What Materials Are PCB Made Of and How to Choose

Printed circuit boards are integral components of modern electronic devices, and understanding their material composition and selection criteria is crucial for designers and DIY enthusiasts aiming to manufacture high-quality electronic products. However, the complex internal structure of PCB, involving considerations such as substrate materials, conductive layers, and insulation layers, can be confusing and challenging. To assist in better comprehension and execution of the manufacturing process, this article will thoroughly explore all aspects related to PCB materials.

PCB Basic Structure

Let’s start with the basic structure of a PCB! Typically, PCB are classified into single-layer, double-layer, and multi-layer types. Single-sided and double-layer have simpler circuit structures, lacking internal wiring layers. In contrast, multi-layer PCB incorporate some wiring within internal layers in addition to the top and bottom layers, enhancing surface area utilization. The following diagram shows the basic stack-up structure of a single-layer PCB, which includes the following layers:

Schematic diagram of the basic structure of a single-layer PCB

  • Substrate Layer: This is the foundational layer of the PCB, usually made of fiberglass-reinforced epoxy resin or other materials. It provides mechanical support and electrical insulation.
  • Copper Layer: A layer of copper is applied over the substrate layer to form conductive pathways. In multi-layer PCB, there are multiple copper layers.
  • Solder Mask: A protective film covering the copper layer, it prevents oxidation of the copper traces and avoids short circuits during soldering.
  • Silkscreen: This top layer is used to mark component positions, test points, and other information, facilitating component identification during manufacturing and maintenance.

PCB Board Manufacturing Materials

Substrate Material

Substrates are the most critical part of a PCB and the choice of material significantly affects the product’s performance and manufacturability. Selecting the appropriate option can enhance both short-term and long-term performance of the PCB and reduce manufacturing faults. When choosing materials, it is essential to consider thermal, electrical, mechanical, and chemical performance. The following table provides common PCB substrate materials and their parameters:

MaterialCTEDk & DfTensile StrengthMoisture Absorption
FR-414-17 ppm/°CDk: 4.5-4.9
Df: 0.02-0.03
340-500 MPa0.10-0.20%
PTFE50-200 ppm/°CDk: 2.1-2.5
Df: 0.001-0.002
10-30 MPa<0.01%
Rogers24-70 ppm/°CDk: 2.2-3.0
Df: 0.001-0.005
40-120 MPa0.01-0.10%
Aluminum Base22-24 ppm/°CDk: 4.0-4.5
Df: 0.02-0.04
150-300 MPa0%
Polyimide30-40 ppm/°CDk: 3.2-3.5
Df: 0.004-0.006
170-200 MPa1-3%

Conductive Material

In PCB design, electric current is typically transmitted through conductor paths and pads, which not only serve as channels for current but also significantly influence the quality and reliability of circuit signal transmission.

Conductor paths refer to the copper traces on a PCB, both visible on the surface and hidden within. Manufacturers start by depositing clean copper foil onto a non-conductive substrate, then use processes like exposure and etching to create defined paths for conductors. Additionally, through-holes on PCB require copper plating to establish electrical connections, often achieved via chemical or electroplating methods.

Pads, on the other hand, are crucial for connecting surface-mount components to the PCB. These pads are typically circular or square metal areas designed to form reliable solder joints with component pins using solder paste or wire. Common pad materials include tin-lead (Sn-Pb) alloys, known for their ease of processing and ability to form reliable solder connections suitable for standard soldering processes. For some high-end and specialized PCB designs, gold-plated pads may be used to enhance conductivity and oxidation resistance.

Other Material

In the manufacturing process of PCB, various materials are used to ensure the functionality and reliability of the boards, in addition to the substrate and conductive materials. For instance, adhesives are used to bond different layers of the PCB together. The choice of adhesive depends on the specific application requirements, ensuring that it can withstand high temperatures without delaminating while providing good electrical insulation. Common adhesives include epoxy resin and acrylic resin.

Furthermore, most PCB surfaces feature a green layer of ink known as solder mask. This layer protects the copper traces from oxidation and corrosion and prevents solder bridges during the soldering process. While green is the standard color for solder masks, other colors can be used as long as they provide clear visibility when paired with the silkscreen layer. Common materials for solder masks include photoimageable solder mask (PSM) and liquid photoimageable solder mask (LPSM).

For flexible PCB, traditional solder masks may not be suitable due to the need to maintain flexibility. Instead, coverlays are used in place of solder masks. Coverlays serve a similar protective function but are made from different materials and applied differently. Common coverlay materials include polyimide film and polyester film.

Considerations When Choosing Materials

Considering only cost when choosing PCB materials is a misconception. The most important aspect is to evaluate the compatibility of the materials with your specific application, and then consider the cost. For example, in household applications, extreme environments are typically not an issue, so cost-effective and stable materials like FR4 can be considered. However, for outdoor applications, environmental factors such as temperature fluctuations, humidity, and UV exposure must be taken into account when selecting materials. In such cases, PTFE might be a better choice due to its excellent weather resistance and waterproof properties.

In some situations, your PCB might need to withstand extreme conditions such as high temperatures, high pressure, or corrosive environments. When selecting PCB materials, it is crucial to ensure they can handle these conditions and perform targeted testing after production is complete.

In addition to environmental factors, electrical performance is also critical, as these PCB will be used in electronic devices, directly affecting the functionality, efficiency, and reliability of the circuits. Parameters related to the electrical performance of PCB include dielectric constant, dissipation factor, insulation resistance, breakdown voltage, and electromagnetic compatibility. Some parameters are related to the material itself, while others are design-related. Therefore, besides selecting the appropriate materials, following PCB design rules is essential for completing the circuit design.

Another often overlooked factor is manufacturability. Most PCB projects are handled by partnered factories, so ensuring these factories have the capability to process the materials you have selected is crucial. Choosing materials that are easy to process and manufacture helps improve production efficiency, reduce production time and defect rates, thereby benefiting the overall progress and cost control of the project.

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