PCB structure

A printed circuit board (PCB) is made up of several layers of conductive and insulating materials arranged together to form a compact panel.

Figure 1: A two-layer printing circuit board

Substrate

The most comon substrate is usually fiberglass, FR4. This solid core gives the PCB its rigidity and thickness.

In addition, some PCBs have special functions, the substrate may be other types, such as aluminum, polymers, RO4350B (for high frequency circuits, antennas), etc. They will be presented later.

Depending on the design requirements, the printed circuit board has different thicknesses. However, the most common thickness for PCB is 1.6 mm (0.063 inch or 63 mil).

Copper

The next layer is a thin copper layer that acts as an electrical conductor. The number of layers depends on the application and the technical requirements of the circuit. Figure 1 illustrates a two-layer printed circuit (two orange parts in Figure 1). In fact, the circuit can have 1 layer, 2 layers, 4 layers or more.

Figure 2: 20 oz-thick-copper layer :))

The copper layer thickness can be varied and It is usually determined by the volume per unit area. The most commonly used oz is ounces per square foot (1 ounce per square foot).

Unite converters are as follows:

- Volume: 1 oz = 28.35 g

- Length: 1 ft = 30.48 cm

- Area: 1 ft2 = 0.093 m2 = 930 cm2

Its specific gravity is 8.9 g / cm3

Therefore, the conventional parameters of the circuit manufacturer is 1oz, that is equivalent to a copper layer thickness of 35 μm or 1.37 mil.

For high power circuits, the thickness of the copper layer can be 2 oz or 3 oz.

Soldermask

On the copper layer is the soldermask layer, roughly translated as welding mask. This name comes from the fact that this layer covers the entire circuit except the pad parts for welding.

Figure 3: Soldermask layer

Layer’s functions are:

- Isolation between the solder pins and the surrounding circuits. This is most likely to happen with hand soldering as shown in Fig. 4, when solders are easily overflowed.

- Isolate between traces in use. For example, the piece of metal falls into the circuit; humidity and dust can cause short circuit.

- Prevent copper oxidation.

- Navigate small SMD components (capacitors, resistors) to the correct position in soldering.

Figure 4: The printed circuit is handmade

This layer is usually green, but may also have other colors such as black, red, blue, yellow, white.

Figure 5: The color of the soldermask layer

Silkscreen

Silkscreen layer is the top layer and It’s color is usually white. It is used to present the value, position of the component, symbol, or any shape or letter what the designer wants to facilitate the assembly, etc

Figure 6: The silkscreen layer

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Author Since: Jul 02, 2018

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