Debonding at the Fiber/Resin Interface in Printed Wiring Boards

Project Number: C94-07

Point of Contact:

Dr. Michael Pecht CALCE EPSC Email: pecht@calce.umd.edu Phone: (301)-405-5323 Fax: (301)-314-9269

Objective

Background

Work Accomplished

Objective

This study was undertaken to examine E-glass reinforcement fiber/resin bonding behaviors for flame retardant (FR-4), bis-maleimide triazine (BT), and cyanate ester (CE) printed wiring boards (PWBs) when subjected to thermal cycling and combined thermal and humidity conditions.

Background

Printed wiring boards (PWBs) provide mechanical support and electrical interconnection for electronic devices. They consist of one of more layers of metal, bonded onto insulating substrates fabricated with glass-fiber-reinforced thermosetting resin. Electrical connection between the different layers of circuitry is achieved by holes either drilled or punched through the substrate and then plated. One of the most challenging reliability problems in multilayer PWBs is the effect of environmental stress cycling on electro-mechanical properties in terms of insulation resistance and interfacial bonding. One observable effect of an increase in temperature and humidity content is separation and debonding between reinforcement plies and the epoxy-resin within the PWB base layers, and/or separation between the base material and the conductive foil.
The PWB bonding property depends on both the raw materials and the manufacturing process. Three different PWB composites were tested in this study: FR-4, BT, and CE. FR-4 board uses an epoxy based on the diglycidyl ether of tetrabromobisphenol A. FR-4 epoxies have standard processibility among PWB producers, excellent adhesion to copper and other metals, low shrinkage during cure, good chemical and moisture resistance, and good performance and relatively low cost. The disadvantage of FR-4 is that is has a high expansion rate when heated to solder temperature. Higher-performance epoxies include BT and CE, which have higher glass transition temperatures and improved chemical and thermal stress resistance. Their disadvantage includes loss of processing ease, increased brittleness, drill wear, and higher material costs.

Work Accomplished

PWB materials were subjected to four thermal cycling experiments, including 25 to 85° C temperature cycling, 25 to 150° C temperature cycling, 25 to 200° C temperature cycling, and combined temperature/humidity cycling of 25° C/20% relative humidity to 85° C/85% relative humidity.

Environmental scanning electron microscopy (ESEM) was used to study degradation effects. Resin/fiber interface debonding was observed to occur primarily on the edges of the glass-fiber bundles. Interfacial degradation modes and mechanisms were identified and explained in terms of moisture sorption of epoxy, mismatch of the coefficient of thermal expansion between the epoxy-resin and glass-fiber, and the geometry of glass-fiber bundles.