Apparatus for rapid non-destructive measurement of die attach quality in packaged integrated circuits

A method and apparatus is disclosed for the non-destructive measurement of die-attach quality in packaged integrated circuit. The apparatus is used in a production line and uses acoustical pulses to generate signals from within the integrated circuit indicative of the die-attach quality.

FIELD OF THE INVENTION 
The invention relates to semiconductor devices and more particularly to 
apparatus and method of detecting voids in the die-attach layer of 
integrated circuit packages and delaminations between the integrated 
circuit chips and the packaging material in plastic packaged integrated 
circuits using acoustic waves. 
DISCUSSION OF THE PRIOR ART 
Innovations in ultrasonic non-destructive evaluation, or NDE, closely 
followed advances in electronics technology. As the capability of 
pulse-echo detection electronics improved, large-scale techniques 
developed in the sea evolved into laboratory ultrasonic NDE devices. The 
field of ultrasonic NDE was launched by Floyd Firestone, a physicist at 
the University of Michigan. In 1942, Firestone received a patent on the 
"Ultrasonic Reflectoscope" which detected voids or cracks inside of 
manufactured parts by the pulse-echo technique using a contact transducer. 
Immersion methods were developed soon thereafter. The development of 
pulse-echo RADAR in 1938 provided the electronics capability for 
Firestone's Reflectoscope. Firestone's original Reflectoscope operated in 
the range of 2-25 MHz which provided a wavelength of 0.2-3 mm in steel. 
Today's commercial NDE devices are essentially mini-SONAR systems, many of 
which can also produce an image, and are remarkably similar in principle 
to the early Reflectoscope. 
Although commercial acoustic microscope manufacturers have offered images 
of the die-attach layer in ceramic-packaged IC's, very little work has 
been done in capitalizing on this capability until quite recently. In 
1986, Raytheon completed a report for the Rome Air Development Center on 
techniques for imaging die-attach in ceramic-packaged IC's. This report 
recommends pulse-echo acoustic imaging above all other techniques 
including x-ray radiography and scanning laser acoustic microscopy (SLAM) 
due to the contrast and reliability of the pulse-echo image. The report 
made this recommendation in spite of the noted lack of automation 
available for this type of evaluation. 
The die-attach layer in ceramic-packaged IC's can be imaged with higher 
frequencies than used for plastic package inspection due to the superior 
sound transmission in the ceramic material. 
The pulse-echo principle of the SONAR system is the basis for the IC 
package acoustic microscopy. In both systems a sound pulse of short 
duration is transmitted at a relatively low repetition rate and the time 
delay before an echo is received is measured. 
In the case of SONAR, this delay indicates the distance to the target. And 
in the reflection SAM, the echo signals are gated in time so that the 
image can be generated of a plane at a specific depth within the sample. 
SUMMARY OF THE INVENTION 
The invention is similar to that disclosed and claimed in copending patent 
application Ser. No. 172,043, filed Mar. 23, 1988, and entitled APATUS 
AND METHOD FOR AUTOMATED NON-DESTRUCTIVE INSPECTION OF INTEGRATED CIRCUIT 
KAGES. 
The present invention provides an in-line process control sensor that will 
report the quality of die attach in a packaged IC averaged over the total 
die attach area. Instead of a focused probe like that used to form an 
image of a sub-surface interface in the above referenced copending patent 
application, the present invention uses a plane wave transducer that 
insonifies a square field the size of the bar inside the particular 
package being examined. This size match will exclude reflections from the 
leads. The data acquisition can result from a single measurement with an 
immersion transducer that produces a sound field the same size as the bar, 
or by a contact transducer that produces a similar sound field. 
If the bar is larger than the largest available transducer size, a point 
scan, line scan, or an area scan (where the area is smaller that the bar 
size) can be integrated over the whole bar to give the same results. A 
scanned array may also be used. 
A plastic packaged integrated circuit is imaged through the top, therefore 
the integrated circuit package is inverted and makes contact to a 
permanent delay contact transducer or to water in the use of an immersion 
transducer. In the case of the permanent delay contact transducer the 
cross section of the delay medium is the same size and shape as the bar. 
In the case of a water immersion system, a short water path is 
established, using a water bath, between the transducer and the package. 
The transducer can be located above or below the integrated circuit 
package in the water bath. In the water immersion system, either the 
transducer is the same size as the bar, or an aperture is placed between 
the transducer and the sample. The aperture adjusts the size and shape of 
the transducer sound field to match that of the bar. The aperture material 
attenuates and/or delays the sound field surrounding the transmitted 
field. 
In the application to plastic packaged integrated circuits, the intensity 
of the reflection from the package/bar interface is compared with the 
intensity from the bar/die attach interface. The reflection from the 
package/bar interface is usually featureless and continuous in a part that 
has seen no thermal testing. The intensity from the bar/die attach 
interface is significantly stronger where the die attach is poor due to 
the reduced acoustic transmissivity. Comparison of this ratio to a 
threshold value based on a certain area fraction of good bonding will 
determine whether or not a part passes. 
Also, this device may be useful for checking delamination at the 
package/bar interface in plastic packaged integrated circuits. This is a 
common mode of failure during thermal cycling, especially with large bars. 
When delamination occurs, the intensity of the package/bar reflection 
increases dramatically and the intensity of the bar/die attach interface 
goes to zero. A similar calibrated ratio test indicates the extent of 
delamination. 
