Behavior of polymeric materials and their effects on high density PWB

In this study, properties of some selected, representative HDI materials (dielectric and solder mask), including dimensional stability (thermal and chemical), tensile behaviors, and fracture toughness have been investigated. Experimental results show that the glass transition temperature of materials cured with the manufacturer recommended schedule did not reach their ultimate, alleged published values, indicating that further curing is needed. Most HDI materials are brittle at room temperature with a low strength and low elongation at failure. It has been found that the resistance to crack propagation of HDI materials is much lower than that of polymer thin films, such as Kapton, widely used in electronic products. It has also been observed that stress-strain behaviors of partially cured materials and fully cured materials are very close at room temperature but very different at higher temperatures. Finite element analyses, however, indicate that low fracture toughness of HDI materials will not be major cause if cracks are observed in the thermal cycling of HDI boards. Rather, it would be due to combined contribution from low strain-at-failure, strong viscoelasticity, and low fracture toughness