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NASA CASE NO. LAR 14581-1-SB

PRINT FIG. 1

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for licensing

in accordance with the NASA Patent Licensing Regulation (14 Code of Federal

Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to grant

licenses to commercial concerns. Although NASA encourages nonexclusive licensing to

promote competition and achieve the widest possible utilization, NASA will provide the

necessary incentive to the licensee to achieve early practical application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 143, Hampton, Virginia 23665-5225.

Serial No.: 07/873,407

04-15-92 LaRC

(NASA-Case-LAR-]4581-1-SB) METHOD

ANO APPARATUS FOR EVALUATING

MULTILAYER O_JECTS FOR

IMPERFECTIONS Patent Application

(NASA) 15 p

N93-12204

Unclas

G3/38 0123668

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METHOD AND APPARATUS FOR EVALUATING MULTILAYER OBJECTS 

FOR IMPERFECTIONS 

AWARDS ABSTRACT 

NASA Case No. LAR 14581 -1 -SB 

Precision design of high speed circuits demands secure mounting of components. 

Multilayer ceramic capacitors serves this function, but often contain critical defects. 

Current nondestructive evaluation techniques are severely limited. 

The present invention permits an evaluation of such objects. A multilayer object 

comprising multiple layers arranged in a stacking direction is evaluated for imperfections 

such as voids, delaminations and microcracks. First, an acoustic wave is transmitted into 

the object in the stacking direction via an appropriate transducer/waveguide combination. 

The wave propagates through the multilayer object and is received by another 

transducer/waveguide combination preferably located on the same surface as the 

transmitting combination. The received acoustic wave is correlated with the presence or 

absence of imperfections by, e.g., generating pulse echo signals indicative of the received 

acoustic wave, wherein the successive signals form distinct groups over time. The 

respective peak amplitudes of each group are sampled and curve fit to an exponential 

curve, wherein a substantial fit of approximately 80-90% indicates and absence of 

imperfections and a significant deviation indicates the presence of imperfections. 

Alternatively, the time interval between distinct groups can be measured, wherein equal 

intervals indicate the absence of imperfections and unequal intervals indicate the 

presence of imperfections. 

Novel aspects of the present invention include use of acoustic waves to nondestructively 

evaluate a multilayer object and the analytical methods employed to evaluate the object 

for imperfections. 

Inventors: Joseph S. Heyman Md. Nurul Abedin 

Address : 130 Indian Springs Road 6 Willowood Drive, # 203 

Williamsbura. VA 231 85 Yorktown. VA 23693 

Employer: NASA LaRC 

Initial Evaluator: Jag J. Singh 

Serial No.: 873,407 

Filed: 04-1 5-92 
-- L

SERIAL NO.: 07/873,407

FILED: 04-15-92

LAR 14581-1 -SB -1- PATENT APPLICATION

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METHOD AND APPARATUS FOR EVALUATING MULTILAYER OBJECTS

FOR IMPERFECTIONS

_Ori.qin of the Invention

The invention described herein was jointly made by employees of the

United States Government and contract employees during the performance of

work under a NASA Contract. In accordance with 35 U.S.C. 202, the

contractors elected not to retain title.

Back.qround of the Invention

1. Technical Field of the Invention

15 The present invention relates generally to nondestructive evaluation of

imperfections in ceramics and more particularly to an acoustic waveguide

technique for detecting delaminations, voids and microcracks.

2O

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30

. Discussion of the Related Art

At present, there is a need to develop improved technology for mounting

very high speed electronic components in order to design circuits more

precisely. That need has been partially fulfilled by the advent of surface mount

chip capacitors. The presence of critical defects can cause premature failure.

These defects can develop in the individual engineering components under

normal operating conditions. Numerous inspection techniques have been

developed for the detection of small and embedded defects that provide

information about the integrity of the electronic materials. Nondestructive

techniques are used to inspect the quality of production at the early stage of

manufacture. The first stage of inspection is visual, and is suited only for
LAR 14581-1-SB -2- PATENT APPLICATION

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relatively large external defects. This visual inspection can be enhanced by

using microscopes to detect small and embedded defects.

A number of nondestructive inspection systems are being considered for

inspecting multilayer ceramic capacitors (MLCC). The various testing methods

used to investigate the MLCC are acoustic emission, X-ray radiography,

neutron radiography, and acoustic microscopy. These techniques each

possess disadvantages. Acoustic emission requires a large load to obtain good

sensitivity which might cause flaws inthe sample. X-ray or neutron radiography

is unable to detect the small cracks in the sample. Acoustic microscopy is very

expensive and difficult to use outside the laboratory.

