Research On Defects Inspection Of Flip Chip Using Active Infrared Thermography Technology

Chip interconnection is one of the key technologies for microelectronic packaging. The flip chip, which uses solder bumps to realize interconnection between chips and substrates, becomes the mainstream technics in microelectronic packaging because of its decreased package size, larger speed of signal propagation and so on. With the development of solde bumps towards higher density and finer pitch, the chip power density will increase dramatically, and the heat dissipation will become a significant problem, the thermal mismatch in the package is also getting serious, which results in solder defects and bonding failures. Defects inspection and characterization of the thermal perfermence for the solder bumps are more difficult as the bumps are hidden in flip chip package. The active infrared thermography technology was applied to defects inspection in microelctronic packaging in this thesis, and the finite element method was also adopted to investigate the heat conduction in the flip chip.The thermal performance of the solder bumps was investigated using the analytical and numerical methods. We constructed a mathematical model for heat transfer in the flip chip structure and provided the solving procedure. A lumped thermal resistance network was derived from the one dimension heat transfer model to which common defects were introduced. The heat conduction in the flip chip was analysed using numerical simulation. The thermal performance of the solder bumps was characterized by using the thermal resistances. The thermal resistances of the reference bump and defective bump were calculated respectively and the relationship between the thermal resistance and the defects size was also studied. The analysis of heat conduction in flip chip and the thermal characterization of the solder bump provide a criterion for package reliability evaluation and defects inspection.We have studied the principle and methods for the active infrared thermography. A novel approach for defects inspection of the solder bumps based on the active infrared thermography technology was proposed and the experimental setup was constructed, in which surface of the die or substrate is heated by the fiber coupled diode laser, and the temperature distribution on the top surface of the die is monitored by the thermal imager. Then the soder defects are distinguished by some characterisctic quantities derived from the thermography processing, which makes the experiments of defects inspection feasible, and offers a guideline for thermography interpretation.Experiments have been carried out to inspect the missing solder bumps of different diameter and pitch. The test vehicle S1 with the solder bumps of 500μm in diameter was detected in transmission way. Techniques of the adaptive filtering, the edge detection and the image segmentation were adopted to decrease the noise in thermograms and to eliminate the influence of emissivity difference between the UBM layer and gaps. The hotspot area over every solder bump and the temperature histogram are used to characterize the defects quantificationally. The test vehicle S2 with the solder bumps of 300μm in diameter was also detected in transmission way. The moving average filter was used to remove the random noise. The source distribution image was created to indicate the spatial nonuniformity of excitation. IR self reference method was proposed that temperature value of every edge point is substract from that of the central point at each time, and the temperature difference was accumulated all time. The defective solders are differentiated by the summation of the temperature difference. The specimen FA 10 with the solder bumps of 135μm in diameter was inspected in reflection way. The spacial and temporal filtering techniques were adopted to improve the signal to noise ratio. The recorded thermograms were input into an adaptive median filter, 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 fast Fourier transform. The defective solder bumps were indentified in the phase map at low frequency.The results demonstrate that the active infrared thermography technology is effective for identification of the missing bumps, and can also be used for inspection of solder balls in CSP and BGA packages, which provides a fast and effective method for reliability evaluation in high density packaging.