Analysis of the Costs and Values ​​of Adhesive-Free Substrates for Ultra-Thin Designs

In the process of FPC moving towards 5G, wearable devices, and foldable screens, "space" has become the most expensive resource. In the past, the mainstream adhesive-coated substrates sandwiched a layer of epoxy resin or acrylic adhesive between a PI film and a copper foil. Adhesive-coated copper is limited by the large coefficient of thermal expansion (CTE) and high dielectric loss of the adhesive, which can cause signal distortion at high frequencies. In contrast, adhesive-free copper uses chemical coating or sputtering processes to directly bond PI and copper, completely eliminating the risks of moisture absorption and delamination caused by adhesives.

1. Why should we discard that layer of "glue"?

Traditional adhesive layers (usually epoxy or acrylate) on laminated sheets, while providing good adhesion, present three main challenges in modern high-density manufacturing processes:

Limitations in Thickness, Thermal Stability, and Electrical Performance

• Increased Thickness: Adhesive layers typically range from 12.5 μm to 25 μm. In increasingly constrained electronic structures, increasing the overall board thickness directly limits the bending radius and other possibilities for fitting within the same structure.
• Poor Thermal Stability: The Tg (glass transition temperature) of the adhesive layer is much lower than that of the PI layer. During high-temperature reflow soldering, the adhesive layer softens first, easily leading to Z-axis expansion and potentially breaking vias.
• Electrical Performance Losses: The high dielectric constant (Dk) and dielectric loss (Df) of the adhesive can cause severe signal attenuation in high-frequency signal transmissions (such as 5G).

2. Manufacturing of Adhesive-Free Copper Substrate: Electroplated Copper (ED) vs. Laminated Copper (RA)

The adhesive-free substrate eliminates the weakest adhesive layer, allowing the copper foil to bond directly to the PI molecules. This is currently achieved primarily through two advanced processes, which significantly improve the heat resistance and dimensional stability of the FPC:

Mainstream manufacturing technology path

• Electroplated copper (ED): A seed layer is formed on PI through vacuum sputtering before electroplating, suitable for ultra-thin applications.

• Laminated copper (RA): PI is coated onto copper foil or directly laminated at high temperature, resulting in excellent structural stability.

3. Value Equilibrium Point

When introducing adhesive-free substrates (2L FCCL), the team had to establish a more evidence-based selection logic between long-term reliability and initial unit price.

Decision-making considerations from a business and technology perspective

• Procurement Perspective: Although the unit price is slightly higher than that of adhesive-based substrates, it improves process yield (reducing bubbling and open circuits) and provides more stable performance in subsequent SMT processes; 4. Using adhesive-free substrates for products subject to frequent dynamic bending effectively reduces long-term warranty and maintenance costs.

• Process Perspective: Adhesive-free structures allow for smaller bending radii (R-angles), and the reliability of plated through-holes (PTHs) is significantly improved. In high-frequency wiring, adhesive-free boards offer more precise impedance control.

4. Key Design Considerations: Performance Differences of Adhesive-Free Structures

Choosing adhesive-free substrates is the core of building a robust supply chain, as their microstructure determines dimensional stability and signal quality at high temperatures.

Conclusion: Choosing glue-free products defines quality.

The evolution from adhesive to adhesive-free materials is not just a simplification of the structure, but also a challenge to the physical boundaries of FPC. For brands that pursue high-frequency 5G transmission and dynamic folding for long lifespan, the investment in adhesive-free substrates will significantly translate into a competitive advantage and reputation for quality in the end market.