An operating window, which is bounded by two temperatures and draw ratios, defines the stable and defect-free stretching region of a polymer film. Physical properties including the coefficient of thermal expansion (CTE), birefringence, and Young's modulus of a recyclable polyimide (PIR) film were measured under stretching conditions. While values of birefringence and Young's modulus increased with increasing stretching stress in the machine direction, the CTE was found to decrease. A semiempirical model for the prediction of birefringence and Young's modulus under stretching conditions was developed, from which the CTE could be estimated from the Young's modulus data. Theoretically evaluated physical properties were found to be in qualitative agreement with the experimental data.
The peeling behavior of polyimide film coated on steel substrates was experimentally investigated and compared with existing models. An operating window for peeling, which is defined as a closed domain for steady and defect-free peeling, is presented in terms of peeling force versus residual solvent content. The window is bounded by two major defects: the film becomes too brittle for peeling at high peeling force, and stick-slip striation defect appears at low peeling force. There exists a critical residual solvent content below which the adhesion between the polyimide film and the substrate is too strong and then peeling is impossible. Existing models for predicting steady peeling and the onset of peeling defects have been modified and applied to setup the boundaries of the operating window. There also exists another operating window for drying of polyimide and is presented in the form of drying temperature versus film thickness.FIG. 12. The SEM images of stick-slip striation defects during peeling. (a) T ¼ 1208C, t ¼ 15 lm, v ch ¼ 10 mm/min, residual solvent content ¼ 11.9 wt% and (b) T ¼ 1808C, t ¼ 28 lm, v ch ¼ 2 mm/min, residual solvent content ¼ 7.3 wt%.
In this study, the polymer/ceramic composites with high dielectric constant (HK=20 substrate) were prepared to investigate the temperature effects on electrical properties. The material properties of HK substrate were compared with those of FR4 substrate and MLCC-NPO capacitor device. Curie temperature of the HK substrate could be obtained from the curves of current densities varied with temperatures. The dielectric strength of the HK substrate was higher than 200kV/cm in the temperature range between 25 o C and 125 o C. The dielectric constants (DK) were slightly decreased but the dielectric losses were increased as the temperature higher than 100 o C. Moreover, an effective value near 0.74 eV of the trap depth and barrier height was predicted by Frenkel-poole and Schottky-controlled process.Furthermore, Electrical properties of HK substrate after thermal treatments were also measured in this study. It was found that the variations of capacitance ( C/C) and dielectric loss ( DF/DF) of HK substrate were about <±3% and <±20% respectively, after thermal cycle of 85 o C for 1000 hrs. The temperature coefficient of capacitance (TCC) of HK substrate near -300ppm/ o C was also obtained as measuring temperatures range from 25 to 125 o C. These results indicated that the HK substrates have excellent electrical properties to meet the applications of embedded capacitors via PCB process.
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