It is effective to use the stack-via-holes method (via-on-via structure), which stacks micro via-holes (MVH), in the development of higher density circuits on build-up printed wiring boards. When micro via-holes are repeatedly stacked, via holes cause an open circuit due to the difference in the thermal expansion of the copper and the glass epoxy base material. In order to verify the reliability of the stack-via-hole connection, we produced a range of test boards with differing numbers of stack-via-hole layers, material types (FR-4, high-Tg FR-4, low thermal expansion material, etc.), and via hole diameters (0.100 mm and 0.075 mm diameters). The boards were examined through DC current induced thermal cycling tests to determine the reliability of the various samples. In respect to the anti-migration properties, we also undertook research on pitches and base material types of micro via- holes. As a result of the evaluation, it was verified that the MVH, then interstitial-via-hole (IVH), followed by the plated through-hole (PTH) achieved the best connection reliability. As for base materials, it is recommended to use a high-Tg material with low thermal expansion qualities. We believe that an MVH diameter of 0.100 mm is preferable to prevent any void attributed from a high aspect ratio. No problem with the insulation reliability was found on any base material as long as the MVH pitch is 0.35 mm or more (or 0.25 mm between the hole walls).
展开▼
