Feature: Scanning technology


Compared to other 3D package types, for


example, stacked dies with through-silicon vias (TSV) require much smaller, finer pitch solder bumps that create additional challenges in defect detection, since any defective chiplets in the package will result in a non-functional package – even if all other modules are functional. Given the combined value of the chiplets,


interposer and other components, a single defective chiplet or poor interconnection can render the entire 3D package non-functional. Tis is driving the requirement for 100% inspection during manufacturing, using non- destructive testing methods.


Non-destructive testing of 3D packages Te challenge today is to perform 100% inspection with relatively high throughput to identify and remove 3D packages or components that do not meet quality requirements. Among the available non- destructive methods, scanning acoustic microscopy (SAM) is the most widely used for testing and failure analysis involving stacked dies or wafers. SAM uses ultrasound waves to non-


destructively examine internal structures, interfaces and surfaces of opaque substrates. Te resulting acoustic signatures can be processed into three-dimensional images that are analysed to detect and characterise device flaws such as cracks, delamination, inclusions and voids in bonding interfaces, as well as to evaluate soldering and other interface connections. Te unique characteristic of acoustic


microscopy is its ability to image the interaction of acoustic waves with the elastic properties of a specimen, and thus examine the interior of an opaque material. Scanning acoustic microscopy works by


directing focused sound from a transducer at a small point on a target object. Te sound, hitting a defect, inhomogeneity or a boundary inside the material, is partly scattered and will be detected. Te transducer transforms the reflected sound waves into electromagnetic pulses, which are displayed as pixels with defined gray values, thereby creating an image. To produce such an image, samples are scanned point by point and line by line. Scanning modes range from single layer


views to tray scans and cross-sections. Multi- layer scans can include up to 50 independent layers. Images from different depths can be combined into a single scan as well, called Tomographic Acoustic Micro Imaging (TAMI). When even higher throughput is required,


up to four transducers can simultaneously scan for higher throughput. Multiple transducers can be used on a single substrate and the images then stitched together, or multiple transducers can simultaneously scan multiple substrates. “Scanning acoustic microscopy provides


non-destructive imaging of defects and delaminations in die and package materials,” says Lisa Logan, Applications Manager, Scanning Acoustic Microscopes for PVA TePla Analytical Systems, a company that designs and manufactures advanced SAMs for both laboratory and production environments. “SAM is particularly useful for inspection of small, complex three- dimensional devices. Te equipment is highly sensitive to the presence of delaminations and air-gaps at sub-micron thicknesses.”


PVA Tepla 302 HD microscopy system


Handling the defects The most common defects in 3D packaging are delamination, cracks in substrate, die tilt, misalignment and void in micro-bumps, bump defects, solder bridging, popcorn cracks, voids in underfill and voids and delamination in TSVs. The resolution of the microscopic images depends on the acoustic frequency, the material properties and the aperture of the transducer. The frequency of the ultrasonic signals generated for 3D package inspection is typically within the broad range of 15-300MHz. Transducers, the heart of all SAM


systems, play such a critical role that manufacturers like PVA TePla Analytical Systems design and manufacture them in a proprietary thin-film technology process. The frequency of the ultrasonic signals


can even be increased into the GHz range, which enables to detect defects in the sub-micron-range. PVA TePla’s high-resolution, GHz-frequency SAM tool, for example, successfully detects voids in TSVs of 5-micron diameter and 50-micron depth, immediately after plating. According to Logan, several leading


suppliers of programmable logic devices have already evaluated and purchased high-resolution SAM equipment for non-destructive analysis of next- generation 3D products to scan for packaging anomalies. “3D chip manufacturers are trying


PVA TePla analytical systems with auto trays


to push the limits on what defects they can detect,” she said. “So, today the evaluation of scanning acoustic microscopy equipment often comes down to which equipment delivers the highest resolution at fastest throughput speeds for 100% inspection.”


www.electronicsworld.co.uk May 2021 37


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