Circuit World, Vol. 32, No. 3, pp. 11-18, May. 2006

A Variant of Conductive Filament Formation Failures in PWBs with 3 and 4 mil Spacings

Keith Rogers and Michael Pecht
University of Maryland
College Park, MD 20742


Purpose: To show how the use of conductor spacings below 4 mil in printed wiring boards (PWBs) can introduce an unanticipated failure mechanism, leading to current leakage and short circuit failure.
Design/methodology/approach: The tests in this study were conducted in accordance with IPC-TM-650 2.6.25, using boards designed with conductor spacings between plated through holes (PTHs) ranging from 6 to 3 mil and from 8 to 3 mil between PTHs and ground planes. The board types and conductor spacings were selected to include current and future printed circuit board fabrication technology.
Findings: For PWBs that may be used in harsh environments where the relative humidity and temperature may approach those of the test environments, even for relatively short periods of time, spacings of 4 mil or less in the materials tested may not be appropriate. However, it is unlikely that the 858C and 85 percent RH conditions are the minimum conditions to induce this failure mechanism. More tests at lower temperatures and relative humidity combinations should be conducted to evaluate conditions at which this type of failure begins.
Originality/value: The value of the paper lies in that the tests show that the IPC industry standard for conductive filament formation (CFF) testing of PTH-PTH conductor spacings of 4 mil or less, at 858C/85 percent RH can introduce a CFF variant failure mechanism, and therefore, may need to be modified to ensure that the test conditions accelerate the CFF mechanism and not other low resistance paths.

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