LTCC substrates for fine pitch (1.0 mm and 0.8 mm) CSP applications have been designed, fabricated, and assembled. The assembly process, including ball grid array (BGA) solder ball attach, die mount, wire bond, and glob top is described. The material and physical design interaction issues that emerged during development are discussed.
The initial CSP design was conventional, with co-fired yellow gold (Au) vias and capture pads and post-fired solderable gold (PtPdAu) pads for solder ball attachment. Because LTCC tape shrinks during co-fire, solder pads were applied post co-fire to ensure proper mating with existing test fixtures and to provide the best alignment relative to the CSP body. Solder pad to capture pad misalignment was visible following solder pad firing. After CSP attachment to a test board, electrical tests revealed opens. Investigation led to the following conclusions. The decreased solder pad diameter necessary to accommodate the fine pitch design was significant relative to the area allocated for the underlying via and capture pad. Misalignment that would have been hidden under larger solder pads was exposed. Even when the capture pad surface was not visibly exposed, the offset solder pad meant less material between the capture pad and the solder ball, less of a barrier to solder leaching. Solder leaching into the yellow gold, observed after CSP removal from the test board, was the cause of the electrical disconnects. In the second design, the capture pad was eliminated in order to discourage leaching by reducing the volume of yellow gold available to alloy with the solder pad during co-fire. Reflow operations still resulted in leached solder pads. A third design replaced the first-layer via yellow gold with a solderable gold. This design proved to be robust.
While developing designs and fabricating these prototypes, it was noted that all ball failures consistently occurred between the solder pad and the LTCC substrate. To investigate adhesion using different metallizations, shear tests were performed on LTCC substrates with either post-fired solder pads or co-fired pads. To investigate how the substrate material affects adhesion, alumina CSPs were also sheared. Shear test results are presented.
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