Conductive anodic filament reliability and failure analysis for halogen-free packaging substrate

Conductive anodic filament (CAF) formation, a failure mode in printed circuit boards (PCBs), which has been reported in 1976, has caused catastrophic field failures on electronic product. With the trend of high circuit density demands in organic packages, the pitch of plated through holes (PTHs) in packages should be reduced, and the amount of CAF failures is expected to be significantly higher. In addition, the trend of restriction in use of halogens in electronic systems has urged the development of halogen-free packaging substrates. The reliability could be affected after removal of halogen, such as CAF failure. In the paper, CAF reliability of halogen-free glass fiber-reinforced (FR-4) packaging substrates was investigated as well as halogen FR-4 substrates. Two types of test coupons, one with halogen-free substrates and the other one with halogen substrates, were both placed in a temperature humidity chamber at 85°C, 85%RH, with bias voltage of 100 Vdc, during 1000 hours. The insulation resistances were monitored online. The failure time for halogen-free substrates was respectively 119 hours (0.4mm pitch), 240 hours (0.5mm pitch) and 610 hours (0.6mm pitch), as well as the time for halogen substrates was respectively 594 hours (0.4mm pitch) and more than 1000 hours (0.5mm and 0.6mm pitch). After comparative analysis for the failure coupons, the root cause for rapid growth of CAF in halogen-free substrates is the separations between glass fibers and resins. The results indicate that the CAF failure should be concerned after removal the halogen from substrates, and, in order to avoid CAF failure in fine-pitch halogen-free substrates, combining force between glass fibers and resins should be improved.