Multi-Layer PCB
Multilayer PCBs are crucial in modern electronics, allowing for more complex and compact circuit designs. They consist of multiple layers of conductive copper traces and insulating substrate material.
Multilayer board parameters:
Number of Layers: Multilayer PCBs can have three or more layers. Layer count is critical and indicates how many individual copper and substrate layers are used in the PCB. Standard multi-tier options include 4, 6, 8 and even higher tiers.
Board Thickness: The overall thickness of a multilayer PCB depends on the number of layers and the thickness of the core and prepreg materials used. Typical thickness range is 0.8 mm to 3.2 mm.
Copper Weight: The inner and outer layers of a multilayer PCB can have different copper weights. Standard copper weights are 1 oz (35μm) for the inner layer and 1 oz or 2 oz for the outer layer.
Materials: Multilayer PCBs use a combination of core and prepreg materials. FR-4 has become a commonly used substrate material due to its good electrical and thermal properties. Materials such as Rogers or Teflon can be used for high frequency applications.
Trace Width and Spacing: The width and spacing of copper traces determine the PCB’s ability to handle high-speed and high-frequency signals. These parameters are critical for impedance control.
Finished Copper Thickness: Final copper thickness is critical to PCB performance and durability. It is determined by the initial copper thickness and the amount of copper remaining after the manufacturing process.
Size: Double-sided PCBs come in a variety of sizes and shapes and can be tailored to the specific requirements of the device in which they are used. Standard sizes (such as 4×6 inches) are typical, but custom sizes are available for unique applications.
Copper Thickness: The thickness of the copper layer on a double-sided PCB is measured in ounces per square foot (oz/ft²). Standard thicknesses include 1 oz and 2 oz, with thicker copper providing better current carrying capabilities.
Hole Size: Double-sided PCBs have holes for component leads or connectors. The accuracy and size of these holes are critical to ensuring proper component alignment and soldering.
Solder mask and silk screen: Solder mask is a protective layer applied to the PCB to prevent solder bridges and corrosion. A silkscreen layer is used for component markings and indicators.
Manufacturing Process of Multi-Layer PCBs:
Design and Layout: The PCB design process involves creating a circuit layout, specifying layer stack-up, determining trace widths and spacing, and adding necessary components. PCB design software is used for this purpose.
Layer Stacking: A multilayer PCB consists of alternating layers of core material and prepreg, with copper foil bonded to each layer. Stacking arrangements are designed to meet electrical and mechanical requirements.
Inner layer processing: Use a photoresist mask and chemical etching to etch the copper layer to create the desired circuit pattern. Then, a thin layer of oxide is applied to prevent the copper from oxidizing.
Lamination: The core, prepreg and copper foil are laminated under heat and pressure to form a single multi-layer structure. This is usually done in a hydraulic press.
Drilling: Precision CNC machines drill holes for through-holes and through-hole components. Laser drilling is used to create micro-holes in high-density boards.
Copper plating: Electroplating chemicals add more copper to the inner layer, ensuring the desired copper thickness.
Circuit Imaging: Using a photoresist and UV exposure process to transfer the PCB design to the outer layers. Use chemical etching to remove unwanted copper.
Solder mask and screen printing: Solder mask is used to insulate and protect copper traces. Silkscreen layers add component labels and reference indicators.
Surface treatment: Surface treatment prevents copper from oxidizing and facilitates soldering. Standard surface treatments include HASL (Hot Air Solder Leveling), ENIG (Electroless Nickel Immersion Gold) and OSP (Organic Solderability Preservative).
Testing: The PCB undergoes various tests, including electrical testing, to ensure that it meets design specifications.
Assembly: Use a placement machine to assemble the components onto the PCB, then reflow the board to connect the pieces.
Final Inspection: The final inspection checks for defects and verifies the proper functionality of the assembled PCB.
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