Sonoscan Final DR 1/3/10 12:36 Page 25
Yield Management
During actual scanning of a flip chip, Figure 4: Time Domain
ultrasound gated on the full height of the solder acoustic image shows
bumps may show that one or more defects are a few voids in the
present, but not their exact vertical locations. underfill, but a
Depths of interest within a flip chip are not Frequency Domains
confined to the solder bump or its connections. image reveals far
There may be cracks in the passivation layer near more voids
the face of the chip, in the low-k dielectric layers
within the layers of metallization on the chip, or
Issue I 2010
within the bulk silicon of the chip. Cracks occur
square4
within the low-k dielectric because the materials
.com
used are both porous and fragile.
Other Techniques
Small dimensions within flip chip configurations
mean that anomalies can be very subtle and still
present a real threat to long-term reliability.
Subtle anomalies may be hard to image
oasiasemiconductor
meaningfully using time domain imaging - i.e.,
.eur
imaging in which echoes are collected from
www
within a specified depth (time window). The
purpose of time domain imaging is to create an
25
image based on the amplitude of return echo
signals. A 300 MHz transducer puts out a pulse
of multiple frequencies ranging roughly 50 MHz
or more on either side of 300 MHz. The return
echoes are distributed along a similar range. Within this range, roughly 20 to 30 different
Software simply selects the highest-amplitude acoustic images might be constructed from the
echo, regardless of its frequency, to determine VRM data. No two images will be identical,
the brightness of the pixel at a given x-y since slightly different frequencies are often
coordinate. The time domain acoustic image is reflected very differently by the same interface.
thus constructed from return echo signals of Among the many VRM images there may be
various frequencies. The invention of the virtual images that reveal details that cannot be
rescanning module by Sonoscan made more imaged by time domain imaging.
extensive analysis of subtle anomalies far easier. Figure 4 shows both the time domain and
The flip chip or other part is scanned to collect frequency domain images made from the virtual
all the acoustic data at every pixel location, and rescanning file of a flip chip. The time domain
the total acoustic data for the part is saved in image shows two voids (marked by the arrow)
one file from which thousands of different next to two solder bumps - a problem because
images can be made. Any depth can be imaged the solder may eventually creep into the void
in focus - something that generally cannot be and break the electrical connection. The
done with an ordinary time domain scan. frequency domain image uses one of the
The virtual rescanning module also permits multiple frequencies into which the signal
frequency domain imaging, a very powerful preserved in the virtual rescanning file was
technique for imaging and understanding subtle decomposed. This image shows the original pair
anomalies. Since the virtual rescanning file of voids and about 25 more that were not
contains all of the return echoes, the data can be detected by time domain imaging.
used to select a very specific depth and to make Acoustic micro imaging employs high
single-frequency images. If the file was made resolution, high magnification and precise depth
from scanning with a 300 MHz transducer, the selection in the non-destructive examination flip
available frequencies with adequate return chips. The data obtained is useful in ensuring
echoes might range from 275 MHz to 325 MHz. the long-term reliability of the devices.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36