PCB layering refers to the arrangement of copper and dielectric layers in a printed circuit board (PCB).
When creating PCB layers, it is crucial to specify:
Number of layers: Determines the overall structure of the board.
Thickness of individual layers: Ensures the required mechanical strength and electrical properties.
Amount of copper: Affects current resistance and thermal characteristics.
Type of prepregs used: Pre-impregnated materials define dielectric properties and adhesion between layers.
Total board thickness: Important for mechanical integration into devices.
A well-designed PCB Layering contributes to lower electromagnetic interference (EMI), better signal integrity, and cost efficiency.
What Are the Different PCB Layers?
Copper Foil
In PCB layering are Copper layers used to create circuit designs such as traces, pads, and copper pours. This metal offers excellent electrical conductivity and facilitates the etching process. The thickness of copper foil varies depending on specific design requirements and is usually measured in ounces.
For example, if we flatten 1 ounce (28.35 grams) of copper to cover an area of 1 square foot (0.093 square meters), the resulting thickness will be 1.4 mils. Therefore, the weight of copper is expressed in ounces per square foot (oz/ft²).
The table below shows the relationship between copper weight, copper thickness, and the equivalent unit length in mils.
Units of Measurement
Ounce (oz) is a unit of weight used in the imperial measurement system, especially in the U.S. and some other countries. One ounce equals approximately 28.35 grams. In PCBs, "ounce" is often used to express the weight of copper per square foot (oz/ft²), indicating the amount of copper used in a specific PCB area. For instance, 1 ounce of copper per square foot corresponds to a copper layer thickness of about 1.4 mils (mil = 1/1000 inch).
Inch (in) is a unit of length primarily used in the imperial system. One inch equals 2.54 cm. This system is commonly used in the U.S., Canada, and some other countries, while most of the world uses the metric system. The inch is frequently divided into smaller units, such as "mil" (1 mil = 1/1000 inch) or "foot" (1 foot = 12 inches). Inches are commonly used to measure heights, widths, object dimensions, or distances, particularly in technical, construction, and engineering applications.
Mils is a unit of length used in the imperial system, mainly in electronics and engineering. 1 mil = 0.001 inches = 0.0254 mm In PCBs, mils are used to express copper thickness or conductor width. For example, copper with a thickness of 1 oz/ft² is approximately 1.4 mils (0.035 mm).
The Impact of Copper Foil on PCB Functionality
The copper layer creates a conductive path for signal transmission across the board. Due to its high conductivity, copper is an ideal material for this purpose.
It also forms strong interconnections between layers, ensuring smooth and efficient electrical signal transmission.
The presence of copper enhances power supply efficiency, leading to more reliable and stable operation.
It effectively reduces ground impedance, and voltage drop decreases noise levels.
Electrolytic copper is a type of copper applied to a PCB through an electrochemical process. This involves immersing the board in an electrolyte containing copper. When an electric current is applied, copper deposits onto the board's surface, forming a thin layer with a vertically deposited grain structure.
This type of copper offers several advantages:
High Precision: Electro-deposition allows for highly controlled copper application, essential for fine and precise PCB circuit patterns.
Flexibility: Suitable for boards with varying copper thickness and distribution requirements.
Cost-Effectiveness: The electro-deposition process is relatively affordable, making it a popular choice for mass PCB production.
Electro-deposited copper is usually applied to internal PCB layers with intricate structures but can also be used for outer layers. This process ensures uniform and firm copper deposition on the desired board areas.
Annealed Copper
Annealed copper undergoes a heat treatment process called annealing, where it is heated to a high temperature, thinned by rolling, and slowly cooled. This results in a horizontal grain structure and a smoother surface, making it ideal for flexible PCBs.
The annealing process impacts copper properties in several ways:
Increased Flexibility: Annealing refines the copper structure, making it more flexible and easier to shape—crucial for flexible PCBs that need to bend and conform to various forms.
Reduced Hardness: Annealing decreases copper hardness, improving its ability to bend or shape without cracking.
Improved Conductivity: Annealed copper may exhibit better electrical conductivity, as the process eliminates internal stresses and defects in the material.
Annealed copper is typically used in applications requiring high mechanical flexibility and extended durability under dynamic conditions
Choosing the Right Copper Foil
The choice of copper foil depends mainly on the required copper thickness, purity, and interface profile between copper and dielectric material.
Copper Thickness: Common thicknesses range from 0.25 oz (0.3 mils) to 5 oz (7 mils). The exact thickness varies by application. For high-power PCBs, a greater copper area is needed to support the required current.
Copper Purity: This refers to the percentage of copper in the foil. Typically, copper foil for electronics has a purity of around 99.7%.
Copper Profile: Low-profile (smooth) copper exhibits lower signal losses at high frequencies due to reduced "skin effect."
Rough Copper Surface (Cu)
When a signal travels through a rough copper layer, signal loss increases with frequency. The higher the frequency of a signal passing through rough copper, the greater the signal path length, resulting in higher signal loss. Therefore, low-profile copper is recommended for high-frequency applications, as it improves signal transmission and reduces losses.
0–3 GHz: Low frequency = short signal transmission path = lower signal loss.
3–7 GHz: High frequency = long signal transmission path = higher signal loss.
10 GHz: High frequency = long signal transmission path. However, copper does not exhibit as deep a skin effect as in the previous case, resulting in less signal attenuation. The signal tends to follow the surface roughness of the copper.
Smooth Copper Surface (Cu)
A rough copper surface causes higher signal losses, especially at high frequencies, because the "skin effect" increases resistance to signal propagation. Conversely, smoother copper enables more efficient signal transmission, which is crucial for high-frequency applications.
Copper Balance in PCB Layering
Proper copper distribution on a PCB is essential for mechanical stability and manufacturing quality. Uneven copper distribution can lead to warping, delamination, or plating issues, negatively affecting the board's functionality and reliability.
PCB Warping: Asymmetrical copper areas can cause board warping due to thermal stress during processes like lamination, soldering, or component assembly. Even copper distribution helps minimize these deformations and ensures mechanical stability.
PCB Delamination: This issue occurs mainly in boards with thicker copper (50 µm and above) and unbalanced copper distribution. Areas with minimal copper coverage may allow excessive prepreg penetration, leading to delamination or interlayer shorts. Adding "dummy" copper areas helps distribute material evenly and stabilize layer connections.
Plating Issues
For proper electroplating, conductive layers must be evenly covered with copper. Uneven distribution can lead to over-etching of thin traces, causing damage or under-etching. This is particularly critical for differential pairs requiring precise impedance control. Balancing copper with additional areas stabilizes the plating process and ensures optimal results.
Copper selection plays a vital role in PCB construction due to its electrical conductivity, thermal performance, and mechanical stability.
And next time, we will focus on selecting the appropriate prepregs!