U.S. Patent No. 4,344,326 to Kahn describes a technique for testing of

laminated capacitors using acoustic emission. The use of acoustic emission is

based upon the premise that defects such as cracks and delaminations will

propagate with the release of acoustic energy when an external stress is

applied to a defective capacitor. In this technique, a capacitor is placed on a

planar base and a ram applies a force in a direction perpendicular to the

laminations of the capacitor. Any acoustic energy signals emanating from the

capacitor under test will be transmitted through the ram, detected by an

attached transducer and forwarded to an acoustic emission processor. The

acoustic energy emitted by the application of the load can be detected,

quantified and used as an indication of the presence and the severity of the

physical defects.

An acoustic waveguide technique has been employed to observe the

sensitivity of the acoustic amplitude of a propagated wave to the epoxy state

and temperature of an epoxy. The wave is propagated through a copper wire

embedded in the epoxy. The use of an acoustic waveguide is promising

because it does not need any electrical contacts. This technique is described

in "Propagation of Acoustic Waves in a Copper Wire Embedded in a Curing

Epoxy," K.J. Sun and W. P. Winfree, Proceedin.qs of IEEE Ultrasonics

Symposium, pp. 439-442, 1987.
LAR 14581-1-SB -3- PATENT APPLICATION

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It is accordingly an object of the present invention to detect imperfections

such as delaminations, voids and cracks in ceramics structure including

multilayer ceramic chip capacitors.

It is another object of the present invention to accomplish the foregoing

object nondestructively and noninvasively.

It is a further object of the present invention to achieve the foregoing

objects using an acoustic waveguide technique.

Additional objects and advantages of the present invention are apparent

from the drawings and specification which follow.

Summary of the Invention

15

The foregoing and additional objects are obtained by a method and

apparatus according to the present invention. A multilayer object comprising

multiple layers arranged in a stacking direction is evaluated for imperfections

such as voids, delaminations and microcracks. First, an acoustic wave is

transmitted into the object in the stacking direction via an appropriate

transducer/waveguide combination. The wave propagates through the

multilayer object in a "W"-type path and is received by another

20 transducer/waveguide combination preferably located on the same surface as

the transmitting combination. The received acoustic wave is correlated with the

presence or absence of imperfections by, e.g., generating pulse echo signals

indicative of the received acoustic wave, wherein the successive signals form

distinct groups over time. The respective peak amplitudes of each group are

25 sampled and curve fit to an exponential curve, wherein a substantial fit on the

order of 80-90% indicates an acceptable absence of imperfections and a

significant deviation indicates the presence of imperfections. Alternatively, the

time interval between distinct groups can be measured, wherein approximately

equal intervals indicate the absence of imperfections and unequal intervals

30 indicate the presence of imperfections.
LAR 14581-1-SB -4- PATENT APPLICATION

Brief Description of the Drawings

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FIG. 1 is a schematic of an apparatus for evaluating a multilayer object

for imperfections according to the present invention;

FIG. 2 is a graph of pulse echo signal groups taken over time for an

undamaged multilayer chip capacitor;

FIG. 3 is a graph of pulse signal groups taken over time for a damaged

multilayer chip capacitor;

FIG. 4 is a graph of experimentally obtained peak amplitudes for each

signal group in FIG. 2 and a model exponential curve;

FIG. 5 is a graph of experimentally derived peak amplitudes for each

signal group in FIG. 3 and a model exponential curve;

FIG. 6 is a perspective of an embodiment of a transducer/waveguide

combination used in the present invention;

FIG. 7 is a side view of another embodiment of a transducer/waveguide

combination; and

FIG. 8 is a side view of another embodiment of a transducer/waveguide

combination.

