INSPECTION


INTERNAL COMPONENT FEATURES IN ACOUSTIC 3D


By Tom Adams


Both X-ray and ultrasound make it possible to nondestructively examine


the internal structure of solid materials. Tere is little correlation between their re- spective abilities to penetrate materials: ul- trasound will pass through lead, but X-ray will not; X-ray will pass through paper, but ultrasound will not. In a great many appli- cations, they are complementary. Ultrasound, and particularly ultrasound


at the frequencies used in acoustic micro- imaging, is partly reflected by all interfaces between different materials, whether the materials are solid or not. When a 100 MHz ultrasonic pulse strikes such an in- terface, a portion of the pulse is reflected back to the transducer that launched it and the other portion crosses the interface and moves deeper into the sample, where it may encounter another interface. This scenario changes significantly if


the pulse strikes the interface between the solid it is traveling through and a gas, typically air. Virtually all of the pulse is reflected. None travels through the gas-filled gap, even if the gap has a verti- cal dimension of a fraction of a micron. Tis high sensitivity is the reason for the widespread use of acoustic tools to image cracks, delaminations, voids, and the like.


C-mode imaging Having struck an interface, a pulse re- turns in the form of an echo to the trans- ducer that launched it. In the great ma- jority of inspection applications, C-mode imaging is used because the user is pri- marily concerned with the amplitude of the echo. A pulse traveling through electronic mold compound and encoun- tering the surface of a silicon die will be about 50 percent reflected; the other 50 percent will travel deeper. Each second the transducer scans the component, it launches thousands of pulses and receives thousands of echoes, each of


16 EVALUATION ENGINEERING JANUARY 2019


which will generate a pixel in the two- dimensional acoustic image. In grayscale C-mode imaging, areas of


the mold compound that are well bond- ed to the die will appear medium gray, the result of the 50 percent reflection. Regions where there is a gap between the die and the mold compound will produce a bright white pixel because the reflection is virtually 100 percent. Black pixels mean that no echo was


received, and occur when a pulse en- counters no interface and is entirely at- tenuated by traveling through a single material such as mold compound. To produce color images, a variety of maps can be used. Cracks and other gaps are often red, but the role of a particular col- or depends on the needs of the user.


Time difference imaging The three-dimensional structure of features within a sample can be indi- cated (but not actually depicted) by an imaging mode known in Nordson SO- NOSCAN’S line of C-SAM tools as the Time Difference mode. Imagine a plas- tic IC package in which the die face is tilted rather than flat. Instead of reading the amplitude data in the return echoes,


as would be done for most production- line inspection, the tool records time in nanoseconds for the echo to travel from the reflecting interface to the top surface of the sample. Te time is then convert- ed into distance, and a color map is used to correspond to the range of distances. The most distant x-y locations sending back data might be colored magenta. The nearest might be blue, with others at their assigned places along the spec- trum. Te percent of the pulse that is re- flected is ignored in this mode, because the goal is to represent the component’s three-dimensional structure. Te result is a flat two-dimensional im-


age that uses color to tell the viewer the relative elevation of various regions on the die face without actually depicting those differences in three dimensions. When a pulse is launched from the transducer, the first material interface


SFigure 1. In 2D imaging, raised solder mask defects are indicated by color changes.


WFigure 2. The same defects in 3D.


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