Introduction
With the advancement of technology and the overall improvement in technical capabilities, an increasing number of electronic devices are moving toward thinner designs and higher integration.
Consequently, the demand for flexible copper-clad laminates has been rising year by year, while also placing higher demands on the substrate’s heat resistance, stability, and reliability.
Development of Adhesive-Free Flexible Copper-Clad Laminates
Compared to adhesive-backed flexible copper-clad laminates, two-layer adhesive-free flexible copper-clad laminates have seen rapid growth in recent years, as they consist solely of polyimide materials that offer high strength, high modulus, excellent heat resistance, and superior electrical properties.
In recent years, driven by the increasing integration of flexible copper-clad laminates and the need to enhance the integration of electronic devices, the use of adhesive-free double-sided flexible copper-clad laminates has become increasingly widespread.
Because adhesive-free double-sided flexible copper-clad laminates feature copper foil on both sides, they are better suited to meet the needs of FPCB manufacturers.
Material Challenges and Dimensional Stability Issues
Currently, in the copper-clad laminate industry, epoxy resins undergo some shrinkage after curing.
When copper foil is etched, the relative positions shift, significantly impacting subsequent PCB drilling.
This can easily lead to defects or even scrap, making dimensional stability one of the most critical performance indicators for copper-clad laminates.
Most flexible copper-clad laminates use polyimide (PI) substrates.
Commercially available polyimide films undergo bidirectional orientation or stretching during the manufacturing process, which effectively enhances their mechanical properties and dimensional stability.
However, as substrates become increasingly thinner, more diverse, and functional, their performance increasingly reflects a company’s technological capabilities.
Although commercial PI films offer good performance, competitors can readily purchase these PI films (or TPI composite films) to produce flexible copper-clad laminates, resulting in insufficient product competitiveness.
Material Innovation and Industry Development
The advantages of coated adhesive-free laminates—including formulation flexibility, customizable product performance, and lower costs—have prompted major manufacturers of flexible copper-clad laminates, such as Nippon Steel, Tai Hong, and Shin Yang, to develop their own products.
Current adhesive-free double-sided flexible copper-clad laminates incorporate both thermosetting polyimide (PI) and thermoplastic polyimide (TPI).
Thermosetting polyimide provides excellent mechanical properties, thermal performance, and dimensional stability, while thermoplastic polyimide acts as a binder, similar to an adhesive.
Currently, the mainstream structures for commercialized adhesive-free double-sided flexible copper-clad laminates are the following two:
Cu/TPI/PI/TPI/Cu and Cu/PI/TPI/PI/Cu. Although coated polyimide offers the above advantages, it places higher demands on technical personnel and process equipment.
Due to numerous influencing factors during the imidization process of coated polyimide, and the difficulty in performing operations such as orientation stretching—unlike film-forming equipment—coated polyimide is prone to a certain degree of degradation in dimensional stability, peel strength, and mechanical properties.
In this paper, we produced a relatively simple, high-performance adhesive-free double-sided flexible copper-clad laminate through a two-step coating process and evaluated its comprehensive performance.
Experimental Section
Materials
Aromatic tetracarboxylic dianhydride, aromatic diamine monomer, SEIKA Co., Ltd., Japan; N-methylpyrrolidone, N,N-dimethylacetamide, industrial grade; electrolytic copper foil, 12 μm.
Synthesis of Thermosetting and Thermoplastic Polyimide Precursors
Accurately calculate the required amounts of each component, precisely weigh the diamine monomer, and dissolve it in a mixed solvent of NMP and DMAc.
Under nitrogen protection, maintain the water bath temperature at 25°C, add the dianhydride while stirring, and stir for 6–8 hours to obtain a polyamic acid solution.
Preparation of Adhesive-Free Double-Sided Boards
The process begins by evenly coating the synthesized thermosetting polyimide precursor solution onto the rough side of the selected copper foil.
The operator controls the coating head gap to achieve the target film thickness. The board then undergoes baking in a forced-air oven at 160 °C for a specified duration.
Next, the synthesized thermoplastic polyimide precursor solution is applied using the same coating process, while the thickness of both the thermosetting and thermoplastic polyimide layers is precisely controlled.
A nitrogen-protected oven carries out heat treatment to ensure complete imidization of the polymer layers.
After curing, the resulting adhesive-free single-sided laminate is laminated with copper foil to form an adhesive-free double-sided flexible copper-clad laminate.
By adjusting the coating gap to regulate the thermoplastic polyimide thickness, the process sets its proportion within the total substrate thickness to 15%, 25%, 30%, and 35%.
Analytical Testing
1. Peel Strength (PS): Tested in accordance with IPC-TM-650 2.4.9.
2. Dimensional Stability: Tested in accordance with IPC-TM-650 2.2.4.
3. Warpage: Place an adhesive-free single-sided flexible copper-clad laminate or an etched PL film (25 cm × 25 cm) flat on a horizontal tabletop, measure the height of warpage at each of the four corners, and calculate the average.
4. Glass Transition Temperature: TA Instruments Q20DSC; heat from 30°C to 400°C at a rate of 10°C/min in a nitrogen atmosphere.
5. Coefficient of Thermal Expansion: METTLER TOLEDO TMA/SDTA840; heat the sample to 300°C; the analysis temperature range is 50–250°C.
Results and Discussion
Glass Transition Temperature
As shown in Table 1, although the TPI thickness ratios vary, they do not significantly affect the glass transition temperatures of thermosetting and thermoplastic polyimides.
