One advantage of non-destructive cross-sectioning is that the same sample can be sectioned acoustically in as many vertical planes as desired with no damage to the sample.
Acoustic micro-imaging tools are usually pictured scanning flat subjects such as plastic-encapsulated ICs, JEDEC trays filled with components, multilayer ceramic chip capacitors or other flat items. The tools use a horizontally-scanning ultrasonic transducer that pulses ultrasound into the subject thousands of times per second and analyses echoes returned by material interfaces to produce pixels for the acoustic image of the subject.
The tools excel at performing scans to produce top-view images of internal structural features, especially sub-surface and internal gaps (cracks, delaminations, voids, etc) that can cause failures. Planar raster scanning of the sample’s x-y area is known as C-mode imaging. The method is non-destructive and a handy compliment to X-ray because it easily provides top-view detection of very thin gaps (down to a small fraction of a micron) that X-ray cannot see.
The acoustic tool can also be used to collect z-dimension data from a vertical plane to produce a non-destructive cross-section through a sample. One advantage of non-destructive cross-sectioning is that the same sample can be sectioned acoustically in as many vertical planes as desired with no damage to the sample. The planes do not need to be parallel to each other; the sample can be rotated as desired and then scanned.
This imaging mode, known as Q-BAM (Quantitative B-scan Analysis Mode), was developed by Sonoscan (now Nordson SONOSCAN) and is used by the company’s C-SAM line of acoustic micro-imaging tools. The operator of the tool first defines an acoustic cut line on the top surface of the sample in a location that will give the best information in a cross-sectional view.
Background: C-mode imaging
In horizontal imaging, the transducer scans across the sample in a straight line, advances the width of one line and scans back across the sample. As it scans, the transducer sends thousands of pulses of ultrasound each second into the sample’s surface and receives from each pulse the returned echoes, if any, before launching the next pulse. When it has scanned the entire area of the sample, it will have collected all data points needed for the acoustic image.
Within the sample, the pulse of ultrasound is reflected only by material interfaces. Homogeneous materials, such as underfill or silicon, may absorb varying amounts of the ultrasound, but being homogeneous these send back no reflection. Interfaces between two solid materials—mould compound to die face, for example, or die face to underfill, or a crack in a die—send back reflections whose amplitude ranges from very low to very high, depending on the properties of the two materials at the interface. But if one of the interfaces is not a solid but a gas such as air, amplitude is extremely high—close to 100 per cent of the ultrasound is reflected as a very high amplitude echo. This is what makes it so easy for a C-SAM tool making horizontal images to find cracks, voids and other gaps, and to ensure that non-gap interfaces are intact and in the right location.
Echoes returning from different depths within the sample will arrive at the transducer at different times. This allows the system to assign a physical depth to the material interface that returned each echo. In most horizontal imaging, echoes are collected only from a depth (known as a gate) of highest interest, while echoes from above and below this depth are ignored. Gating on only part of the sample’s thickness avoids having two features in the same x-y location but at different depths superimposed in the acoustic image.
In Q-BAM imaging the transducer scans back and forth only along the single line defined by the operator. As in C-mode imaging, it sends thousands of pulses into the sample each second and receives return echoes where there are material interfaces. On the first pass, only the echoes from a very thin gate at the bottom of the sample are collected and used for imaging. All of the echoes above this thin gate are ignored.
At the end of this first scan along the line, the transducer moves slightly upwards to change the focus and then the transducer heads back along the same line. Once again, echoes are returned from material interfaces at all depths, but only echoes from this second very thin gate will be collected to become pixels in the cross-sectional acoustic image. After numerous back-and-forth scans and altitude adjustments, the transducer reaches the top of the sample and the cross-sectional image is complete. The image will be dimensionally identical to an optical image made after sawing through the sample along the line and then photographing the exposed face. The same internal features, including defects, will be in the same places.
Fig. 1 is the top-view image of a plastic-encapsulated IC that has suffered a popcorn crack. Popcorn cracks typically originate in or near the die attach material and expand downwards or (as here) upwards. This image was gated on a depth from just above the die face to just below the die attach. Note that the gate from which echoes are accepted does not extend all the way to the top surface of the package.
The circular red-and-yellow feature is the top surface of the upwardly-slanted popcorn crack. Red-and-yellow features are gaps. On the face of the die, small red areas are horizontal delaminations between the die and the mould compound.
In Fig. 2, the Q-BAM cross-sectional image through the package is at the top and replaces part of the C-mode image. For vertical measurements, the 10 bars to the left of the Q-BAM image divide the thickness of the scan depth into 10 segments, each 0.028mm in vertical extent. The horizontal white line near the bottom of the C-mode image marks the vertical plane from the bottom to the top of which the transducer collected return echo signals.
Starting at the left side of the C-mode image, the white line passes through the lead finger, tape and tie bar. These features reflect little ultrasound and can be seen as a very faint sequence of features in the Q-BAM image at the top. Farther along, the white line passes through the red popcorn crack; the Q-BAM image of the crack shows that, along this line, the right end of the crack extends upwards. Beyond this point only a few faint structures are visible.
The white line passes through the die face in Fig. 3. The small red features in the C-mode image are small (but dangerous) delaminations between the die face and the mould compound. These appear yellow in the Q-BAM image. To the right of the die, the popcorn crack extends a short distance upwards.
The Q-BAM image has been moved to the bottom in Fig. 4 to avoid impinging on the white line. The lead finger and tie bar reflections are faintly visible here, as is a part of the popcorn crack at the left of the die. At the right in the Q-BAM image, the popcorn crack can be seen extending all the way to the top of the package. In the C-mode image at the top, the crack does not appear to extend so far to the right, but a close look at the C-mode image shows a dark region to the right of the yellow crack. The crack extends above the top of the gated depth, but the black region is that vertical portion of the gate from which no echoes were received.
In Fig. 5, the white line runs along the very edge of the die. The Q-BAM image reveals—faintly—the die face delaminations, as well as the upward crack at the right. The crack here reaches the surface above and once again is in part displayed in the C-mode image as a black area from which no echoes were received. Note that the two mould marks at the top corners of the package are also black because these lie above the gate.
The IC package imaged here by the Q-BAM method could be imaged along other lines parallel to these lines, if desired, or the package could be rotated and imaged along a line in any horizontal direction. Two advantages of the method are lack of damage to the part and sensitivity of ultrasound to very thin gaps. In C-mode imaging, a delamination for only a small fraction of a micron thick shows up strongly. In Q-BAM, the cross-section through the same delamination will show up just as strongly.
Q-BAM is sometimes used in conjunction with destructive physical analysis, often because a very thin gap may be difficult or impossible to see optically after physical sectioning. It is frequently used to plan locations for destructive physical analysis. C-mode and Q-BAM techniques image and locate the gap in three dimensions before physical sectioning is performed.
Tom Adams is a consultant at Nordson SONOSCAN