Application of photothermoelastic effect to noncontact inspection of soldered connections

Recently, thermoelastic vibration due to periodic laser irradiation on the plate-like sample has been observed in the photoacoustic experiments, and it has been used for the nondestructive evaluations of layered materials and clamped samples. By using the effect, we propose a new inspection method for soldered connection, in which the mechanical condition of soldering can be tested nondestructively. Surface of the IC package pin is irradiated periodically by heating laser to cause the thermoelastic vibration of the pin. In order to measure the vibration, probe laser beam impinges on the surface of the pin. The micro-translation of the speckle pattern, which is produced by the scattering reflection of the probe laser, is detected by a photo-diode. Unbonded pin of the IC package is distinguished from the well-bonded pins by the enhanced vibration. The characteristics of the pin vibration are examined experimentally under several conditions, and also they are considered theoretically from the thermoelastic bending point of view. The proposed technique will be useful for the automated inspection of the soldered connection used in the printed circuit board.