Thin Film on LTCC for Connectivity and Conductivity

Abstract

The topic of this paper is the development, and characterization of multilayer thin films on Low Temperature Co-fired Ceramic (LTCC) for the purpose of attaining robust connectivity and improved conductivity, aimed at the RF single digit GHz frequency range. The aspect of connectivity primarily covers both standard SnPb eutectic solder as well as Au wire/wedge connections to the same thin film materials. This multi-purpose film ages well, as do solder joints—even after rework. In circuit development, this allows the re-appropriation of solder pads for wire bond purposes and vice versa, greatly enhancing the flexibility of the substrates. The useful lifetime of such substrates has been extended significantly by this work. The photolithographically-defined circuits further enable smaller features and tolerances and better geometrical uniformity. Vias received special attention due mainly to topography encountered at via boundaries and at the top of vias. Preparation and pattern generation for the thin film traces and pads will be discussed with attention to each material and thickness. Ion milling and post-processing for solder masking as well as unique soldering properties of the film edges are also presented. Results for adhesion and performance with respect to the various functions will be presented, as well as a detailed study of the thin film integrity and robustness.