X-Ray Inspection of LTCC Devices: Theory and Practice

Abstract

The appeal of the LTCC (low temperature cofired ceramic) process lies in the possibility of creating multilayer (3D) structures, integrating conductor paths between passive elements, such as inductors, resistors, and resonance cavities. Unfortunately, the very nature of a cofired device makes postfiring optical inspection of buried elements impossible, as the ceramic material is opaque to visible light. This limitation, however, does not exist at x-ray wavelengths. The aim of this paper is to provide a practical overview of the application of high resolution x-ray imaging for nondestructive inspection and fault detection in multilayer LTCC structures. First, we present a simplified mathematical description of x-ray absorption inside an LTCC structure and demonstrate that due to the physical properties of the substrate (glass/Al2O3 ceramic), the conductor material (silver), and the cavity fill (air), a high contrast image of the investigated structure can be obtained. Next, we show the application of a commercial off the shelf industrial x-ray system for imaging various faults in LTCC structures, such as via voids (caused by inadequate filling of a via hole with conductor material), microcracks, paste creep (during lamination, excess via conductor leaks out of the via hole and in between the tape layers, shorting the via to an adjacent circuit), interruptions in conductor paths, and alignment errors. We also demonstrate the application of computed tomography to verify 3D geometry of buried resonance cavities and detect tape delaminations. Finally, we discuss the limitations of the method, related to structure thickness (number of layers), material composition, imaging geometry, and equipment characteristics, such as detector resolution and spatial noise.