Study on the factors which affecting the conductive anodic filament reliability for packing 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. The mechanism for CAF is the transfer of copper (Cu) ions and the deposition of Cu salts along the epoxy-glass interface in the presence of moisture and voltage bias influenced by concentration and pH gradients, from anode to cathode. Recently, following the increase of CAF failure in electronic assembles, CAF reliability concerns in the industry have increased as board designs have advanced in terms of decreasing dimensions and/or higher voltages. This study was first from an actual failure case. The resistance was about 5 kΩ between two circuits which should be insulation. Then the leakage area was confirmed between two neighboring plated through holes (PTHs) by circuit analysis. With cross section analysis by metallography microscope and SEM&EDS (scanning electron microscope and energy dispersive spectroscope), the result indicated the cause of leakage is CAF formation in the packing substrate. Furthermore, the reliability research was performed to evaluate the factors of designs for CAF resistance. Test coupons, which manufactured with the same packing substrate material of the failure PCB sample, were placed in a temperature humidity chamber at 85°C, 85%RH. Another type of coupons with CAFResistance packing substrate material were also placed in the chamber at the same test condition. A voltage of lOY, SOV or 100V (DC) was applied to the test coupons during the test. Each coupon had hole-to-hole spacing of 0.4mm, 0.5 or 0.6mm. The insulation resistances were measured online during the test. According to the expected product life, the temperature humidity test should be performed 1000h. The TTF (time to failure) results show that the holes closer together or the voltage higher are more susceptible to CAF, but the type of substrate is key factor in designs for CAF resistance. As a result, in order to avoid CAF failure in the expected product life, the attention should be mostly focused on more reliable packing substrate materials.