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.

中文翻译：

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公共交通通信系统中印刷电路板 (PCB) 故障的根本原因分析

印刷电路板 (PCB) 是实际电路应用的基本要素，其故障会造成巨大的财务成本甚至安全问题，尤其是在 PCB 过早和意外发生故障的情况下。了解PCB故障的故障模式甚至故障机制，并不足以保证在后续不同批次的PCB操作中不会再次发生相同的故障。确定根本原因对于防止再次发生同样的故障至关重要。在这项工作中，针对故障是 PCB 烧坏的实际行业案例，描述了从客户退回和库存复制板到根本原因识别的分步方法。发现失效机制是导电阳极灯丝 (CAF)，即使 PCB 是抗 CAF 的。根本原因是PCB去惩罚。根据已确定的根本原因进行可靠性验证，以确保纠正措施的有效性，以完成案例研究。这项工作表明，耐 CAF 的 PCB 并不一定保证没有 CAF，并且 PCB 工艺可以使其耐 CAF 失效。