20 Detailed Description

Referring to FIG. 1, a schematic diagram is shown of the apparatus used

in the present invention. A computer 10 is provided which is connected to and

controls an oscilloscope 12, preferably a digital oscilloscope such as the LeCroy

25 9400A which is commercially available from LeCroy Corporation. A line out

from the oscilloscope is connected to a function generator 14 for generating a

radio frequency (RF) pulse which is amplified by RF power amplifier 16. For

example, the computer can command that a RF pulse with a center frequency

of 1 MHz be generated. The frequency chosen is dependent upon the material

30 properties and structural dimensions of the test specimen TS.
LAR 14581-1-SB -5- PATENT APPLICATION

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The amplified RF pulse is applied to the multilayer test specimen TS,

which may be a multilayer ceramic chip capacitor (MLCC). The pulse is applied

by a transmitting transducer 18 which is acoustically coupled to the TS via a

first tapered buffer rod 20. Buffer rod 20 is uniformly cylindrical from a first end

coupled to the transducer 18throughout its body. The opposite end is tapered

to a fine point in order to produce a sharp acoustic contact by reducing the

interface area between this pointed end and the TS. The resulting acoustic

wave propagates across the TS and is picked up by a second tapered buffer

rod 22. Second tapered buffer rod 22 is similar in shape to first rod 20. By

way of example, both rods may be copper and have an untapered diameter of

approximately 7001_m.

A receiving transducer 24 is coupled to the untapered end of second

buffer rod 22, detects the propagated acoustic wave, and converts it into an

electrical signal which is transmitted to a low pass filter 26 and then amplified

by amplifier 28. The amplified electrical signal is then supplied to the

oscilloscope 12. Oscilloscope 12 permits a visual display of the received

acoustic signal and transmits this data along with temporal data to the computer

10 as discussed in greater detail below with reference to FIGS. 2-5. This

permits the evaluation of the MLCC, as discussed below.

Both the transmitting and receiving transducer/waveguide configurations

are preferably on the same "top" or "bottom" surface of the multilayer test

specimen, wherein the respective layers of the test specimen are stacked one

atop the other from top to bottom. Accordingly, the propagated wave will travel

through the layers in the stacking direction in a "V" or "W" pattern a significant

number of times before being picked up by the receiving transducer/waveguide

configuration, whereby an extensive survey of multilayer integrity can be

performed as discussed below. The transmitting and receiving configuration

can also be located on opposite sides of the TS so that the wave travels in a

"N"-type pattern.

Table 1 summarizes results of oscilloscope readings of a test of a C-
LAR 14581-1-SB -6- PATENT APPLICATION

3216-CH-1H102K MLCC commercially available from TDK Corporation. The

RF pulse was 1 MHz and amplitude readings were taken every 10_s.

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Table 1

Amplitude (mV) Time (t) (l_s)

8.84 96.6 (to)

8.18 106.6 (tl)

7.21 116.6 (t2)

6.0 126.6 (t3)

5.31 136.6 (t4)

3.71 146.6 (ts)

2.68 156.6 (ts)

1.93 166.6 (tT)

The MLCC was determined to be relatively defect free prior to testing

based on known manufacturing quality and microscopic inspection. FIG. 2

depicts pulse echo signals versus time for the same type of "good", i.e.,

relatively defect free, capacitor. The C-3216-CH-1H102K MLCC has typical

dimensions of 3.2 mm x 1.6 mm x 0.85 mm. The buffer rod points were placed

at the length ends so that the pulse traveled a typical length of 3.2 mm.

Both Table 1 and FIG. 2 clearly show a waveform consisting of an initial

transmitted pulse (A in FIG. 2) followed by a series of undistorted echoes

spaced at equal intervals (B_in FIG. 2) for a "good" MLCC. This information is

digitally transmitted to the computer 10, where it is stored for subsequent

analysis. Based on the time between subsequent echoes (approximately 101LS)

and the known path length (6.4 mm), the velocity of sound in the capacitor is

determined by the computer or manually to be 640 m/sec.

Next a damaged MLCC of the same type and manufacture was

examined according to the present invention. The damage was caused by
LAR 14581-1-SB -7- PATENT APPLICATION

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applying 1.7 kV of RF power across the terminal T in the acoustic propagation

direction resulting in microcracks indicative of parametric or catastrophic failure.

The microcracks were observed with a microscope. The damaged capacitor

was then evaluated according to the method described above. FIG. 3 depicts

the acoustic pulse echoes versus time for the damaged capacitor. Unlike the

results of FIG. 2, FIG. 3 shows distorted echo trains which are unevenly

spaced. Accordingly, the acoustic profile of a MLCC indicates the presence of

damaging imperfections such as delaminations, voids, and microcracks. This

acoustic profile can be evaluated as a function of time, i.e., computer sampled

pulse groups or computer sampled peak amplitudes of each group which occur

approximately periodically (equal intervals of time) indicates an acceptably

undamaged MLCC whereas unequal intervals indicate an unacceptably

damaged MLCC. Alternatively, the acoustic profile can be evaluated as

discussed below in reference to FIGS. 4 and 5.

