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100 SMT Magazine • August 2016 By F. Sarrazin, et al.* FLEXTRONICS INTERNATIONAL LTD Abstract Ceramics packages are being used in the electronics industry to operate the devices in harsh environments. In this paper we report a study on acoustic imaging technology for nondestructively inspecting underfill layers connecting organic interposers sandwiched between two ceramics substrates. First, we in- spected the samples with transmission mode of scanning acoustic tomography (SAT) system, an inspection routine usually employed in assem- bly lines because of its simpler interpretation criteria: flawed region blocks the acoustic wave and appears darker. In this multilayer sample, this approach does not offer the crucial infor- mation at which layer of underfill has flaws. To resolve this issue, we use C-Mode Scanning in reflection mode to image layer by layer uti- lizing ultrasound frequencies from 15MHz to 120MHz. Although the sample is thick and contains at least five internal material inter- faces, we are able to identify defective underfill layer interfaces. Introduction Since the introduction of scanning acoustic tomography (SAT) also known as C-Mode scan- ning acoustic microscopy (CSAM) technology to the semiconductor package manufacturing more than two decades ago, several thousand pieces of equipment have been serving the indus- try as essential quality assurance tools. Acoustic imaging offers inspection of imperfect material joints containing non-metal structures, such as delamination between silicon-metal joint glues, which in turn are difficult if not impossible to detect with X-ray imaging approach. Therefore, the device package failure analysis engineers routinely utilize both X-ray and acoustic imag - ing technologies as their complimentary non- destructive analysis tools. The SAT technology does have its own limi- tations derived from the physical nature of acoustical wave: requirement of liquid medium to transfer ultrasound energy, requirement of flat and smooth package surface, difficulty in designing transducers, low resolution at low- er ultrasound frequencies, less penetration at higher ultrasound frequencies, and slow acqui- sition speeds, etc. ARTICLE

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