For readers new to printed circuit board (PCB), the initial complexity can be intimidating. However, by starting with the fundamental concepts and gradually delving into the finer details, you can navigate the learning process more effectively. In this article, TechSparks will introduce you to a core component of PCB: PCB substrate. Through conceptual explanations and comparisons of various foundational materials, we aim to help you better understand PCB and successfully complete your projects.
What is a PCB Substrate and How is it Manufactured
The most common explanation we hear about printed circuit board is that they are like an “electronic sandwich.” They are constructed by stacking different layers together through a lamination process, as illustrated in the typical example of a 4-layer PCB structure shown below.
In this structure, copper layers serve as the routing layers, allowing current to flow and connecting various electronic components. The innermost part is made of pre-preg, which is a prepreg material serving as an adhesive between the core (copper foil + substrate + copper foil) and the core. The substrate layers are typically made of insulating materials, acting as the circuit’s insulating layers, always positioned between the two copper layers, providing insulation and structural support.
The PCB manufacturing process typically starts with the inner layers. Manufacturers choose the appropriate substrate material based on design requirements and cut it to the required size. Then, they thoroughly clean the substrate to ensure that the surface is free of dirt or oil residues. Next, copper foils are applied to both sides of the substrate. This process is repeated as needed to build a multilayer circuit board.
Common PCB Substrate Materials
In modern electronics, a one-size-fits-all approach to design doesn’t apply. Users need to select the right PCB substrate material according to their requirements. Here, TechSparks introduces several of the most common PCB substrate materials types:
First, it’s essential to clarify that FR-4 is not a traditional material name but stands for Flame Retardant-4. It typically refers to a type of substrate material that can self-extinguish in flames, usually composed of a combination of fiberglass cloth and epoxy resin. FR-4 substrate is the most prevalent substrate material due to its high process compatibility, ease of processing, and cost-effectiveness. Notably, FR-4 exhibits high process compatibility, allowing manufacturers to alter material compositions to achieve high Tg values based on specific requirements.
Ceramic substrate is typically made from materials like aluminum oxide (alumina) or aluminum nitride ceramics. Due to factors such as raw material cost and fabrication complexity, ceramic PCB is relatively expensive and is not commonly used in consumer electronics. It finds application in industrial, communication, and military sectors. Ceramic PCB is considered worthwhile in high-end applications where circuit boards face more stringent challenges. It excels in high-temperature environments, aiding in thermal dissipation, offering low loss, and stability, meeting the demands of high-frequency and microwave circuits. Ceramic substrate also exhibits high chemical stability, protecting against chemical corrosion threats.
Polyimide is a high-performance polymer material widely used to manufacture flexible PCB, meeting the demands of complex three-dimensional spatial requirements. This material is typically composed of polyimide monomers, featuring a chemically stable imide group that maintains excellent performance at elevated temperatures.
Polyimide substrate encompasses four generations: low-flow polyimide, second-generation pure polyimide, third-generation polyimide, and filled polyimide. Despite the addition of extra materials in their original formulations, they are fundamentally all polyimide. This substrate is known for its reliability and suitability for flexible circuits.
Metal Core Substrates
Metal core substrate uses metals like aluminum and copper. Its primary advantage lies in excellent thermal performance, making it ideal for high-power applications, aiding efficient heat dissipation. Compared to traditional substrates, metal materials typically have thermal conductivity at least ten times higher, quickly transmitting and dispersing heat generated by the circuit to the surrounding environment, preventing hotspots. Moreover, metal core PCB exhibits outstanding dimensional stability, reducing the likelihood of warping or delamination issues, ensuring circuit reliability. However, it’s important to note that constructing multi-layered boards with metal materials is challenging and is usually limited to ten layers or less, even for professional manufacturers.
Key Substrate Material Properties
The primary concern when selecting a substrate material is its electrical properties, which directly affect the circuit’s performance and behavior. Key electrical properties include the dielectric constant and dielectric loss.
- Dielectric Constant: Describes the capacitive properties of a dielectric or material relative to a vacuum. When an electric field is applied to the dielectric, electrons separate, increasing the material’s ability to store charge. A lower dielectric constant generally indicates better insulation performance, reducing signal interference and the capacitance of circuit interconnects.
- Dielectric Loss: Describes the phenomenon of energy loss in a dielectric or material during the propagation of electromagnetic waves due to molecular friction, polarization loss, and conduction loss. A lower dielectric loss suggests that the material experiences less energy loss during electromagnetic wave transmission. This is particularly important in high-frequency electronic devices to ensure accurate signal transmission and stable performance.
The thermal performance of substrate material significantly impacts both PCB assembly and application.
- Assembly: High-temperature soldering is a critical step in assembling bare boards into a finished PCBA board. High-temperature soldering involves applying molten solder paste to the PCB and then cooling it to form secure solder joints. The selected material must be compatible to ensure that high-temperature soldering does not damage the PCB.
- Application: During PCB operation, current flows through traces and components, generating heat. Additionally, the normal operation of components releases heat. If this heat cannot be effectively dissipated, it can affect the circuit’s performance.
So, three key aspects of thermal performance need attention:
- Thermal Conductivity: Reflects the material’s ability to transfer heat, measured as the rate of heat transfer per unit area over a temperature gradient in a unit of time. High thermal conductivity indicates more efficient heat transfer and dissipation.
- Coefficient of Thermal Expansion: Describes the material’s change in dimension, whether length, area, or volume, in response to temperature fluctuations. When selecting materials, matching the CTE of the substrate and components is crucial, as mismatch can lead to thermal stress, mechanical stress, or soldering issues.
- Glass Transition Temperature: The temperature at which a material transitions from a solid glassy state to a high-temperature liquid state. For PCB substrates, Tg is typically above 100°C and is considered in conjunction with the soldering process.
Evaluating a substrate’s ability to maintain its characteristics during manufacturing and application is crucial. Some key aspects of mechanical performance include:
- Peel Strength: Describes the adhesive strength between layers or between components and the PCB. It represents the external force required to separate these components. High peel strength ensures strong adhesion between PCB layers and components, preventing them from becoming loose or detached during use.
- Flexural Performance: Measures a material’s bending capability and resistance to bending. For flexible PCB, dynamic and static bending capabilities are discussed, depending on the application. Rigid PCBs discuss flexural resistance, ensuring that the material maintains its performance under external forces.
- Impact Resistance: Evaluates a material’s performance when subjected to impact or shock loads. This is particularly important in applications like automotive PCB, where the operating environment includes vibrations, necessitating specialized testing after manufacturing.
Comparison of Substrate Material Properties
|3.8 – 4.8
|6 – 10
|3.2 – 3.8
|4.0 – 5.4
|0.25 – 0.40 W/(m·K)
|24 – 180 W/(m·K)
|0.2 – 0.4 W/(m·K)
|1 – 9 W/(m·K)
|0.25 – 0.35 W/(m·K)
|12 – 18 ppm/°C
|5 – 10 ppm/°C
|20 – 70 ppm/°C
|14 – 18 ppm/°C
|130 – 210°C
|250 – 350°C
|120 – 130°C
|RF, high frequency, microwave circuits
|Available in either rigid or flexible configurations
|High power applications
|Low-end low-cost electronics
|0.1 – 0.2%
|0.4 – 0.5%
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