One recent breakthrough in acoustic micro imaging makes it possible to collect and store all of the acoustic data from the whole volume of a flip chip, and to reconstruct the flip chip later from the data – i.e., to construct a "virtual" flip chip at will. In some respects, this representation is analogous to such representations by other energy forms - for example, a CAT scan, but it differs sharply in other respects because ultrasound is mechanical energy and not a portion of the electromagnetic spectrum.

The practical utility of the acoustic representation, which has acquired the name "virtual sample", is more obvious. The virtual sample is a matrix file. It is not an image, but it can be used to make at least 10,000 different acoustic images of the object, and in some instances many more than 10,000.

For makers and users of Flip Chips and other semiconductor packages, the value of the virtual sample lies chiefly in the fact that a package can be "scanned" after the physical Flip Chip has been disposed of in testing or used in production. The effort involved in scanning the physical sample to make the matrix file, and the cost of storing the matrix file, are small compared to the value of having the virtual sample on hand when a question or problem concerning the physical sample arises.

Multinational companies have begun to send that virtual sample representing a "golden part" to all of their manufacturing facilities to use as a standard. This use enables the manufacturer to compare differences resulting from process changes, variations in molding materials, and tolerance variations.

The virtual sample is made by using the Virtual Rescanning Module™, a hardware/software enhancement for C-SAM® acoustic micro imaging systems (Sonoscan). The Virtual Rescanning Module, or VRM, uses the same ultrasonic transducers and the same scanning protocol as conventional (i.e., planar) acoustic imaging, but collects data in a different fashion.

In both planar image and VRM, the transducer raster-scans the Flip Chip, pulsing and receiving return echoes thousands of time a second. Because ultrasound makes the round trip to the depths of the Flip Chip in a microsecond or two, the transducer is able to scan rapidly and yet collect data over what amounts to a grid of x, y positions.

The key to VRM lies in the details of the pulse-echo process at each x,y location. Ultrasound that has been pulsed into a Flip Chip is reflected by internal interfaces. The first interface is typically the top surface of the silicon at the back side of the chip. If the Flip Chip has been overmolded, then the top surface of the molding compound will be the first interface, and the molding compound/silicon will be the second interface.

At each interface, a portion of the ultrasound is reflected back to the transducer, and a portion travels deeper, when it is in part reflected by the next interface. Each echo contains intensity and polarity information.

If you consider the whole depth of the Flip Chip beneath a single x,y coordinate, you can picture echoes returning from several depths. From a given interface, the transducer will collect numerous echo values. The number of values available for collection at each x,y coordinate is very large.

Each x,y coordinate that is scanned can thus be envisioned as having a large number of potentially collectable echo values positioned beneath it at various depths. These echoes can also be represented as a waveform, and this waveform is displayed on-screen when the operator is making a planar image.

In making a virtual sample, the Flip Chip is scanned several times. At each scan, the electronic gate is set deeper. The successive gates encompass the entire thickness of the Flip Chip. The focus and gain are optimized for each gate, just as they are optimized for the single gate used in making a planar image.

This explains why the virtual rescanning method does not produce at its conclusion a single image. Instead, it produces the matrix file from which thousands or tens of thousands of individual images can be made. An engineer can "rescan" the matrix file in the absence of the Flip Chip to produce any planar image he wishes to define - at any depth, showing any internal feature. A planar image made from the matrix file will look much like a planar image made from the physical Flip Chip.

But there is a dark line near the edges of the delamination (arrows). This line marks the edge of a thin layer of underfill still sticking to the chip face. In the areas marked by this line, the delamination is between the bulk of the underfill below and the thin layer of underfill remaining on the chip face. But the two areas (delamination-underfill vs. underfill-delamination-underfill) are very nearly the same shade of gray, and are not easily distinguished in this planar image.

A 230 MHz transducer might pulse a frequency range from 170 to 260 MHz, for example. In making a planar image, the highest echo value for each x,y position is selected, without regard to the frequency of the echo.

In rescanning a matrix file, the operator may elect to perform FFT separation on the image from a desired depth showing, for example, the bond of the solder bumps to the pads on the chip face. The result will be some dozens of individual FFT images each showing what that interface looks like at a single frequency - 170 MHz, 171 MHz, etc. Some of the FFT images will look similar, but the contrast will differ, sometimes strikingly, from one image to the next. The differences in contrast are often helpful in seeing in great detail the fine points of the Flip Chip package construction. The all-frequency planar image will show defective solder bonds. The individual FFT images will also show the defective bonds, but may give more information about the defects.

Summary

The Virtual Rescanning Module makes and stores a virtual sample containing all of the acoustic data of the physical sample. In the absence of the physical sample, the virtual sample can be rescanned to produce any of thousands of planar image, three-dimensional images, and single-frequency FFT images.

Contact information:

E-mail: info@sonoscan.com