Ligno-cellulose fibers are disintegrated into ligno-cellulose fibrils by high shear forces mechanical grinding in the presence of water. The degree and homogeneity of fibrillation is verified by electron microscopy. Figures 2A and 2B show images of fibers before disintegration and images of the fibrillated LCNF after processing. The cellulose fibers, originally several millimeters long and more than 10 µm thick (Fig. 2A), are disintegrated into a network of fibrils with lengths of several micrometers and diameters of a few nanometers (Fig. 2B). The absence of fiber residues and larger fibril agglomerates are an indication of complete disintegration of the fibers and, therefore, high homogeneity of the suspension.

As has already been shown in earlier studies on lignin-free CNF, the water that stabilizes the fibril network can be removed by pressure and temperature. This results in hornification, i.e., a reconstruction of the hydrogen and van der Waals bonding network of the cellulose with each other, and the fibrils densify to almost the theoretical density of 1.5 kg/cm³ of cellulose. Reaching this density in the case of LCNF requires the free accessibility of the surface of the fibrils their bonding interfaces as well as their mobility and reconfigurability in a densely packed network, both of which are given by the complete disintegration of the original fibers (Fig. 2B) but is partially hindered by the presence of lignin still in the system. After complete dewatering, densification and drying of the LCNF, a uniform stiff solid mass in the form of a round plate was formed (see Fig. 1). Electron microscopy of the surface and fracture surface of the plate provide information about its structure and internal morphology.

The surface of the densified LCNF substrate, which is later used as a substrate in the PCB application, is essentially non-porous and flat but covered with individual micrometer-sized particles (Fig. 2C). This surface structure is also significantly defined by the polishing quality of the steel mold. During dewatering of the LCNF suspension in the steel mold, the fibrils are oriented into layers by the outflowing water. In addition, surface uniformity of LCNF substrate was analyzed (Fig. 2E) to understand printability and assembly properties. Analysis shows that LCNF substrates are macroscopically flat which works very well with inkjet printing process, however, to achieve successful assembly of bare die chip the surface inequities needed to be tacked to obtain flat surface within area marked in yellow (Fig. 2E left-side image). The contact electrodes are equipped with higher bumps to assure contact with the electrode of bare die chip. Otherwise, in general electronics applications the surface uniformity of LCNF applicable.

The cross-sectional SEM images Fig. 2D provided insights into the internal structure of the LCNF substrates. The images show a layered cellulose fibrils deposition pattern, which is associated to the arrangement inducted by flow and hornification processes that occur during the substrate forming process. These processes involve the reorganization and bonding of cellulose fibrils, resulting in a dense, well-ordered stiff internal structure. The hornification process ²⁷, which involves drying and re-bonding of the cellulose fibers, contributes to the mechanical robustness and dimensional stability of the substrates. This structured internal morphology not only enhances the mechanical strength of the substrates which exceeds required standards²⁸ but also plays a vital role in their overall performance in the future electronic applications.

Lignin and hemicelluloses naturally presented in the starting fibers and resulting fibrillated materials have an influence on the physical and chemical properties of the LCNF substrate as well as its dewatering and hornification. An unquantified observation is that the dewatering rate of the LCNF suspension in the mold is higher than that of a lignin-free CNF suspension. It is assumed that the retention capacity of the cellulose is reduced by the attached more hydrophobic lignin, allowing the water to drain faster from the fibril network.

To understand the nature of the easier dewatering during the board forming process for LCNF compared to when lignin free CNF ²⁰ is used ATR-FTIR measurements were performed to study the intramolecular bonding.

The ATR-FTIR spectra presented in Fig. 2F contain two samples: lignin free CNF and LCNF.

In the 3400–3200 cm⁻¹ region, a broad peak is observed in both CNF and LCNF samples, indicative of O–H stretching vibrations commonly associated with hydroxyl groups (–OH) in cellulose^29,30. The 3000–2800 cm⁻¹ region shows weaker peaks attributed to C-H stretching vibrations, typical of aliphatic compounds present in cellulose and lignin^31,32.

The peaks in the 1750–1690 cm⁻¹ region are associated to C = O stretching and 1450–1580 cm^-1 due to -C = C- stretching are related to aromatic skeletal vibrations typical for lignin and hemicellulose and weaker in lignin free CNF sample^29,33. The 1200–1400 cm⁻¹ region displays strong peaks in the CNF and LCNF samples, indicative of C-H stretching vibrations or angular strain of CH (cellulose and lignin)³⁴. The peak below 900 cm⁻¹ mainly in LCNF can be attributed to out-of-plane bending modes, often associated with aromatic rings in lignin³⁴.

