Epoxy/$BaTiO_{3}$ composite embedded capacitor films (ECFs) have been developed for the fabrication of embedded capacitors with high dielectric constant and low capacitance tolerance(＜ ±5%) in organic substrate. This paper presents the materials and process development of $epoxy/BaTiO_{3}$ composite ECFs and the investigation of the properties of ECFs such as adhesion strength, dielectric constant, leakage current, and relative permittivity in microwave region.
In terms of materials formulation, ECFs are composed of high dielectric constant $BaTiO_{3}$ powder, specially formulated epoxy resin, and latent curing agent. And in terms of coating process, a roll coating method is used for obtaining film thickness uniformity in a large area. Typically, capacitors of 12 ㎛ thick film with 8 nF/ ㎠ capacitance, 90 dielectric constant, and less than ±5% capacitance tolerance were successfully demonstrated on PCBs using ECFs.
Effects of materials and process on the interfacial adhesion strength between ECF and Cu foil were investigated using peel strength measurement. From various experiments, it was found that residual solvent removal was very important. Use of the solvent with high volatility such as MEK and drying in a vacuum oven improved adhesion strength. $BaTiO_{3}$ particle content and size greatly affected adhesion strength. As $BaTiO_{3}$ particle content increased and $BaTiO_{3}$ particle size decreased, adhesion strength decreased. The decrease was attributed to the increase of the number of particles per unit area which act as singular points at Cu foil/ECF interface where stress is concentrated. 3-layer ECF where adhesion layers are coated on both side of core dielectric layer was proposed to solve week adhesion of high $BaTiO_{3}$ particle loading ECFs.
The dielectric constant variation of the $epoxy/BaTiO_{3}$ ECFs with BT particle size was investigated. Tetragonality of BT powder decreases as particle size decreases. The highest dielectric constant is obtained from the 0.83 ㎛ powder. The decrease of the dielectric constant of the epoxy/BT ECF below 0.83 ㎛ is due to the decrease of tetragonality, and the decrease of the dielectric constant above this size is presumably due to the reduction of domain wall.
Theoretical equations for the prediction of the dielectric constant of polymer/ceramic composites were compared with experimental results about the dielectric constant of ECFs with various particle size and contents. The experimental results were well fitted to Lichtenecker equation and Jayasundere-Smith equation. By fitting the experimental results to the equations, the dielectric constants of $BaTiO_{3}$ powders were estimated.
Changes in the dielectric constant of the epoxy/BT ECF with BT particle volume fraction from 0 to 65 vol% was observed. The dielectric constant of the epoxy/BT increases ECF as BT powder volume fraction increases and maximum values are observed at 55~60 vol% for each powders. However, more powder addition above these lowers the dielectric constant of the ECFs. This is due to the voids or pores formed by excess powders, which was confirmed by SEM observation and film density measurement. Using bimodal combination, dielectric constant of ECF was further increased. From the combination of C1 (0.06 ㎛) + S5 (0.916 ㎛), 75vol% packing density was achieved and dielectric constant 90 was obtained.
Leakage current changes with $BaTiO_{3}$ particle content and size were investigated. As the number of $BaTiO_{3}$ particle grows, distance between the particles become shorter. Beyond certain critical point, percolation occurs and large current can flow easily. Leakage current of composite films increased as particle size increased. This is presumably due to the decrease of the number of particles per unit length, resulting in the decrease of the number of BT powder/epoxy/BT powder, acting as a potential barrier.
Dielectric constant of $epoxy/BaTiO_{3}$ composite ECFs at microwave region between 500 MHz and 3 GHz was measured using cavity-mode resonator method. In this range, the dielectric constant slightly decreased as frequency increased, but dielectric relaxation phenomena was not observed. The dielectric constants measured in this range were smaller than those measured at 1 MHz. This decrease is due to the decrease of epoxy dielectric constant, and the dielectric constant of $BaTiO_{3}$ powder is preserved up to 3 GHz without regard to particle size and content.
