October_AMP_Digital

other gap-type defect will virtually all be reflected to the

transducer and will produce a bright white pixel in the

acoustic image. Bright white features indicate gap-type de-

fects, whereas nongap features appear as shades of gray.

Gaps are often pseudocolored red to make them stand out.

No portion of the pulse crosses the defect (which has two

interfaces) to go deeper into the sample.

The process described here illustrates how planar

acoustic images are made. They usually display the internal

features of the whole area of a sample, which might be any-

thing from an electronics component to a medical pack-

age seal to a synthetic diamond cutting tool material. The

process for creating a nondestructive cross-section differs

chiefly in that the transducer does not scan the entire

x-y

area of the sample, but rather scans back and forth along

the single straight line that sets the location for the verti-

cal plane.

Integrated circuit case study

Figure 2 is a planar acoustic image of a plastic-encap-

sulated integrated circuit (IC). The fine lines radiating in all

directions are metal lead frame fingers while the four heav-

ier lines extending to each corner are tie bars. The red cen-

ter marks the location of the silicon die; red (highest

amplitude return echoes) indicates that the die is delami-

nated from the plastic encapsulation above it. The pale

gray square near the outer ends of the lead fingers is tape

that holds the lead fingers in place during IC package man-

ufacturing. To learn more about the delamination, and per-

haps to determine its cause, cross-sectional images were

made along a line intersecting the die near its lower edge,

and along a line at the center of the die.

The cross-sectional image made along the first line,

near the edge of the die, is shown in Fig. 3. The strong red

and white horizontal lines at the top indicate the interface

between the plastic (solid) and the water (liquid) at the top

surface of the IC package. Working downward from the

surface, there are no echoes and no material interfaces

until the horizontal red line (arrow) that marks the delam-

ination between the die face and the plastic above it. De-

lamination extends along the entire width of the die and

reflects >99.99% of the ultrasonic pulse.

Figure 4 is the cross-sectional view along the second

line at the center of the die. In this vertical slice, the red

delamination appears to be warped upward. But the red

line only images the interface between the mold compound

and the empty gap; ultrasound does not penetrate into the

gap. The gap probably formed when the curing mold com-

pound pulled away from the silicon and formed a shallow

bubble that is thickest above the center of the die.

While most samples imaged by this method are flat,

cylindrical samples can also be imaged by using a fixture

that rotates them. The straight line that defines the verti-

cal plane is drawn around the circumference of the cylin-

der at a given spot along its length. The transducer remains

stationary just above the sample, and scans the deepest

area of interest while the cylinder rotates through 360°. It

then scans the next deepest level until the surface of the

cylinder is reached.

Figure 5 shows the planar image (top) and cross-sec-

tional image (bottom) of a plastic tube that is bonded by a

polymer adhesive to a metal rod. In the planar image, the

red-black features are bubbles in the adhesive. As part of

the process modification, it was important to learn the

depth at which the bubbles lay.

To create the Q-BAM image, the transducer scanned

along the bottom edge of the planar image. The strong red

line at the top of the cross-sectional image represents the

interface between the plastic tube and the water couplant,

Figs. 3 and 4. There is very little difference in acoustic

ADVANCED MATERIALS & PROCESSES •

OCTOBER 2014

Planar view

Top side

15 MHz

Fig. 2 —

Planar acoustic image looking down into a plastic-

encapsulated integrated circuit. The two lines indicate the

locations for cross-sectioning.

Cross-section

Fig. 3 —

Acoustic cross-section along line 1. The horizontal red line in the

lower half of the image is the delamination that separates the die from the

plastic encapsulant above it.

Fig. 4 —

Sectioning along line 2 suggests that the delamination is thickest

over the center of the die.