Formation and microstructure of heterogeneous Al/PI interface. (a) Proposed mechanism for the formation of a heterogeneous Al/PI interface through dehydration reaction at 150 °C. (b) High-resolution TEM image of the as-bonded Al/PI interface after bonding at 150 °C for 10 min. Diffraction patterns of the Al matrix and the interface reveal that the interface, where some new fine crystals are formed, grew after crystal orientation of the Al matrix. (c) and (d) show the interfacial microstructure after storage at room temperature for 3 and 6 months, respectively. The symbols ▲ and point out the initial interface where Al and PI surfaces were brought into contact and the Al/PI interfacial region, respectively.