The ATR-FTIR analysis reveals that the presence of lignin in LCNF introduces aromatic structures, as evidenced by peaks in the 1750–1690 cm⁻¹ and 1450–1580 cm⁻¹ regions, corresponding to C = O and C = C stretching vibrations, respectively. These aromatic structures are associated with increased dewatering efficiency during the board-forming process compared to lignin-free CNF. Lignin’s polyphenolic and hydrophobic nature reduces the water-binding capacity of the fibrils by obstructing hydrogen bonding sites. Consequently, water retention within the fibril network is minimized, facilitating more efficient water removal and quickening the substrate formation process.

In electronic applications such as PCBs, the presence of moisture can significantly affect the performance and durability of the materials used. Moisture absorption can lead to reduced insulation resistance, increased dielectric constant, and mechanical degradation, which in turn impact the reliability of electronic components. When cellulose-containing materials, like LCNF, are used in eco-friendly PCB substrates, their inherent hygroscopic nature makes it essential to thoroughly examine its behavior for optimizing material properties to ensure stable performance in electronic applications.

To test the LCNF substrate stability at different environmental water vapour absorption tests were performed (see Fig. 3A).

The results indicated that LCNF substrate samples stored at 85% RH reached a constant water vapor absorption value more quickly than those stored at 50% RH. At equilibrium, samples stored at 85% RH absorbed approximately 9.5% moisture, while those stored at 50% RH absorbed up to 4% of water vapor. This faster stabilization at higher humidity levels suggests a more rapid equilibration process under conditions of higher moisture availability.

For a better understanding and to compare the properties of the LCNF substrate and conventional glass fiber reinforced epoxy substrates PCB substrates, liquid water absorption following the IPC-TM-650²³ standardized test for electronics was carried out. Using the protocol, substrates with LCNF showed a 34.2% of moisture absorption (SD = 0.321), surpassing the minimum stated requirements of 5.6%²⁸.

A result of the water uptake through vapor or liquid water immersion is that water changes the internal structure of the substrates through plasticization. In general, a loss of dimension stability must therefore also be expected in such systems. The evaluation of the dimensional stability for LCNF substrate was done in comparison to commercial PCB materials following the ICP-TR-483 standard²⁴. Table on Fig. 3B shows the dimensional stability measurements for LCNF substrate samples. Each sample was subjected to a controlled environment, and the percentage change in dimensions was recorded. The mean values, standard deviations, and confidence levels at 95% were calculated. According to the standards the high-performance PCBs require dimensional stability under 0.1%, while for general electronic applications requirements in the range up to 0.5%²⁸.

The significant difference in water absorption between the two environmental conditions has important implications for the use of LCNFs substrates in indoor applications. The higher water absorption at 85% RH indicates that LCNF substrates are sensitive to ambient humidity levels, which could affect their dimensional stability, mechanical properties over time and reduce insulation properties.

The LCNF_1 sample set exhibited a dimensional change from 0.07% to 0.11% under the test conditions of 23 °C and 0% RH, with a mean of 0.07—0.11% and a standard deviation of 0.05—0.07%. This sample set fall within the 95% confidence level, indicating high statistical reliability. The same behavior showed samples set LCNF_2 stored at 50% RH and 23 °C with dimensional change of 0.11 – 0.13%.

However, LCNF_3 sample set stored at 20 °C and 85% RH shows a dimensional change of 0.81% to 0.95%, this sample significantly exceeds the typical standard values²⁸, indicating substantial instability in high humidity conditions.

In bio-based materials, such as cellulose, polyesters, polyamides, etc., the plasticizing property of water molecules, as soon as they penetrate the inner structure of the material, also becomes noticeable through a reduction in its mechanical strength and modulus. To investigate this effect, the mechanical properties under bending and tension of various LCNF substrates were analyzed after storage at different humidity levels and temperatures.

The results of the three-point bending test (Fig. 3C) showed that LCNF substrates have significant flexural strength and modulus, indicating good resistance to bending forces. The average flexural strength was found to be 133.25 ± 9.9 MPa, and the flexural modulus was 11.33 ± 0.6 GPa for samples stored at 0% RH 23 °C. Conditioning at 50% RH 23 °C resulted in the reduction of the flexural strength to 114.27 ± 11.2 MPa and the flexural modulus to 9.94 ± 2 GPa. A significant reduction of the mechanical properties was observed for the samples stored at 85% RH and 20 °C. The flexural strength value dropped to 63.23 ± 1.7 MPa and the flexural modulus to 6.22 ± 0.2 G Pa. These values suggest that the LCNF substrate can effectively withstand mechanical stresses encountered in typical electronic applications and exceeds significantly minimum requirements which is 83 N/mm² (83 MPa) for cellulose reinforced PCBs and 415 N/mm² (415 MPa) for epoxy glass fiber reinforced PCBs ²⁸.

