Selecting the Best PCB Substrate for Enhanced Performance

In the ever-evolving world of electronic manufacturing, the selection of the right Printed Circuit Board (PCB) substrate material is of paramount importance. It plays a pivotal role in defining the fundamental attributes and performance of PCBs. To improve the functionality and efficiency of your circuit boards, optimizing the substrate material is the first and most crucial step. In recent years, numerous innovative substrate materials have emerged, aligning with new technologies and market trends.

The landscape of the printed circuit board market has witnessed a profound shift in focus, transitioning from traditional hardware products like desktop PCs to the realm of wireless communication, servers, and mobile terminals. Mobile communication devices, exemplified by smartphones, have driven PCB technology towards achieving high-density designs, reduced weight, and multifaceted functionalities. It’s imperative to recognize that the performance of PCBs is inextricably linked to the selection of the appropriate substrate material. As such, substrate material selection plays a pivotal role in shaping the quality and reliability of both PCBs and the end-products they are intended to serve.

Meeting the Demands of High Density and Fine Lines

• Copper Foil Requirements

  The pursuit of higher density and finer lines, particularly in the case of High-Density Interconnect PCBs (HDI PCBs), necessitates specific considerations. A decade ago, HDI PCBs were defined as having line widths (L) and line spacings (S) of 0.1mm or less, according to IPC standards. Today, these dimensions have shrunk significantly, with L and S values reaching as low as 60μm and, in advanced scenarios, even 40μm.

  Traditionally, circuit patterns were formed through imaging and etching processes, achieving a minimum L and S value of 30μm using thin copper foil substrates with thickness ranging from 9μm to 12μm. However, due to the challenges associated with thin copper foil Copper Clad Laminate (CCL), many PCB manufacturers now prefer an etching-minus-copper-foil approach, where the copper foil thickness is increased to 18μm. Despite its use, this method is not recommended as it involves numerous complex procedures, challenges in thickness control, and increased costs. Consequently, ultra-thin copper foil with a copper thickness ranging from 3μm to 5μm is considered a superior alternative.

• Low Roughness Copper Foil

  Achieving low roughness on the copper foil surface is imperative. This facilitates improved bonding between the copper foil and the substrate material, ensuring the peel strength of conductors. To achieve optimal results, it’s essential to reduce the surface roughness of copper foil to less than 3μm, or even as low as 1.5μm.

• Insulating Dielectric Laminates

  High-Density Interconnect PCBs heavily rely on the Building Up Process. While Resin Coated Copper (RCC) and prepreg epoxy glass cloth combined with copper foil lamination have traditionally been used for fine circuitry, emerging techniques such as Semi-Additive Process (SAP) and Modified Semi-Additive Process (MSPA) are gaining prominence. These methods involve insulating dielectric film lamination with chemical copper plating to create copper conducting planes, enabling the production of fine circuits.

  The choice of laminating dielectric material is critical. It must possess the requisite dielectric performance, insulation properties, heat resistance, and bonding characteristics compatible with HDI PCB technology.

Meeting the Requirements of High Frequency and High Speed

The progression from wired to wireless communication technology and the transition from low-frequency, low-speed transmission to high-frequency, high-speed transmission represent significant developments. The shift from 4G to 5G technology in smartphones underscores the demand for faster data transmission and increased data capacity.

To meet the requirements of high-frequency and high-speed transmission, the selection of high-performance materials is essential. A primary consideration is the dielectric constant (Dk) and dielectric loss (Df) of the substrate material. Substrate materials with a Dk below 4 and Df below 0.010 are classified as middle Dk/Df laminate boards. For even higher performance, low Dk/Df laminate boards with a Dk below 3.7 and Df below 0.005 are preferred.

Several types of substrate materials are available for high-frequency circuit boards, including fluorine-series resin (e.g., PTFE), PPO or PPE resin, and modified epoxy resin. PTFE, known for its excellent dielectric properties, is suitable for products operating at frequencies of 5GHz or higher. In contrast, modified epoxy FR-4 or PPO substrates are suitable for frequencies ranging from 1GHz to 10GHz.

