*Article* **Root Cause Analysis of a Printed Circuit Board (PCB) Failure in a Public Transport Communication System**

**Cher-Ming Tan 1,2,3,4,5, Hsiao-Hi Chen 1, Jing-Ping Wu 1, Vivek Sangwan 1,\*, Kun-Yen Tsai <sup>1</sup> and Wen-Chun Huang <sup>1</sup>**


**Abstract:** A printed circuit board (PCB) is an essential element for practical circuit applications and its failure can inflict large financial costs and even safety concerns, especially if the PCB failure occurs prematurely and unexpectedly. Understanding the failure modes and even the failure mechanisms of a PCB failure are not sufficient to ensure the same failure will not occur again in subsequent operations with different batches of PCBs. The identification of the root cause is crucial to prevent the reoccurrence of the same failure. In this work, a step-by-step approach from customer returned and inventory reproduced boards to the root cause identification is described for an actual industry case where the failure is a PCB burn-out. The failure mechanism is found to be a conductive anodic filament (CAF) even though the PCB is CAF-resistant. The root cause is due to PCB de-penalization. A reliability verification to assure the effectiveness of the corrective action according to the identified root cause is shown to complete the case study. This work shows that a CAF-resistant PCB does not necessarily guarantee no CAF and PCB processes can render its CAF resistance ineffective.

**Keywords:** 3D X-ray; bias temperature-humidity reliability test; conductive anodic filament (CAF); de-penalization; finite element analysis