The LCNF substrates are a multi-material compound of fibrils, lignin, hemicelluloses, and water. In previous studies on lignin-free CNF substrates, a flexural strength of 200 MPa and a flexural modulus of 11 GPa were measured²⁰, whereby the sample was previously conditioned at 50% RH and 23 °C. The LCNF substrates achieve lower values for strength and modulus under the same conditions, which is likely due to the lignin and hemicelluloses²¹.

The same trend in the measured values can also be observed in tensile tests on LCNF samples at different humidities and temperatures (see Fig. 3D). Samples of the LCNF substrate showed a tensile strength of 95.3 ± 4 MPa, a Young’s modulus of 13.05 ± 0.8 GPa, and an elongation at break of 1.2% for the samples stored at 0% RH 23 °C. Samples that was stored at 50% RH 23 °C showed in average a tensile strength of 68.64 ± 3.2 MPa, a Young’s modulus of 10.2 ± 0.5 GPa, and elongation at break of 1.36%. Mechanical properties of the samples that were stored at 85% RH 20 °C however shows drastic reduction of the tensile strength to 36.94 ± 0.6 MPa, a reduction of Young’s modulus to 5.98 ± 0.7 GPa, with an elongation at break of 1.87%.

The measured strengths and moduli are compared with the properties of technical plastics and fiber reinforced composites. Ashby diagrams are a useful tool to compare material properties and identify material classes rapidly and effectively. Fig. 3E shows an Ashby diagram with modulus and material density values of different material classes including the measured values of LCNF substrates assuming the density is 1484 kg/m³. At this density, LCNF substrates achieve a comparable stiffness to various lighter woods and the heavier composite materials used for PCBs. In contrast, the stiffness at the same density of LCNF is significantly higher than that of technical polymers.

TGA is important for PCB material substrate characterization to evaluate the thermal stability and decomposition behavior, determining the material’s resistance to the elevated temperatures during processing and operation. In Fig. 3F the TGA analysis results shows that the LCNF substrate undergoes an initial weight loss due to moisture evaporation till 100 °C followed by a substantial degradation phase starting around 205 °C initiating 2% weight loss and 5% weight loss appears at 285 °C followed with a peak of full decomposition at 358 °C. The final residue, about 18.02% of the initial weight suggests the presence of non-volatile inorganic materials and not completely pyrolyzed carbon due to the nitrogen atmosphere.

The thermal conductivity of the LCNF substrate was measured on proprietary guarded hot plate devise showed results in the range of 0.245—0.302 ± 0.4 W/mK. This falls within the Level A²⁸ standard requirements for PCB materials (see Supplementary Information Table 1). For comparison, previous studies have shown that epoxy glass fiber-reinforced substrates typically exhibit a thermal conductivity of approximately 0.343 W/mK under standard conditions³⁵.

The resistance (\(\rho\)) for LCNFs stored in different environmental conditions for 30 days was determined as follows:

\({\rho }_{\text{LCNF at }0 { \% RH}}\)= 23.9 × 10³ \(\Omega\) ·cm.

\({\rho }_{\text{LCNF at }50{ \% RH}}\)= 14 × 10³ \(\Omega\) ·cm.

\({\rho }_{\text{LCNF at }85 { \% RH}}\) = 9 × 10³ \(\Omega\) ·cm.

These samples exhibits excellent performance according to the standard for cellulose reinforced systems which is 10³ \(\Omega\) ·cm²⁸. For comparison, FR4 epoxy glass fiber reinforced PCB substrates typically exhibit a volume resistance of approximately 10⁸ – 10⁹ Ω·cm under similar conditions²⁸.

The measurements showed consistent resistance values across different samples, indicating homogeneous material properties and effective processing techniques. Nevertheless, LCNF substrates exhibit lower but competitive resistance compared to FR4 substrates at all tested humidity levels. The resistance of LCNF substrates decreases significantly with increasing humidity. This indicates that LCNF substrates based on CNF can potentially match the electrical performance of FR4 substrates under specific environmental conditions, supporting their viability as sustainable alternatives in the electronics industry.