Layout and process characteristics of LTCC substrate for microwave module

With the development of wireless communication, LTCC substrate technology has been utilized to manufacture components and modules working in higher frequency with more applications ahead. For microwave modules working in Ku-band, special requirements are needed for LTCC substrate structure design and fabrication process. In order to avoid discontinuity of microwave signal transmission, higher VSWR and performance degradation, which originate from the disconnection of wire-bonding from chip surface to the under plate LTCC substrate surface, recess cavities for accommodating die-bonding chips must be fabricated with good quality. Here thus comes a challenge to LTCC substrate process. In green substrate lamination undergoing a pressure as high as 3kpsi, the technique of rubber- insert- aided cavity forming was experimented for finding a suitable rubber with expected elasticity and hardness and optimizing the clearance between cavity and inserts. As for integrating high-power devices on LTCC substrate, thermal-via is commonly used for heat dissipation with the thermal conductivity of substrate from improved from 2W/m·K up to 50W/m·K, taking Ferro A6-M as an example. A higher density of thermal- via helps dissipate heat while excessively increases the discontinuity of the substrate´s ceramic material, therefore cracks or warpage may occur after firing. The topology of via, punching process parameters and formula of via paste were regulated to achieve a good trade-off between substrate thermal management capability and mechanical properties.