Via to Via Isolation vs. Quality of Via Formation in Organic Substrates

Market driven demands for smaller IC packages in space-sensitive applications, have forced designers to provide substrates with smaller and closer conductive features. For example, today's organic PBGA substrates or FCBGA substrate cores may have lines/spaces approaching 35/35μm and mechanically drilled vias as small as 100μm in diameter on 250μm pitch. These fine design features create major challenges for substrate manufacturers to maintain high isolation values between vias. The high density interconnections may result in substrate failures due to electrical leakage or shorts between vias, which are believed to be caused by the formation of a conductive anodic filament (CAF), when the organic substrates are exposed to a critical temperature / humidity / bias voltage environment, such as THB. Such was the case when this phenomenon was discovered during a failure mode analysis (FMA), which was performed to determine the cause of failure of an ASIC BGA Device that failed after 360 hours of temperature, humidity and bias voltage (THB) testing. Although the FMA determined the cause of failure to be die related, the analysis also discovered electrical near-shorts between the mechanically drilled vias in the core of the organic substrate. Metal growth was observed that is believed to have been caused by the poor drilling quality of the vias, which were 150μm in diameter on 325μm pitch. It should be noted that the substrate was not manufactured by Japan Circuit Industrial Co. (JCI). This paper addresses the FMA findings and aims to prove that the electrical near-shorts were triggered by the CAF phenomena as a result of the poor quality of the drilled vias, their proximity to each other, and the critical THB environment in which the device was tested. The FMA was comprised of cross sectioning, horizontal lapping and material analyses of the substrate. The techniques used in this FMA were similar to those described in the IC packaging reliability book "Failure-Free Integrated Circuit Packages", Charles Cohn / Charles A. Harper Editors, McGraw-Hill Inc., 2005. In summary, the analysis results show that the quality of the drilled vias is paramount in obtaining highly reliable substrate interconnections.