A novel approach for flip chip solder joint inspection based on pulsed phase thermography

Surface mount components have been extensively used in microelectronic packaging. However, it brings great challenge for defect inspection with the development of solder bumps towards ultra-fine pitch and high density. Traditional nondestructive detection methods are insufficient for solder joint assessment due to their own disadvantages. Therefore, it is necessary to explore new methods for solder joint inspection. A novel approach based on the pulsed phase thermography was investigated for identifying the defects of solder joints. In this approach, the test chip was stimulated by a thermal pulse, and the consequent transient response was captured by a commercial thermal imager. The spacial and temporal filtering techniques were adopted to improve the signal to noise ratio. The recorded thermograms were input to an improved median filter with a 5×5 mask, and the temperature evolution of each pixel was extracted and smoothed by the moving average operation. Then the temperature–time curve was fitted with an exponential function. To eliminate emissivity variations and heating non-uniformity, we converted the fitted temperature values in time domain to the phase information in frequency domain using the fast Fourier transform. In low frequency range, the phase–frequency curve of the defect area was differentiated from that of the sound area. The results demonstrate that this approach is effective for identification of the missing bumps, and can be used in the solder joint inspection in high density packaging.