Detection of delaminations in sub-wavelength thick multi-layered packages from the local temporal coherence of ultrasonic signals

Interface delamination caused by thermo-mechanical loading and mismatch of thermal coefficients is one of the important failure modes occurring in electronic packages, thus a threat for package reliability.Scanning acoustic microscopy is widely employed in the inspection of microelectronic devices. However, for advanced packaging formats such as LFBGA with flip-chip technology, whose boundaries dimensions are comparable to the acoustic wavelengths, ultrasonic echoes are often noisy and overlapped. Therefore, a more rigorous approach is required to detect anomalies, such as delaminations.A new signal processing methodology based on local temporal coherence is proposed to detect delaminations in sub-wavelength thick multi-layered packages.This work presents local temporal coherence analysis as a means of comparing two waveforms in order to provide a quantitative measure of the shape change of a signal compared to a reference as a function of time. Application of this procedure for delamination analysis in complex microelectronic packages is demonstrated.