With the continuous development of electronic technology, the complexity of electronics is increasing. Traditional single-layer PCB have become inadequate for these advancements, leading to the emergence of multi-layer PCB. In addition to enhanced functionality, the manufacturing process undergoes significant changes.
In a single-layer PCB, the entire circuit exists on one plane, whereas a multi-layer PCB has multiple inner layers tightly bonded together in a specific arrangement. The bonding process is known as the PCB lamination process, which roughly involves four steps: stacking→pre-pressing→lamination→cooling. Although this process is mainly machine-driven, unexpected errors can still occur, leading to issues such as bubbles, misalignment, and delamination. In this article, TechSparks will delve into this process to help you gain a more accurate understanding and implementation in production.
Table of Contents
How the PCB Lamination Process is Conducted
Lamination is somewhere in the middle of the PCB manufacturing process, following the circuit pattern design and transferring onto copper foil. Before commencing lamination, it’s essential to prepare these materials and clean away any residual plating and etching solutions.
Next, stack these materials in the predetermined sequence to form a multi-layered circuit structure. The focus during this stage is ensuring sufficient contact area and adhesion strength between each layer of copper foil and insulation material to avoid delamination issues.
Place the stacked PCB into a lamination press for pre-pressing. The lamination press gradually increases temperature and applies slow pressure to remove air and moisture between layers, enhancing the bonding of inner layers.
Subsequently, when the temperature and pressure in the lamination press reach specific thresholds, which are related to the properties of the materials used and input at the initial stage, a reaction occurs between the copper foil and insulation material, creating a unified structure in the high-temperature and high-pressure environment.
Finally, due to the elevated temperature, the Pre-Preg material becomes molten, requiring a cooling phase for solidification.
PCB Laminating Materials
The choice of laminating materials for PCB is based on the functionalities required for the project. Since lamination involves a high-temperature, high-pressure process, it’s crucial to understand some key attributes of laminates to set parameters, one of which is the Glass Transition Temperature (Tg), indicating the temperature at which the material changes from a hard, brittle state to a soft, pliable one.
As a general-purpose material, its Tg value is approximately in the range of 130-140°C. For specific applications, the material composition of FR-4 may change, achieving a Tg value of up to 210°C.
CEM-1 and CEM-3
These belong to composite epoxy materials and are cost-effective choices. CEM-1 is a paper-based laminate with a phenolic resin, while CEM-3 uses woven glass fiber. The Tg values for both materials are typically in the range of 105-120°C.
BT Epoxy Resin
As a high-performance resin material, it is commonly used in advanced circuits. It serves not only as the core layer for HDI PCB, providing structural rigidity, but also as an external HDI build layer for advanced circuit boards. Its Tg value is approximately 180℃.
This is a high-performance polymer material used for constructing flexible PCB, offering excellent flame resistance, corrosion resistance, and a low coefficient of friction. The Tg value for Polyimide is around 250°C.
Types of PCB Lamination
Double-Sided PCB Lamination
Due to the lamination process aiming to bond different layers, the pressure parameters are related to the number of PCB layers. The more layers there are, the greater the pressure required. In the case of a double-sided PCB, which only has two copper layers and uses one insulation layer to isolate them, smaller pressure is sufficient for the process.
Multi-layer PCB Lamination
Multi-layer PCB boasts a sophisticated structure, aiming to achieve higher functional density and performance. This design typically involves multiple levels of copper foils and substrates. Taking a typical 4-layer PCB as an example, its structure is as follows:
- Top Layer
- Copper Layer
- Substrate Layer
- Copper Layer
- Pre-Preg (Pre-impregnated Core)
- Copper Layer
- Substrate Layer
- Copper Layer
- Bottom Layer
In this structure, Pre-Preg is used to bond different copper layers, securing them together through a high-temperature and high-pressure lamination process. While theoretically, the number of layers in a PCB can be unlimited, in practice, manufacturing high-layer-count PCB requires more advanced technology and equipment. As the layer count increases, the complexity and failure rate of manufacturing also rise accordingly. Currently, some top-tier manufacturers may achieve PCB with up to 120 layers, but for most manufacturers, around 50 layers might already be a technological limit.
Sequential PCB Lamination
In PCB, there are different types of vias, and buried vias are located inside the PCB without being exposed on the top and bottom layers. Clearly, this cannot be achieved using traditional drilling processes, hence the support provided by sequential lamination technology. In the sequential lamination process, the inner layers of the PCB are broken down into multiple subsets, each completed in a separate manufacturing process. These subsets are then combined through lamination techniques to form a complete PCB.
Issues During PCB Lamination
Incorrect lamination processes or parameter settings can lead to a range of problems affecting the quality of the circuit board. Below are explanations of these issues along with corresponding solutions:
Problem Description: Bubbles appear on the surface of the PCB, possibly caused by material moisture absorption or uneven stacking.
Solution: Ensure materials remain dry before the lamination process starts, and apply materials evenly during stacking.
Problem Description: Hollows or pores appear on the surface of the PCB, possibly due to material evaporation or the presence of moisture.
Solution: Use low-volatile lamination materials, conduct pre-pressing before lamination to remove moisture and air.
Problem Description: Insufficient bonding of internal layers after lamination, leading to delamination. This could be caused by inadequate pressure, uneven temperature, interlayer impurities, etc.
Solution: Set reasonable parameters, ensuring sufficient pressure and even temperature. Pay attention to the importance of cleaning the PCB, as even fingerprints can cause delamination.
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