Analytical calculations of mechanics for strain energy release rate, bending strain and phase angle are reported for a new procedure to test thin film mechanical properties. In the Microwedge Indentation Test (MWIT), a symmetric wedge-shaped probe extends fully across a wide thin film interconnect line, so that analysis under the assumption of plane strain is appropriate. As the indentation proceeds, an interfacial crack between the interconnect line and the substrate develops if the interface is weak. From this, adhesion can be calculated. Eventually, the interfacial crack becomes long enough that buckling ensues, giving rise to large thin film bending strains. Consequently the phase angle changes continuously from 53 to -37°. Depending on whether spallation occurs by fracture in the center of the beam or at the end of the beam at the interfacial crack tip, a thin film critical bending strain or grain boundary fracture toughness may be determined.