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

Chip interconnection is one of the key technologies for microelectronic packaging. Theflip chip, which uses solder bumps to realize interconnection between chips and substrates,becomes the mainstream technics in microelectronic packaging because of its decreasedpackage size, larger speed of signal propagation and so on. With the development of soldebumps towards higher density and finer pitch, the chip power density will increasedramatically, and the heat dissipation will become a significant problem, the thermalmismatch in the package is also getting serious, which results in solder defects and bondingfailures. Defects inspection and characterization of the thermal perfermence for the solderbumps are more difficult as the bumps are hidden in flip chip package. The active infraredthermography technology was applied to defects inspection in microelctronic packaging inthis thesis, and the finite element method was also adopted to investigate the heat conductionin the flip chip.The thermal performance of the solder bumps was investigated using the analytical andnumerical methods. We constructed a mathematical model for heat transfer in the flip chipstructure and provided the solving procedure. A lumped thermal resistance network wasderived from the one dimension heat transfer model to which common defects wereintroduced. The heat conduction in the flip chip was analysed using numerical simulation.The thermal performance of the solder bumps was characterized by using the thermalresistances. The thermal resistances of the reference bump and defective bump werecalculated respectively and the relationship between the thermal resistance and the defectssize was also studied. The analysis of heat conduction in flip chip and the thermalcharacterization of the solder bump provide a criterion for package reliability evaluation anddefects inspection.We have studied the principle and methods for the active infrared thermography. Anovel approach for defects inspection of the solder bumps based on the active infraredthermography technology was proposed and the experimental setup was constructed, inwhich surface of the die or substrate is heated by the fiber coupled diode laser, and thetemperature 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 thethermography processing, which makes the experiments of defects inspection feasible, andoffers a guideline for thermography interpretation.Experiments have been carried out to inspect the missing solder bumps of different diameter and pitch. The test vehicle S1with the solder bumps of500μ m in diameter wasdetected in transmission way. Techniques of the adaptive filtering, the edge detection and theimage segmentation were adopted to decrease the noise in thermograms and to eliminate theinfluence of emissivity differnence between the UBM layer and gaps. The hotspot area overevery solder bump and the temperature histogram are used to characterize the defectsquantificationally. The test vehicle S2with the solder bumps of300μ m in diameter was alsodetected in transmission way. The moving average filter was used to remove the randomnoise. The source distribution image was created to indicate the spatial nonuniformity ofexcitation. IR self reference method was proposed that temperature value of every edge pointis substract from that of the central point at each time, and the temperature differnence wasaccumulated all time. The defective solders are differentiated by the summation of thetemperature differnence. The specimen FA10with the solder bumps of135μ m in diameterwas inspected in reflection way. The spacial and temporal filtering techniques were adoptedto improve the signal to noise ratio. The recorded thermograms were input into an adaptivemedian filter, and the temperature evolution of each pixel was extracted and smoothed by themoving average operation. Then the temperature-time curve was fitted with an exponentialfunction. To eliminate emissivity variations and heating non-uniformity, we converted thefitted temperature values in time domain to the phase information in frequency domain usingfast Fourier transform. The defective solder bumps were indentified in the phase map at lowfrequency.The results demonstrate that the active infrared thermography technology is effectivefor identification of the missing bumps, and can also be used for inspection of solder balls inCSP and BGA packages，which provides a fast and effective method for reliabilityevaluation in high density packaging.