February, 2018
www.us-tech.com
Page 69
Measuring Surface Flatness with Acoustic Micro Imaging Tools Continued from page 66
material of lower acoustic velocity to a material of higher acoustic velocity, and negative if the reverse. The arrival time tells the distance of each x/y coordinate from the transducer, and thus its altitude on the sample. Sonoscan calls this method Acoustic Surface Flatness™ (ASF). Figure 2 is the ASF image of an
unpopulated FR-4 PCB measuring 7.5 x 3.5 in.(19 x 8.8 cm). It was imaged by a C-SAM system using a 100 MHz transducer and a focal length of 0.5 in. (1.3 cm) The color map at the left border shows the range of elevations. If the board were perfectly flat, or even acceptably flat, the entire board might display only a single color. But that is not the case here. The lowest surface regions are dark red, along the upper left edge. The most elevated surface
regions are near the bottom-center and are pink. The maximum differ- ence in elevation between two sur- face points in these regions is 0.06 in. (1.5 mm). Holes where the surface is absent are black. The white rectan- gular feature at the lower left of the image is a hole in the board. It is white because it is so far from the other values on the color map. Each ultrasonic pulse struck a
single x/y coordinate and reported the distance to the coordinate. This distance was converted into color. But the echo also reported the ampli- tude of each signal. Surface features large and small differ slightly in dis- tance and/or amplitude from the sur- rounding area. These small differ- ences make many surface features visible. In some regions, however, the resulting contrast is very slight and few details are visible. There are few details in the green region, for example.
Causes of Warpage Warped PCBs fall into two cate-
gories. Many are warped over their entire surfaces. Some, however, have strictly local warpage, typically caused by a defect within the body of the board, such as a bubble between layers.
Surface flatness is also meas-
ured in many plastic-packaged com- ponents. Although any component package can be warped, warping is especially critical in thin, large-area components, such as BGAs. Warpage may simply be the result of forces encountered during production of the component, creating packages that are dished (the edges are the highest points) or domed (the center is the highest point). Warpage will likely be more complex and less symmetrical if an internal gap, such as a void, is present. Package-on-package (PoP) as -
sem blies can also display surface dis- tortions caused by warpage. De - viations from flatness are often seen in packages where a memory chip rests on top of a processing chip. Like individual components,
wafers also have flatness issues, but the nature of the risk is not the same. A significantly non-flat component is unlikely to be used in populating a board, but a significantly warped wafer may contain nothing but flaw- less components. But, the wafer still needs to be
scanned acoustically to image and evaluate the features within each device. The acoustic micro imaging
See at ATX / MD&M West, Booth 1821
tool’s transducer expects a wafer to be flat. If it is not, regions above and below the expected focal plane will
not be imaged. Sonoscan solved this problem by incorporating distance-measuring
technology to automatically adjust the transducer during the scanning of wafer. The resulting image of a warped
wafer will show, acoustically, a flat wafer, but all anomalies will have been found. The tool can even take the tech-
nology one step farther: if the devices in the wafer are destined to be used in a stack, it can measure the warpage of individual devices to determine their fitness for stacking. Contact: Sonoscan, Inc., 2149 E
Figure 2: Acoustic map showing elevation difference on a PCB.
Pratt Boulevard, Elk Grove Village, IL 60007 % 847-437-6400 fax: 847-437-1550 E-mail:
info@sonoscan.com Web:
www.sonoscan.com r
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