The intensity from the primary subsurface interface will pass through a 
minimum due to the cancellation of the reflection from the bonded area by 
the inverted reflection from the delaminated area. The primary subsurface 
interface is defined here as either the package/bar interface or the 
package/lead frame interface. The fraction of the total primary subsurface 
interface area that is delaminated can be determined by (1) taking the 
ratio of the amplitude of the primary subsurface interface area to the 
amplitude of a deeper interface reflection (such as the die attach 
interface reflection, for example); and (2), by measuring the general 
phase (i.e. inverted or non-inverted) of the primary subsurface interface 
reflection. 
The technical advance represented by the invention as well as the objects 
thereof will become apparent from the following description of a preferred 
embodiment of the invention when considered in conjunction with the 
accompanying drawings, and the novel features set forth in the appended 
claims.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 illustrates one configuration of the apparatus of the present 
invention using, for example, a water immersion transducer. The integrated 
circuit device, for example a plastic packaged integrated circuit, is 
inverted in fixture 11 so that the top of the package 19 is placed into 
the water bath with immersion transducer 17. Acoustic waves 18 are 
transmitted from transducer 17 through the water bath and plastic package 
15 to the semiconductor bar 12. Bar 12 is attached at 14 to the lead frame 
bar pad 13. Each of the interface junctions will reflect waves back to the 
transducer 17. 
There will be a reflection at the package interface 19, the semiconductor 
bar/package interface and the interface where the semiconductor bar 12 is 
attached to the bar pad 13. 
FIG. 2 shows the portion of the acoustic pressure signal that includes the 
reflections from a plastic-packaged integrated circuit. The abscissa 
(Time(nsec)) is the time delay relative to the generation of the acoustic 
pulse at the transducer. The ordinate (Amplitude (arb)) is reflected pulse 
intensity in arbitrary units. The first pulse to arrive at the transducer 
between 17.5 and 18 microseconds delay is from the package surface. At 
about 1 microsecond later, the reflections from inside the package arrive 
at the transducer. These internal reflections are from interfaces such as 
the package/bar interface and the bar/die attach interface. In this 
particular figure (FIG. 2), a transducer center frequency of 10 MHz was 
used, and the package/bar and package/die attach interface reflections 
partially overlap in the time domain signal. The amplitudes of 
non-overlapping portions of these reflections are measured. The amplitudes 
are used to form a ratio for comparison with a threshold value to indicate 
the fraction of the total die attach area that is bonded. 
The package/bar and bar/die attach interface reflections in FIG. 2 are 
shown separated in FIG. 3 with an expanded scale. This separation is 
performed by mathematical subtraction of an inverted copy of the acoustic 
pulse reflected from a delaminated portion of the package/bar interface. 
This reflection from a delaminated portion of the package/bar interface is 
shown in FIG. 4. Note that this reflection is inverted, or 180 degrees out 
of phase with the reflection from a bonded portion of the package/bar 
interface. It should be noted that the reflections in FIG. 3 are 
completely temporarily resolved if a transducer of higher center 
frequency, such as 25 MHz is used. 
It should also be noted that when the intensity of an interface reflection 
is measured, it is the absolute value, or rectified, intensity that is 
measured. This is a due to the inversion of the reflected pulse, for 
example, at a delaminated portion of the package/bar interface as compared 
to that from a bonded portion of the interface. 
The application of this concept to determining die attach area in 
ceramic-packaged IC's is more straightforward. In the case of using a 
permanent delay contact transducer, ceramic-packaged parts are examined by 
placing the package on the solid delay line of the transducer 
right-side-up due to the air gap under the lid (FIG. 5). 
Instead of a permanent delay contact transducer, a water immersion system 
can be used and the transducer can be above or below the IC package. The 
key reflections from a ceramic-packaged part are from the ceramic/bar 
interface and from the bar surface. The die attach adhesive transmits the 
acoustic signal very well where the bonding is good. These reflections are 
strong, well resolved and in a reciprocal relationship to each other. 
FIG. 5 illustrates apparatus used in testing ceramic dip packages using a 
permanent delay contact transducer, for example. Water immersion can also 
be used with ceramic packages. Device 30 resides on fixture 31 and over 
transducer 32. Acoustic waves 33 of between 5 and 100 MHz travel through 
the ceramic package to the ceramic/bar interface at 36 and through the 
semiconductor bar 38 to the bar surface 35. The device is enclosed with 
lid 34, but this part of the package has no affect on the test. 
FIG. 6 shows an acoustic image formed from reflections from several 
sub-surface interfaces in a ceramic DIP using an acoustic microscope. 
FIG. 7 is an image of the die attach area in FIG. 6. This image is formed 
from only the ceramic/bar reflection. 
FIG. 8 is a similar image formed from only the bar surface depth. Note how 
one is the opposite of the other. The acoustic pulses transmitted through 
the well bonded areas are reflected at the bar surface. A ratio of the 
amplitudes of these two reflections will give a sensitive measure of 
average die attach area. 
The apparatus illustrated in FIGS. 1 and 5 may be used in an automated 
production line. The devices to be tested may be transported to a position 
over the transducer and then moved as the next device to be test is 
brought into position, no operator is needed since the input and is 
totally automated. Each reading should be very fast, about 10 seconds per 
device. 
The reflected pulse detector will automatically locate the appropriate 
reflections. In the case of a solid delay line, a coupling grease may be 
necessary. With the water path, nothing other than the water is needed.