The choice among these high-frequency substrate materials involves a trade-off between cost, dielectric properties, water absorption, and frequency characteristics. Fluorine-series resin offers exceptional dielectric performance but at a higher cost. On the other hand, epoxy resin is more cost-effective but lags in dielectric performance.

In cases where products operate at frequencies exceeding 10GHz, fluorine-series resin becomes the material of choice. It’s essential to note that PTFE substrates may have downsides such as high cost, poor rigidity, and high thermal expansion coefficients. To address these challenges, inorganic materials like silicon dioxide can be used as fillers or glass cloth can be added to reinforce substrate rigidity and reduce thermal expansion.

Unique Insulating Resins and Surface Roughness of Copper

Beyond the choice of the substrate material, other factors come into play for high-frequency signal transmission. The surface roughness of copper conductors significantly affects signal transmission loss due to the Skin Effect phenomenon. The Skin Effect occurs when electromagnetic induction at high frequencies forces current to concentrate on the surface of a conductor, resulting in increased signal loss.

To minimize signal loss, copper conductor surface roughness must be controlled. At the same frequency, higher surface roughness leads to more significant signal loss. Therefore, copper foil roughness should be kept as low as possible, ideally below 1μm, especially for signals above 10GHz. Copper foil with ultra-low roughness (0.04μm) is highly beneficial. Proper oxidation treatment and adhesive resin systems are crucial to achieving the desired surface roughness.

Addressing High Heat Resistance and Dissipation Needs

As electronic devices become smaller and more powerful, they generate more heat. Effective thermal management is essential to ensure optimal device performance. Metal Core PCBs (MCPCBs) or Insulated Metal Substrate (IMS) PCBs offer excellent heat dissipation properties.

Aluminum is a cost-effective and thermally conductive material commonly used in MCPCBs. It provides excellent heat resistance and dissipation capabilities. The key to effective thermal management lies in ensuring a strong adhesion between the metal core and the circuit plane.

Selecting Substrate Materials for Specialized PCBs

The ever-increasing application of rigid PCBs and flex/rigid PCBs in various fields introduces new requirements in terms of number and performance. Different types of substrates are emerging to meet these demands.

Polyimide films, available in various forms such as transparent, white, black, and yellow, offer high heat resistance and low thermal expansion coefficients. These materials cater to different applications’ needs.

Mylar substrates, known for cost-effectiveness, exhibit characteristics such as high elasticity, dimensional stability, surface quality, photonic coupling, and environmental resistance, making them versatile choices for diverse requirements.

High-speed, high-frequency signal transmission is essential for flexible PCBs (Flex PCBs). The dielectric constant and dielectric loss of flex substrate materials must be carefully considered. Polyimide and advancing polyimide substrates, as well as substrates with inorganic additives, can be tailored to meet specific needs, such as low Dk/Df for high-speed transmission or high power conductors for applications with large currents.

Rely on Highleap Electronic for Expert Material Selection and PCB Manufacturing

Choosing the right substrate material for your PCB is a critical decision, and it requires a deep understanding of the various attributes involved. If you find yourself navigating the complexities of substrate material terminology and performance criteria, there’s a cost-effective solution available to help you make informed choices.

Highleap Electronic, a leading global provider of bare board fabrication, PCB assembly, and component sourcing services, specializes in customizing optimal PCB solutions tailored to your project’s unique requirements, budget, and performance expectations. Our experienced engineers take into account factors such as the application environment, functionality, and your budget to guide you through the substrate material selection process.

With over a decade of experience and a track record of successfully completing hundreds of thousands of PCB projects, Highleap Electronic is your trusted partner in selecting the perfect substrate material and fabricating high-performance PCBs that meet and exceed your expectations.

For PCBs with standard FR4 substrates, you can obtain an instant online quote through our website. If your project requires specialized substrate materials such as Flex PCBs, Rogers PCBs, or Aluminum-based PCBs, we encourage you to reach out to us directly for a custom quotation. Rest assured, Highleap Electronic will help you make the right substrate material choice for your project and deliver PCBs that perform at their best.