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.
Highlights
Equipment and devices can fail during their operation in the field and this may occur prematurely
Burn-out area at the bottom edge of the printed circuit board (PCB) found through naked eyes
Cracks are present at the PCB edge from where the moisture penetrates inside the PCB, which causes the short-circuit; the burned-out area can be observed
Summary
Equipment and devices can fail during their operation in the field and this may occur prematurely. Its initiating mechanism starts when a resistive path forms between two traces or planes in a PCB that are at a different electric potential. Slee et al [1] described various c2aoufs1e3s of propagating faults in PCBs; namely, resistive heating, interconnect overheating, contamination, electrochemical migration, tin whiskers, insulation failure, and component. CCuurrrreennttllyy,, tthheerree aaarrreee vvvaaarrriiiooouuusss tttyyypppeeesss ooofff dddeee---pppeeennnaaallliiizzzaaatttiiiooonnn;; nnnaaammmeeelllyyy,,,hhhaaannndddbbbrrreeeaaakkk,,,pppiziizzzzzaaacccuuuttttteeerrr///VV--ccuutt,, ppuunncchh,, rroouutteerr,,, sssaaawww,,,aaannndddlllaaassseeerrr[[[555]]]...IIInnnttthhhiiissswwwooorrrkkk,,,aaa pppiiizzzzzzaaacccuuuttttteeerrr///VV--ccuutt wwaass eemmppllooyyeedd ffoorr tthhee ddeee---pppeeennnaaallliiizzzaaatttiiiooonnn. Failure symptoms of a CAF range from intermittent resistive faults under current limit conditions to catastrophic propagating circuit board faults if the fault current is not sufficiently limited [1]. From our failure analysis observations, the failure mechanism was likely to be a CAF as the short-circuit paths were not on the surface this PCB was CAF-resistant
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