Nondestructive Evaluation of Soldered Joints Using Photothermoelastic Effect

This paper describes photothermoelastic effect occurring at the IC lead's soldered joints. Irradiation of chopped LASER light on a lead frame produces flexural vibration caused by the thermoelastic bending. Conditions of soldered joints were modelled into 3 categories, that is "nomal soldering", "lifted leg", "touching". Flexural resonance was found in "lifted leg". And the deflection of a lead frame varied according to the mechanical conditions of soldered joints, when irradiation spot position moved along the surface of a lead frame. Precise analysis of amplitude and phase of the vibration gave information on the conditions of soldered joints. In order to explain the experimental results, a calculation model was made. The thermoelastic bending was calculated under the assumption that the temperature difference existed along the thickness of the shell element. Difference between the phenomena occurring from the PT effect and the conventional photoacoustic effect were also observed through the detection of acoustic wave and optical deflection method.