A new application of acoustic micro imaging: screening MCM-C multilayer defects

Abstract

Acoustic Micro Imaging (AMI) has been successfully applied for detecting soldering and underfill failures when analyzing surface mounted devices (SMD), ball-grid array (BGA) and flip-chip mounting, as well as to find internal defects, discontinuities, delaminations, and leakages in plastic packaged ICs. This paper highlights new possibilities of AMI in analyzing multilayer defects in thick-film structures. Studying a model system of multilayer thick film, it has been found that C-mode Scanning Acoustic Microscopy (C-SAM) technique seems to be a good candidate for differentiating not only the location surface defects, subsurface bubbles but also short circuit locations, nondestructively. Low Temperature Cofired Ceramic Multichip Module (MCM-C LTCC) interconnection substrates and thick-film multilayer structures are generally built up from relatively high-glass-content dielectric and conductive materials, causing one of the most important problems of the technology. In the recent failure analysis investigations, it has been demonstrated that not only rude blistering effect or pinholes can cause short circuit failures leading to decreased yields, but ionic migration and dendritic growth through the melted glass electrolyte can also occur. Short circuit locations remain generally undetected in conventional morphology studies of the surface performed with optical and scanning electron microscopy. Thus, they can only be analyzed by destructive methods, after polishing or cross-section preparation, even then the right position can be located only with difficulties. A special method has been developed by analyzing optical and acoustic multilevel picture for revealing the short circuit locations nondestructively. Performing conventional destructive investigation followed by scanning electron microscopy (SEM) and electron microprobe analysis proved the latter finding. This may be a relevant contribution for future multilayer failure analysis processes.