Fabrication of Microvias for Multilayer LTCC Substrates

Abstract

Advances in screen printing and photoimageable paste technologies have allowed low-temperature cofired ceramic (LTCC) circuit densities to continue to increase; however, the size of vias for Z-axis interconnections in multilayer LTCC substrates have been a limiting process constraint. In order to effectively exploit the 50-100-/spl mu/m line/spacing capabilities of advanced screen printing and photoimageable techniques, microvia technologies need to achieve 100 /spl mu/m and under in diameter. Three main steps in fabrication of microvias include via formation, via metallization or via fill, and layer-to-layer alignment. The challenges associated with the processing and equipment for the fabrication of microvias are addressed in this paper. Microvias down to 50 /spl mu/m in diameter with spacings as small as 50 /spl mu/m are achieved in 50-254-/spl mu/m-thick LTCC tape layers through the use of a mechanical punching system, whereas the minimum size of 75-/spl mu/m via/spacing is obtained using a pulse laser-drilling system in the LTCC tape layers with the same thicknesses as those for the punching test. The quality of punched microvias and laser-drilled microvias will be presented as well. Layer-to-layer alignment is crucial to the connection of vias in adjacent LTCC tape layers. Through a stack and tack machine with a three-camera vision system and an adjustable precision stage, less than 25-/spl mu/m layer-to-layer misalignment is achieved across a 114.3/spl times/114.3 mm (4.5/spl times/4.5 in) design area. In a six-layer LTCC test substrate (152/spl times/152/spl times/0.762 mm), microvias of 50, 75, and 100 /spl mu/m in diameter are successfully fabricated without the use of via catch pads. The cross section of fired microvias filled with silver conductor pastes at various locations of this substrate demonstrates a minor layer-to-layer misalignment in both X and Y directions across the substrate.