High Frequency Electrical Performance and Thermo-Mechanical Reliability of Fine-Pitch, Copper-Metallized Through-Package-Vias (TPVs) in Ultra-Thin Glass Interposers

This paper demonstrates the high frequency performance and thermo-mechanical reliability of through vias with 25 µm diameter at 50 µm pitch in 100 µm thin glass substrates. Scaling of through via interconnect diameter and pitch has several electrical performance advantages for high bandwidth 2.5D interposers as well as mm-wave components for 5G modules. This paper focuses on the assessment of thermo-mechanical reliability, of high aspect ratio TPVs at ultra-fine pitch, metallized using a via–first approach, and the accurate electrical modelling of TPVs and transmission lines with TPVs up to 40 GHz, using ANSYS HFSSTM. Test vehicles consisting of through via daisy chain structures were designed and fabricated on 100 µm thick glass, laminated with a 5 µm epoxy dry film polymer on both sides. Fine pitch TPV arrays were subjected to Thermal Cycle Testing (TCT) between -55 °C and 125 °C, and the majority of TPV chains passed 1000 cycles with less than 15% change in DC resistance. The impact of pitch scaling on the reliability was studied by varying the spacing of neighboring TPVs using 3D quarter-symmetric finite element models. A novel approach based on wave dimensional analysis was investigated to accurately capture the electrical parasitics of the vias in mm wave frequency bands. The resistance and inductance of a single signal TPV at 28 GHz were estimated to be 93 m and 60 pH respectively. Using Voltage Standing Wave Ratio (VSWR) calculations, it was shown that smaller via diameters are preferable for transitions from a 50 impedance matched planar to vertical interconnection.