Printed circuit boards are subject to increasingly hostile environment, for example, to a higher reflow temperature with the new lead-free solders. Consequently, thermal stability of copper clad laminates, a major component in printed circuit boards, is a critical issue in electronic packaging industries. In order to understand thermal degradation behavior, popcorn blister tests, were performed at high temperatures for three different brands of copper clads laminated with FR-4 epoxy-saturated glass fabrics. Regardless of the brand, all the specimens became defective with blisters at about 250degreesC. Detailed examination of fracture morphology with a scanning electron and an optical interferometric microscope revealed that popcorn blisters started not at the interface between copper foil and epoxy resin but at the interface between epoxy resin and glass yarns inside the FR-4 fabric. Sizing agent, an organic lubricant used for glass fibers, is suspected for the initial breakdown of the interface between glass yarns and epoxy resin. As epoxy resin degraded through decomposition of amino-alcohol portion or some other reaction, the gas products then diffused to the yarn-resin interface area. When the temperature reached about 250degreesC, the pressure of much gas pockets became strong enough to rupture the laminate under both tension and shear stresses or to advance the fracture through the epoxy via cleavage fracture. Fracture patterns displayed both river and hackle markings, typical characteristics of epoxy fracture. The river markings seemed to indicate cleavage fracture of the epoxy (mode I) which was in close contact with glass yarns. The hackle markings, on the other hand, suggested rupturing of epoxy under both tension and shear stresses (modes I and II), which were most likely caused by a change in the maximum principal tensile stress direction due to interlocked copper nodules.