| Created: 5/21/95 |
Updated: 8/05/98 |
Conductive Filament Formation in Printed Wiring Boards
Project Number: C93-12
Point of Contact:
Dr. Michael Pecht
CALCE EPSC
Email: pecht@calce.umd.edu
Phone: (301)-405-5323
Fax: (301)-314-9269
Objective
To investigate the effects of different printed wiring board (PWB) characteristics
and environmental stresses on conductive filament formation (CFF) failure
mechanism.
Background
Multi-layer organic laminates used in printed wiring boards and laminated
multichip modules (MCM-L), can develop a loss of insulation resistance
between two traces, between a trace and a via, and between two vias, due
to the growth of conductive filaments along the epoxy resin/glass interface.
The growth of the filaments is a function of temperature, humidity, voltage,
laminate material, manufacturing processes and the geometry and spacing
of the conductors. In order to develop a model which can be used
to establish both design guidelines for the prevention of conductive filament
formation, and tests for product qualification, a design of experiments
study was conducted. Temperature, humidity, and a voltage were the
stress parameters, and conductor spacing, conductor geometry, laminate
material (FR-4, BT and CE) and surface coating (presence and absence of
solder mask, solder plate, and post coat) were the laminate parameters.
Work Accomplished
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The effect of the variables (temperature, moisture content, voltage bias,
geometry, spacing, surface condition of the board) on time to failure was
analyzed.
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Among the three materials tested, it was found that BT and CE boards have
better resistance to filament formation than FR-4 boards.
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An empirical model was developed to predict the occurrence of CFF based
upon all known contributing parameters (temperature, moisture content,
high voltage bias, spacing, surface condition of the board).