Fabrication of microvias for multilayer LTCC substrates

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-μm line/spacing capabilities of advanced screen printing and photoimageable techniques, microvia technologies need to achieve 100 μm 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 μm in diameter with spacings as small as 50 μm are achieved in 50-254-μm-thick LTCC tape layers through the use of a mechanical punching system, whereas the minimum size of 75-μm 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-μm layer-to-layer misalignment is achieved across a 114.3×114.3 mm (4.5×4.5 in) design area. In a six-layer LTCC test substrate (152×152×0.762 mm), microvias of 50, 75, and 100 μm 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.