Analysis of Catastrophic Field Failures Due to Conductive Anodic Filament (CAF) Formation
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W. J. READY, B.A. SMITH, L.J. TURBINI and S.R. STOCK School of Materials Science & Engineering Georgia Institute of Technology, Atlanta, GA 30332-0245 http://reliability.marc.gatech.edu/
ABSTRACT Under certain environmental conditions, printed wiring boards (PWB) respond to applied voltages by developing subsurface deposits of copper salts extending from anode to cathode along separated fiber / epoxy interfaces. The formation of these deposits, termed conductive anodic filaments (CAF) require high humidity (80%RH) and high voltage gradient (5V/mil). The humidity exposure during the storage environment may cause the failure in the use environment. CAF formation is enhanced by the use of certain hot air solder leveling (HASL) fluids and / or water soluble flux constituents. In this work, two catastrophic field failures were analyzed. Both failures were related to boards produced in a manufacturing process, which included HASL. One CAF failure occurred between a component through-hole and power plane held at a potential difference of 40V with a 0.005" nominal spacing. The other occurred on an inner layer of a multi-layer board (MLB) between a via and ground plane held at a potential difference of 320V with 0.015" nominal spacing. The nature of the CAF was analyzed using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). Ion chromatography (IC) was used to identify
residue extracted from a failed board. The failure phenomena known as CAF poses serious longterm reliability concerns in electronic products exposed to adverse and hostile environments, especially those with closely spaced conductors. BACKGROUND In 1976 a new failure mode was observed as a result of a study of high voltage circuitry on printed wiring boards (PWB) and the hostile environments in which these circuits were required to perform [1]. This failure mode is characterized by an abrupt, unpredictable loss of surface insulation resistance between conductors that are held under a bias voltage. The loss of resistance is caused by the growth of a subsurface filament from anode to cathode. The filament, termed conductive anodic filament (CAF), forms in an electrochemical process in which corrosion of copper at the anode results in a copper-containing salt, which proceeds along a separated fiber / epoxy interface [1-5]. This paper will review two different field failures that were caused by CAF formation. Data from scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and ion chromatography (IC) will be used to relate the hot air solder leveling (HASL) fluid to these failures.
45 Mat. Res. Soc. Symp. Proc. Vol. 515 *1998 Materials Research Society
EXPERIMENTAL Field Failure #1 Serial Sectioning & SEM/ EDS Observations Several field returns of electronic product revealed a failure between a through-hole and power plane with a 0.005" nominal spacing held at a potential difference of 40V. A failed board was examined to determine the extent of damage possible. Figure 1, a cross section of the MLB reve
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