Evaluation Engineering - 26

AUTOMATED TEST

Front View
In a TO-247, a completed acoustic image made from the front
side will reveal the die, any extruded die-attach material around
the die, any delaminations of the mold compound from the heat
sink, any voids in the encapsulant, as well as the leads and the
wires bonded to the leads. The leads are imaged separately because echo collection by the receiver is generally gated on (i.e.,
restricted to) a specific depth range of interest. A gate wide
enough to encompass vertically both the leads and the die in
a TO-247 would likely be out of focus at one of those depths.
Figure 1 shows approximately where each gate may lie.
Except where they're bonded (or not bonded), the wires produce
few echoes because their curved surface scatters ultrasound in
various directions.
Figure 2 is the Gate 2 topside image of a TO-247 imaged by
a 15-MHz transducer having a focal length of 0.75 inch. The
die and much of the heat sink are within the image at the top
of Figure 2. The most obvious defect is the red areas marking delaminations of the encapsulant from the heat sink. The
delaminations may contain air or possibly a vacuum, both of
which block ultrasound from crossing the gap, even if its vertical
extent is a tiny fraction of a micron.
Acoustic evaluation of TO-247s and other high-power components considers the likelihood of expansion of structural defects.
In this device, the delaminations are already substantial and
could conceivably reach the die. Red was used to color them
because the the air-to-solid (mold compound) produces the
highest possible echo amplitude (>99.9%). Red is at the bottom
of the color map along the left side of Figure 2.
Red regions mark negative reflections, where the pulse
encounters a material of higher acoustic impedance (= density × acoustic velocity) followed by a material lower acoustic

impedance. In this color map, the reverse situation-positive
polarity-would be white.
There are additional defects above and below the die. The
white features aren't voids but rather excess-a foreign substance
that somehow was incorporated during assembly of the device.
They're white because an arriving pulse went from a material
(mold compound) of lower acoustic impedance to a material of
higher acoustic impedance. This reflection has positive polarity.
The lower feature has a tiny red void next to it. The upper
feature has a tiny area that appears red, not because it's a void,
but because the curved surface of the excess die-attach material
is round enough to scatter the arriving ultrasound.
Using a shallower gate (Gate 1), the leads to which the wires
from the die are attached were also imaged (bottom of Figure
2). The wire at left has the desired image: a light region near the
edge of the lead, and a darker region below it. The light region
indicates good bonding. The wire on the lead at right may be
less securely bonded, although there are no red areas that would
indicate an air-filled delamination.

Figure 3: This image, created with echoes from Gate 1, shows the wires on
the leads.

Back View
The TO-247 was then flipped right to left and imaged from its
back side, through the heat sink (Fig. 3). Most of the regions
that are red in the front-side image are white here because a
grayscale color map was used. Gray areas are well-bonded. A
few tiny voids are also in the die attach.

Imaging Another TO-247

Figure 2: Made with echoes originating in Gate 2, this image shows the die
and heatsink.

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The top view of a second TO-247 (Fig. 4) was also imaged at 15
MHz by a transducer having a focal length of 0.75 inch. The die
at left, which is somewhat obscured, is slightly larger in area
than the die at the right. The red regions are delaminations of
the mold compound from the heat sink and (at left) the die.
Yellow regions may be partially bonded or may be tilted. Both
conditions will reflect slightly less ultrasound than the fully
delaminated regions. In a high-power device, defects of this
magnitude make early field failure very likely.
The two leads are imaged at the bottom of Figure 4. The wire
at far right has the same appearance-a pale region near the

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