The glass transition temperature of this TPI is above 280°C, which is comparable to that of TPI composite films from leading manufacturers such as Chugoku and Ube.
表1
Impact on Dimensional Stability
In flexible copper-clad laminates (FCCLs), thermosetting polyimide-based FCCLs tend to expand after the copper foil has been fully etched, whereas pure thermoplastic polyimide tends to contract.
Therefore, combining thermosetting and thermoplastic polyimide in different thicknesses can effectively address the issue of poor dimensional stability caused by excessive expansion or contraction.
表2
As shown in the data in Table 2, pure TPI shrinks significantly after etching, and this shrinkage worsens after baking.
Since the IPC-4204 standard requires shrinkage to be within 2000 ppm, TPI cannot be used alone in adhesive-free laminates.
Pure PI undergoes a certain degree of expansion after etching and baking and is also not suitable for standalone use.
However, combining thermosetting polyimide with thermoplastic polyimide can significantly improve dimensional stability, keeping overall expansion and contraction within a narrow range.
Effect on Peel Strength
Due to its thermal non-processability, thermosetting polyimide is difficult to further laminate to produce adhesive-free double-sided boards.
However, if thermoplastic polyimide is applied, adhesive-free double-sided boards can be produced by high-temperature lamination with copper foil.
Because of the asymmetric structure of the board, the peel strength differs between the two sides.
Thermoplastic polyimide contains a higher proportion of flexible monomers, resulting in greater affinity with copper foil and higher peel strength.
Thermosetting polyimide, due to its more compact molecular arrangement, exhibits significantly lower peel strength with copper foil compared to thermoplastic polyimide.
Currently, a peel strength of 0.7 N/mm in adhesive-free laminates is sufficient to meet practical requirements.
表3
As shown in Table 3, the bond strength between thermoplastic polyimide and copper foil is higher than that between thermosetting polyimide and copper foil;
However, the TPI content has no significant effect on peel strength.
Since TPI fully wets and fills the copper pores of the copper foil after melting at high temperatures, the peel strength tends to stabilize as long as the TPI layer thickness is equal to or greater than the height of the copper pores.
Impact on the Coefficient of Thermal Expansion
Both rigid copper-clad laminates (CCL) and flexible copper-clad laminates (FCCL) have stringent requirements for the coefficient of thermal expansion.
When the substrate’s coefficient of thermal expansion is greater than or less than that of the copper foil (17 ppm/°C), repeated temperature changes during use can easily cause the insulation layer to separate from the copper foil, leading to failure.
Therefore, the closer the thermal expansion coefficient of the substrate is to that of the copper foil, the longer the service life of the material will be.
As shown in Table 4, pure TPI has a higher thermal expansion coefficient because its flexible molecular chains are more prone to movement when heated.
Due to the high rigidity of the monomers used in thermosetting polyimide, the molecular chains are arranged more compactly, and chain segment movement requires higher temperatures;
Therefore, the coefficient of thermal expansion is lower before the glass transition temperature.
The coefficients of thermal expansion of pure TPI and pure PI differ significantly from that of copper foil, whereas when PI and TPI are used in combination, the coefficient of thermal expansion of the composite film approaches that of copper foil.
表4
Impact on Warping
Flexible copper-clad laminates are more prone to warping than rigid copper-clad laminates.
During production, factors such as temperature, line speed, and imidization significantly affect the warping of the laminates.
Downstream FPCB manufacturers require the boards to be flat during the cutting process and to remain flat even after partial etching.
If the boards curl after exposure and development, it will severely impact subsequent processes and lead to a decrease in yield.
表5
As shown in Table 5, although pure TPI remains flat after full etching, the sheet struggles to maintain its flatness and curls into a tube due to stress.
Pure PI with copper remains relatively flat, but the PI film curls into a tube after etching; therefore, neither is suitable for use.
Combining TPI with PI significantly reduces the curling of both the sheet and the composite film.
Generally speaking, PI, which has a low coefficient of thermal expansion, curls toward the copper foil, while TPI curls toward the PI side.
Therefore, when properly combined, the stresses from the two materials partially offset each other, achieving near equilibrium, resulting in a relatively flat final composite film.
Effects on Mechanical Properties
Thermosetting polyimides have attracted attention for their excellent high strength and high modulus.
Due to the high rigidity of the monomers and the relatively compact packing of the molecular chains, they exhibit high strength and high modulus.
Thermoplastic polyimides, on the other hand, use a higher proportion of flexible diamine monomers, resulting in relatively lower strength and modulus.
表6
As shown in Table 6, the strength and modulus of pure TPI are relatively low and do not meet corporate standards; however, combining TPI with PI significantly improves both strength and modulus.
The PI in the composite film serves as a reinforcing layer, ensuring the film’s mechanical properties.
Although the introduction of TPI reduces mechanical properties to some extent, the overall performance still meets the relevant requirements.
Conclusion
Based on the above test results, adhesive-free double-sided flexible copper-clad laminates containing only thermoplastic polyimide or thermosetting polyimide in the insulating layer exhibit poor overall performance.
To achieve double-sided flexible copper-clad laminates with better overall performance, a composite film was created by introducing TPI into the surface layer of PI.
When the TPI content was 15–25%, the composite film achieved a relatively effective balance in dimensional stability, curl, peel strength, thermal expansion coefficient, and mechanical properties.
This structure is relatively simple to manufacture, has a low cost, and offers good overall performance, making it suitable for the production of adhesive-free double-sided flexible copper-clad laminates.
