Sonoscan Final DR 1/3/10 12:36 Page 24
Yield Management
and a crack within the body of the solder bump.
All three, along with the cracks shown at other
depths within the solder bump, are threats to
the long-term condition of the connection. At
the left are gates (the depths from which echoes
are used to make the acoustic image) that could
be used to image items #1, #2 and #3. In
practice, the gates could be much narrower than
shown here - for example, in order to separate
two cracks that are vertically very close to each
other. Practically speaking, there is no hard and
fast rule for the optimum thickness of a gate, but
making a gate too thin at a very high acoustic
frequency might result in too few return echo
signals to make an acoustic image.
White Bumps
“White bumps” are solder bumps that appear
white, or at least brighter than surrounding
Figure 2: Imaging this transmitter of ultrasound, but the thinning of bumps, in the acoustic image. Ultrasound
flip chip at the chip- chips is a helpful coincidence for imaging at very pulsed into a flip chip sends back its highest
24
to-bump depth high ultrasonic frequencies. As the frequency amplitude signals from the interface between a
revealed “white goes up, the depth of field decreases rather solid material and a gap, such as may occur
www
bump” defects dramatically. What this means is that a 300 MHz when there is a crack or a void inside a solder
.eur or 400 MHz transducer, without “gating” of the bump. Solder bumps without cracks or voids
oasiasemiconductor
echoes, returns in-focus data only from a depth have interfaces with bond pads at the top and
that may be less than the height of the solder bottom, but these are solid-to-solid interfaces
bump. This depth can be further narrowed by between similar materials that reflect ultrasound
excluding all of the echoes except those from a only at moderate amplitudes. They are typically
very closely defined depth of interest. gray in the acoustic image. When a flip chip is
The result is that today acoustic micro scanned at a depth that includes a crack or a
.com
imaging systems are using higher-frequency, void, the solder bump will appear white because
higher-resolution ultrasound to image smaller the gaps reflect >99.99% of the arriving pulse.
anomalies in or adjacent to smaller solder Figure 2 is the highly magnified acoustic
square4
Issue I 2010
Figure 3: Acoustic bumps. In addition to the use of higher image of one area of a flip chip. The depth of
imaging shows no frequencies, the technology has also added interest was limited to the chip-to-bump
defects at chip-to- advanced modules and techniques that raise interface because it was thought that defects
bump interface (left), imaging to even higher levels. might exist at this depth. The defects in the
but imaging at the Some of the features that may be of interest image are at the under bump metallization of
bump-to-substrate when imaging flip chips are shown in the the chip. In some areas you can see that the
depth shows bump diagram in Figure 1. Items #1, #2 and #3 are, “white bump” areas extend beyond the area of
defects (arrows) and respectively, a crack along the passivation layer, the bump bond site. Figure 3 shows an area of a
voids a disbond between the solder bump and its pad, flip chip imaged acoustically with two different
gate settings. At left, ultrasound was gated on
the chip-to-bump interface, and only echoes
from this depth were used to create the acoustic
image. This is the depth at which the solder
bump connections are bonded to the pads on
the chip face. There are no bright white features
that would indicate defects. The solder bump
interfaces are uniformly gray, which is what one
would expect from well-bonded connections.
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