The Lamb waves are guided modes which propagate in a plane layer. The A[inf i] antisymmetric and S[inf i] symmetric (i=1,2,...) Lamb waves have an infinite phase velocity in low frequencies (cutoff frequencies), and the phase velocity limit, in high frequencies, is equal to the velocity of the shear wave in the isotropic solid material. The thickness resonance of an elastic plate normally insonified by an ultrasonic plane wave is related to the cutoff frequencies. From these resonance frequencies, the plate thickness can be easily deduced. In this presentation, a focused ultrasonic transducer with a large bandpass to characterize an electrical solder point between two steel sheets is used. This solder is abundantly used to manufacture cars. An experimental ultrasonic pulse method (pulse-MIR) allows us to plot acoustic spectra. The solder is placed in the focal spot of the beam. The transducer can be moved in a parallel direction to the surface of the sheet steel. Many time signals are recorded along a diametrical line with a small step. For each time signal a spectrum is computed. The results are shown with a graphical software. The false color images allow us to evaluate the solder quality.