Mechanical Strength Enhancement Of Low Temperature Co-Fired Multilayer Ceramics By Introducing Residual Stress

In recent years,electronic components are developing towards miniaturization,integration and high reliability.The harsh work conditions of automobile,aviation and aerospace put forward higher strength requirements for the packaging substrate.As an important high integration electronic packaging material,low temperature cofired ceramics?LTCC?has been widely used.However,the brittle glass in LTCC materials make it hard to increase the strength higher than 350 MPa.In this thesis,LTCC multilayer process is adopted to obtain the residual stress by using materials with different thermal expansion coefficients in the multilayered structure,and the enchancement of LTCC substrate is realized.Research work on theoretical calculation,stress field simulation of multilayered structure by finite element method,material selection and preparation,thermal expansion coefficient?TEC?regulation of LTCC materials,laminate structure design and preparation,have been carried out.The main results of this thesis are as follows:The two LTCC materials,DP3 glass/Al₂O₃ and Ca-Al-B-Si-Zn-O glass?CABSZ?/Al₂O₃,were prepared by using alumina with two particle sizes of 1?m and 2?m,respectively.High strength,high TEC?330 MPa,6.6 ppm/K?DP3/Al₂O₃ and high strength,low TEC?346 MPa,5.67 ppm/K?CABSZ/Al₂O₃ LTCC composite material were obtained.And the D8C2/Al₂O₃ material?337 MPa,5.9 ppm/K?was obtained by mixing the two kinds of glass mentioned above.The fracture of multilayered material was analyzed according to the first strength theory.Combined with the stress distribution of multilayer structure in the process of service,the optimal thickness ratio was solved by theoretical calculation.The analyse shows that a higher Young's modulus in surface material and a higher strength in inner material contributed to a larger enhanced strength with smaller optimal thickness raito.Based on the fracture criterion of multilayered material,the optimal thickness ratio and maximum strength enhancement effect were obtained by finite element method.The optimal thickness ratio of 0.185-0.227 and the maximum strength of 398.36-404.49MPa can be achieved when the material parameters of D8C2/Al₂O₃ and DP3/Al₂O₃were applied in the model.According to the results of theoretical calculation and finite element analysis,several multilayered composites with different surface layer to inner layer thickness ratios of 0.07,0.17,0.19,0.26 and 0.41 were designed.The LTCC multilayer composite substrate material with the maximum strength of 404 MPa was obtained at the thickness ratio of 0.19.When the thickness ratio was greater than 0.19,the fracture began at the surface and the effect of material reinforcement was similar to that of surface compressive stress.When the thickness ratio was less than 0.19,the fracture began at the interface and the effect of surface compressive stress enhancement was weakened.The thermal expansion coefficient of Ca-Al-B-Si-Zn-O glass was regulated by changing the CaO/SiO₂ ratio.It was found that the thermal expansion coefficient of glass increased linearly with the increase of the CaO/SiO₂ ratio,and the effective thermal expansion coefficient of CaO was about 12.3 ppm/K.The glass with different CaO/SiO₂ contents was co-fired with alumina.It was found that,with the increase of CaO/SiO₂,the precipitated phase changed from the anorthite?CaAl₂Si₂O₈?to the gehlenite?Ca₂Al₂SiO₇?,and the sintering temperature was 850 ^oC,the strength was 315MPa,and the TEC was 7.66 ppm/K.The multilayer structure was prepared using this material by laminating with CABSZ/Al₂O₃ material with thickness ratio of 0.22 and enhanced strength of 462 MPa was obtained.