The low Dk, Df surface modification of silica filler in the high speed and high frequency applications

With the continuous development of electronic communication technology, the 5G communication involving GHz signal (millimeter wavelength) transmission has met the challenge in signal delay and loss due to the large Dk and Df of the dielectric materials in the communication devices. In order to meet the less signal delay and low loss requirements in 5G technologies, the copper clad laminate (CCL) for high frequency and high speed applications has attracted intense interests. As a critical components in the CCL, silica fillers could reach the filling ratio ~ 60% in mass, and it is an effective approach to improve the high speed/frequency performance of CCL via reducing the Dk and Df of the silica fillers with surface modification. Through the surface modification via grafting the low or none-polar surface coupling agents (SCAs), apart from the reduced Dk and Df, the silica fillers could also be benefitted with better dispersibility, larger adhesion energy with the resin matrix as well as the lower water absorption ratio, which could give rise to a better high speed/high frequency performance of the CCL.In this paper, three SCAs with specific molecular structure were used to perform the surface modification of silica fillers. By optimizing the content and type of SCA, silica filler with low dielectric constant and dielectric loss could be reached. The vector network analyzer was used to test the Dk and Df of the SiO2-Epoxy composites (pseudo CCL), and the Dage4000 was used to test the adhesion strength of the composites on the copper film. The coefficient of thermal expansion (CTE) and modulus of the composites were measured by thermomechanical and dynamic thermomechanical analyzer, respectively. Moreover, the interface adhesion between the silica filler and the epoxy matrix was characterized by the cross-section view in a scanning electron microscopy. The water absorption test of the composites was measured with a high-pressure cooking method. The results showed that the molecular structure and the adding mass ratio of SCAs played the vital role in tailoring the dielectric performance of the surface modified silica fillers, and the best high speed/high frequency performance with low Dk, Df, low water absorption ratio as well as the high copper adhesion strength could be attained after the optimization of SCA molecular structure and the surface modification process.