FIG. 4 shows the amplitude of the acoustic signal versus time for the

undamaged "good" MLCC. Specifically, the peak amplitudes of each distinct

wave envelope or group of FIG. 2 is sampled by the computer, plotted as a

data point and connected to other data points via a dashed line. A model

exponential curve fit, represented by a solid line, is applied to the experimental

data curve via the computer executing a conventional curve fitting program.

The data in this case closely follows the exponential curve fit. Thus, it is seen

that the amplitude of the acoustic signal for the "good" MLCC decreases over

time. Similarly, FIG. 5 depicts the experimental amplitude peaks versus time

for the MLCC test results of FIG. 3 connected via a dashed line. Once again,

a model exponential curve fit is applied to the data points. The experimental

data clearly does not follow the exponential due to damage caused by the

applied RF power. Accordingly, the presence of distorted wave trains and

deviation of peak amplitude from an exponential curve fit indicates a damaged

MLCC.

From the foregoing, it is clear that a substantial curve fit to the
LAR 14581-1-SB -8- PATENT APPLICATION

exponential indicates a relatively undamaged MLCC and a deviation from the

curve fit indicates the converse. The percentage of fit indicative of a relatively

undamaged state is of course application specific. A fit of approximately 80-

90% has been found to be a good indication of an acceptable undamaged state

5 and a deviation of 10-20% to indicate an unacceptable damaged state.

FIG. 6 shows the use of a spacing device 30 which maintains the

physical, electrical, and acoustic separation of the tapered ends of the buffer

rods 20 and 22 respectively coupled to transmitting and receiving transducers

18 and 24. Spacing device 30 is depicted as a box-like structure wherein the

10 buffer rods are attached to perpendicular faces of the structure. The buffer

rods should be spaced so as to contact the terminal ends T of the MLCC test

specimen TS. Of course, various alternative holding devices could be

employed, e.g., an adjustable spacer located between the buffer rods.

FIG. 7 depicts an embodiment wherein the buffer rods are replaced with

15 buffer cones 32 and 34. The transmitting and receiving transducers 18 and 24

are respectively coupled to the bases of cones 32 and 34.

In a preferred embodiment both the cone 38 and transducer 40 are

fabricated from the same rod of a piezoelectric material, as shown in FIG. 8.

Specifically, the rod is ground as shown and an electrode band 42 is bound

20 around the ground object above the cone 38. The object is then placed in an

electric field and raised above its Curie temperature T c to pole the electrode

and create an active piezoelectric material between the electrode 42 and the

opposite end of transducer 40. The cone 38 is inactive in a piezoelectric sense

and continues to act as a waveguide with the same acoustic properties as the

25 piezoelectric transducer.

The present invention is nondestructive and does require an acoustic

couplant to perform an ultrasonic evaluation. Rather, a well spatially defined

point source is used to transmit and receive ultrasonic waves. As

demonstrated, the resulting pulse echo trains are indicative of the presence or

30 absence of microcracks, delaminations and voids.
LAR 14581-1-SB -9- PATENT APPLICATION

Many improvements, modifications and substitutions will be apparent to

the skilled artisan without departing from the spirit and scope of the present

invention as described herein and defined in the following claims.
LAR 14581-1-SB PATENT APPLICATION

METHOD AND APPARATUS FOR EVALUATING MULTILAYER OBJECTS

FOR IMPERFECTIONS

5

10

15

2O

Abstract of the Disclosure

A multilayer object comprising multiple layers arranged in a stacking

direction is evaluated for imperfections such as voids, delaminations and

microcracks. First, an acoustic wave is transmitted into the object in the

stacking direction via an appropriate transducer/waveguide combination. The

wave propagates through the multilayer object and is received by another

transducer/waveguide combination preferably located on the same surface as

the transmitting combination. The received acoustic wave is correlated with the

presence or absence of imperfections by, e.g., generating pulse echo signals

indicative of the received acoustic wave, wherein the successive signals form

distinct groups over time. The respective peak amplitudes of each group are

sampled and curve fit to an exponential curve, wherein a substantial fit of

approximately 80-90% indicates and absence of imperfections and a significant

deviation indicates the presence of imperfections. Alternatively, the time

interval between distinct groups can be measured, wherein equal intervals

indicate the absence of imperfections and unequal intervals indicate the

presence of imperfections.
SERIAL NO.: 07/873,407

FILED: 04-15-92
NASA CASE NO. LAR 14581-1

JOSEPH S. HEYMAN ET AL.

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