Exploring the Layers of a PCB: Composition and Structure Unveiled

A PCB is somewhat like a layered cake, as it consists of alternating layers of different materials that are heat-bonded together to form a single entity.

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1. FR4

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The substrate material is typically made of fiberglass, often referred to as "FR4." This solid core provides rigidity and thickness to the PCB. Flexible PCBs are also made using high-temperature plastic materials like Kapton or equivalent materials.

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Less expensive PCBs and perforated boards (as shown above) are made from other materials such as epoxy resin or phenolic plastic. These lack the durability of FR4 but are much more cost-effective. You may often detect a distinct odor while soldering, and these types of substrates are commonly used in low-end consumer electronics. Phenolic resin has a lower thermal decomposition temperature, and when the soldering iron is left on the circuit board for too long, it can result in delamination, smoking, and charring.

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2. Copper Layers

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The next layer consists of thin copper foils, which are laminated onto the circuit board?using heat and adhesive. In standard double-sided PCBs, copper is applied to both sides of the substrate. In lower-cost electronics, the PCB may have copper on only one side. When we refer to double-sided or 2-layer boards, we are specifying the number of copper layers (2) in the stackup. This can range from as few as 1 layer to as many as 16 layers or more.

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The thickness of the copper can vary and is specified in ounces per square foot. The vast majority of PCBs have 1-ounce copper per square foot, but some PCBs used for high-power applications may use 2 or 3 ounces of copper. Each ounce per square foot is roughly equivalent to about 35 micrometers or 1.4 thousandths of an inch of copper thickness.

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3. Solder Mask Layer

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The top layer of the copper foil is known as the solder mask layer, often giving the PCB its characteristic green color (though other colors are available). The solder mask layer covers the copper traces to prevent unintended contact with other metals, solder, or conductive surfaces. This layer helps guide users in soldering to the correct locations and prevents solder bridges.

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In the example below, the green solder mask layer is applied to most of the PCB, covering the fine traces but leaving exposed silver rings and SMD solder pads for soldering.

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The most common color for the solder mask is green, but many other colors are available, including blue, red, and white.

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4. Silkscreen Layer

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A white silkscreen layer is applied on top of the solder mask layer. Silk screen printing adds letters, numbers, and symbols to the PCB to aid in assembly and provide indications for better understanding of the circuit board. Silk screen labels are often used to indicate the function of each pin or LED.

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White is the most common color for silkscreen, but any ink color can be used. Black, gray, red, and even yellow silkscreen colors are widely used but are generally less common, especially on PCBs with multiple colors.