Study of key failure modes of PTH in high density printed board and case study

Abstract

Over the last several decades, the multilayer printed boards, especially the high density substrates, have been widely used in electrical industry. Generally the different layers of printed boards are interconnected by the plated-through holes (PTH) and buried/blind vias. The quality and reliability of PTH significantly affect the reliability of printed boards, even the performance of the whole electrical system. So the research of quality, reliability and failure analysis to the PTH is important to make products reliable and to maintain its performance under different loads. The purpose of the paper is to introduce the methods of quality control of PTH and the key failure modes of PTH in high density printed board. The key failure modes of PTH such as plating voids, plating crack and conductive anodic filament (CAF) were presented. At the same time, several typical PTH failure cases were used to show how the failure occurred. We also investigated the failure mechanisms and found the root causes by using some conventional methods of failure analysis such as metallography microscope inspection and SEM&EDS analysis. The plating crack is mainly driven by the mismatch between the different material properties, such as coefficient of thermal expansion (CTE), hygro-swelling and vapor pressure induced expansion. For CAF failure, the mechanism is an electrochemical transport of ions across an electrical potential between anode and cathode. This paper also introduced an accelerated evaluation method to identify the CAF failure. Finally, for reliability improvement, several suggestions were given to prevent the PTH failure.