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Examination of Glass-fiber and Epoxy Interface Degradation in Circuit Boards



Examination of Glass-fiber and Epoxy Interface Degradation in Circuit Boards
Failures in PCBs account for a significant percentage of field returns. Studies have found that path formation is often along the glass fiber to epoxy matrix interface.
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Authored By:


Bhanu Sood, Michael Osterman, Michael Pecht
Center for Advanced Life Cycle Engineering
University of Maryland
College Park, MD USA

Summary


Multi-layer organic laminates, which make up over 90% of the interconnecting substrates in electronics (standard FR-4 represents 85% of the substrates used for laminates), can develop a loss of electrical insulation resistance between two biased conductors due to conductive filament formation. The probability of conductive filament formation is a function of the temperature, moisture content, voltage bias, manufacturing quality and processes, materials, and other environmental conditions and physical factors.
With increases in design density and tighter spacing between conductors, the probability of failure due to conductive filament formation (CFF) in printed circuit board (PCB) electronic assemblies has increased.

CFF is a failure observed within glass- reinforced epoxy PCB laminates that is caused by an electrochemical process involving the ionic transport of a metal through or across a non-metallic medium under the in fl uence of an applied electric field. The growth of the metallic filament is a function of temperature, humidity, voltage, laminate materials, manufacturing processes, and the geometry and spacing of the conductors. The growth of these filaments can cause an abrupt loss of insulation resistance between the conductors under a DC voltage bias.

A statistical examination of field returns and root cause analysis performed at Center for Advanced Life Cycle Engineering at University of Maryland shows that failures in PCBs account for a significant percentage of field returns in electronic products and systems. Studies on CFF have found that path formation in a PCB is often along the glass fiber to epoxy matrix interface.

Conclusions


The two step process of conductive filament formation includes the resin/glass fiber bond degradation and pathway formation, followed by an electrochemical reaction between the conductors. One method by which the pathway forms is due to breakage of the organosilane bonds at the glass and resin interface. This breakage occurs by hydrolysis (adsorption of water at the fiber glass/epoxy resin interface) or by repeated thermal cycling, which induces stresses at the interface due to coefficient of thermal expansion mismatches. This paper reviews the organosilane-resin bonding process and summarizes processing and environmental phenomenon that can result in breakage of the bonds. The paper also motivates the
requirement for systematic investigation of mechanical properties at the glass/resin interphase, and to track the influences of physio- chemical stresses such as moisture and pH that can affect the strength of this vital interface.

Initially Published in the IPC Proceedings

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