compact designs.

The number of layers in a multi-layer PCB does not necessarily indicate the number of independent routing layers. Special “void” layers may be added to control board thickness. Typically, the number of layers is even, with the two outermost layers always included. Most motherboards are designed with four to eight layers, although technically, PCBs can be manufactured with nearly 100 layers. High-end systems, such as supercomputers, used to employ multi-layer boards, but with modern cluster computing, such excessive multi-layer boards are becoming less common. Since the layers in a PCB are tightly bonded, it’s often difficult to identify the exact number of layers just by looking at the board. However, with a careful inspection, you can sometimes spot the layers in more complex designs.

Now, let’s dive into the production process of multi-layer boards, focusing primarily on the treatment of the outer layers.

1. Brown Oxidation (Browning Process)

The first step in the production of multi-layer PCBs is the brown oxidation process. This step is crucial for increasing the roughness of the copper surface, enhancing its adhesion to the prepreg material used during the lamination process. The copper areas are treated to ensure they bond properly with the insulating materials. For the areas that are not copper-plated, such as the etched regions, the substrate’s surface is naturally rough from the previous etching process. At our factory, the brown oxidation line not only roughens the inner layers but also thoroughly cleans the surface to remove any dirt or contaminants.

2. Lamination

Next comes the lamination process, where materials like copper foil, prepreg (PP), and the inner PCB layers are stacked together. For example, in the case of a four-layer board, prepreg layers are placed above and below the inner layer, followed by copper foil on both sides of the prepreg. The copper foil acts as the outer layers of the multi-layer PCB.

In a vacuum lamination press, heat and pressure cause the prepreg to melt and bond the copper foil to the inner layers. This process also ensures the inner layers and copper foil act as electrical insulators. After cooling, the prepreg solidifies, firmly holding the layers together, resulting in the completed four-layer board.

Before lamination, the PCB layers are stacked together in a clean room with carefully controlled temperature and humidity to prevent defects like bubbling, delamination, or warping. It’s crucial to avoid moisture absorption during this step, so sometimes the boards undergo baking before lamination. Once stacked, the boards should be laminated as soon as possible to prevent moisture reabsorption and oxidation of the inner copper surfaces.

During the lamination process, kraft paper is used. This material ensures that the pressure during lamination is evenly distributed across all layers. Additionally, it serves as a thermal buffer to manage temperature increases during the lamination process. The kraft paper also helps increase friction between layers, preventing slippage during lamination.

The temperature and time settings are critical, as they must match the Tg (glass transition temperature) of the B-stage resin. B-stage resins with different Tg values require specific heating times and temperatures to ensure that the resin bonds well with the copper foil and inner layers without forming gaps. If the B-stage is overheated, it may result in insulation defects and other issues that lead to scrapping.

3. Drilling

The next step is drilling the PCB. Most holes, including NPTH (Non-Plated Through-Hole) and PTH (Plated Through-Hole) holes, are drilled after lamination. Drilling the PTH holes requires compensating for the diameter to account for shrinkage during copper plating and electroplating. For larger holes, a relief hole is drilled first.

The holes serve two main purposes: they connect different PCB layers and provide points for component insertion and positioning. The type of substrate material affects how well holes are drilled. For example, thicker glass fibers make it harder to drill clean holes, as they increase the likelihood of misalignment and rough hole walls. Materials with a higher Tg also affect drilling, with tougher, high-Tg boards causing more wear on the drill bits and rougher hole walls.

Drill bits are typically made of tungsten carbide, a hard and brittle material, and the quality of these bits directly impacts the hole quality. After use, the drill bits are inspected and sharpened under a microscope to make sure they can be reused effectively. Smaller drill bits typically wear out faster than larger ones due to the higher stress during drilling. As a result, drill bits for smaller diameters cannot be reused as much as those for larger diameters.

During drilling, the PCB layers are stacked together with an aluminum sheet placed on top. This aluminum sheet has several functions: it centers the drill bit, prevents the drill bit from overheating, avoids burrs around the hole edges, and ensures that no copper traces are damaged. The board is also supported by a paper layer at the bottom to prevent damage to the surface.

4. Deburring

After drilling, there are often burrs left on the edges of the holes, particularly when the drilling is done with machines. These burrs need to be removed before copper plating and dry film lamination to ensure smooth processes.

5. Cleaning

Because the copper foil on the inner layers of the PCB is very thin, it’s easily contaminated by residues from the drilling process. Even small amounts of debris can affect the copper’s ability to bond during the copper plating process, which would result in poor electrical conductivity in the holes. To prevent this, the boards are cleaned thoroughly to ensure that no residue remains on the copper.

Conclusion

The production of multi-layer PCBs is a complex and precise process that requires attention to detail at every stage. From brown oxidation to lamination, drilling, deburring, and cleaning, each step plays a crucial role in ensuring that the final product meets the required standards for performance and reliability. Whether you’re making a simple four-layer board or a highly complex multi-layer design, understanding each phase of the process is essential for creating high-quality PCBs.