Conductive Anodic Filament Failures in Fine-Pitch Through-Via Interconnections in Organic Package Substrates

Abstract

Failures due to conductive anodic filament (CAF) formation in copper-plated through-vias have been a concern in printed wiring boards since the 1970s. With the continuous reduction in through-via pitch to meet high circuit density demands in organic packages, the magnitude of CAF failures is expected to be significantly higher. In this study, an accelerated test condition [130°C, 85% relative humidity (RH), and 100 V direct current (DC)] was used to investigate CAF in two organic package substrates: (1) cyclo-olefin polymer–glass fiber composite (XR3) and (2) epoxy–glass fiber composite (FR4). Test coupons with through-via spacing of 100 μm and 200 μm were investigated in this study. CAF failures were not observed in either substrate type with spacing of 200 μm. With spacing of 100 μm, insulation failures were observed in FR4, while XR3 exhibited stable insulation resistance during the test. The substrates were characterized using gravimetric measurement, and XR3 was found to exhibit significantly lower moisture absorption compared with FR4. The CAF failures in FR4 were characterized using scanning electron microscopy and energy-dispersive x-ray spectroscopy. The results suggest a strong effect of moisture sorption of organic resins on CAF failure at smaller through-via spacing in package substrates.