Design limitations related to conductive anodic filament formation in a micro-world

Abstract

The drive toward more compact, highly functional, and portable electronic devices has forced a reduction in conductor spacing. According to the 2006/2007 IPC Technology Roadmap, today’s drilled holes are a minimum of 0.20 mm in diameter with via pitch at 0.60 mm. State-of-the-art is 0.15 mm with 0.40 mm via pitch and future trends in the near future reduce these to 0.10 mm holes and 0.35 mm vias. A direct reliability concern with reduced pitch designs is an electrochemical failure mode in printed wiring boards (PWBs) known as conductive anodic filament (CAF) formation. CAF—a copper-containing salt which grows subsurface from the anode to the cathode has been identified as Cu2(OH)3Cl. When the CAF bridges between the two holes, catastrophic failure can occur because this salt has semiconductor properties. CAF growth is enhanced by higher voltage gradient and closer spacing. CAF test coupon designs must take into account a number of factors in the board manufacturing process which will affect the reproducibility of the coupons. These include glass fiber style, resin type, and content, as well as the drilling parameters such as drill bit wear, speed, diameter, and drilling sequence. The closer spacing identified in the IPC Roadmap creates hole-edge to hole-edge conductors that can be problematic. Cracking between barrels can occur during the drilling process, especially for more brittle laminate materials. During the plating process, residues can accumulate in the cracks. Subsequent testing under humidity and bias conditions will lead to shorts which can be mistaken for CAF. This paper will identify the limitations in designing a PWB for CAF reliability testing.