Nondestructive methodology for standoff height measurement of flip

IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 24, NO. 2, MAY 2001
163
Nondestructive Methodology for Standoff Height
Measurement of Flip Chip on Flex (FCOF) by SAM
C. W. Tang, Y. C. Chan, K. C. Hung, and D. P. Webb
Abstract—Flip chip technology is the emerging interconnect
technology for the next generation of high performance electronics. One of the important criteria for reliability is the width of
the gap between the die and the substrate, i.e., the standoff height.
A nondestructive technique using scanning acoustic microscopy
(SAM) for the standoff height measurement of flip chip assemblies
is demonstrated. The method, by means of the implementation of
a pulse separation technique, time difference of the representative
signals of the die bottom and water interface and water and
substrate surface interface from the A-scan image can be found.
Then, the corresponding standoff height can be calculated. When
compared to the traditional destructive measurement method
(SEM analysis on sectioned sample), this nondestructive technique
yields reliable results.
Index Terms—Filler particles, flip chip, nondestructive, scanning acoustic microscope, standoff height, underfill.
I. INTRODUCTION
T
O MEET the demands of higher density, greater performance, and lighter weight in the electronics industry, flip
chip technology is the emerging interconnect technology for the
next generation of high performance electronics [1], [2]. The
most important advance in improving the flip chip reliability has
been by filling the gap between chip and substrate with an appropriate underfill encapsulant. The underfill provides dramatic
fatigue life enhancement by dissipating thermally induced stress
between the die and the substrate. However, one of the important criteria for the reliability issue is the size of the gap between the die and the substrate, i.e., standoff height. Control of
the standoff height is necessary for formation of well shaped
solder joint and of a constant fillet shape for a fixed volume of
underfill. Additionally, if the standoff height is too small, the
filler particles in the underfill may become trapped and the not
be evenly distributed, affecting the thermal performance. Moreover, if the gap is too small, some of the flux residue may remain
even after cleaning, potentially causing underfill delamination.
Any defects such as void or delamination in the underfill layer
may result in solder fatigue failure and ruin the whole flip chip
package.
Traditionally, the standoff height was measured by the
method of contact measurement or SEM measurement of
sectioned samples. However, these methods have the respec-
Manuscript received March 29, 2000; revised March 12, 2001. This work was
supported by the City University of Hong Kong under Strategic Grants (Project
7000955) and the City University of Hong Kong.
The authors are with the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong (e-mail: cwtang@ee.cityu.edu.hk).
Publisher Item Identifier S 1521-3323(01)04312-X.
Fig. 1.
Six samples of flip chip on flex substrate (FCOF) assembly.
tive disadvantages of lack of accuracy and being more time
consuming.
Ultrasonic techniques [3] have been used successfully for
thickness measurement and material characterization in several
applications primarily because they are nondestructive in nature
and can yield reliable results for simple geometries. Acoustic
microscopy techniques, in particular, are attractive for IC packaging applications because they afford the potential to perform
these measurements over a small, localized area [4]. Scanning
acoustic microscope (SAM) is used extensively throughout the
microelectronics industry to inspect flip chip packages for delamination or cracking [5]. In this paper, we will discuss how to
use this technology to measure the standoff height of a flip chip
assembly. Moreover, we verify the results by the SEM measurement of the sectioned and polished samples. Our research results propose a more efficient method of nondestructive standoff
height measurement.
II. EXPERIMENT
Flip chip on flex assemblies, as shown in Fig. 1, were used.
Three types of samples were studied: assemblies with underfill
(filler particles inside) after the curing process, assemblies
without underfill and assemblies with underfill (without filler
particles) after the curing process. The schematic of the flip
chip packages investigated in this study are shown in Figs. 2
and 3.
Scanning acoustic microscope (SAM) was used for data acquisition with a transducer of 230 MHz. Five positions of each
1521–3323/01$10.00 © 2001 IEEE
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IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 24, NO. 2, MAY 2001
Fig. 2. Schematic of flip chip assembly without underfill.
Fig. 5. (a) Schematic of ultrasonic wave hitting the package. (b)
Corresponding A-scan image of package in (a).
Fig. 3.
Schematic of flip chip assembly with underfill.
Fig. 4. Five measurement points on each sample.
sample were scanned, as shown in Fig. 4. The sample under
study was placed in the water tank of the SAM. The transducer
is focused at the die bottom and water (or underfill interface).
The transducer was moved to a location where the thickness of
the standoff height is to be determined. Moving the transducer
over any particular location (position 1 to position 5) and repeating the data acquisition can collect data from multiple locations of interest. Ultrasonic thickness is made possible by the reflection of ultrasound at interfaces between dissimilar materials.
When ultrasound waves propagating in a material encounter an
interface with a dissimilar material (with a different acoustic
impedance), a portion of the ultrasonic energy is reflected back.
Thus, when ultrasound in the acoustic microscope impinges on
an assembly shown in Fig. 5(a), a “typical signal” appears on
the oscilloscope. The pulse separation technique describe above
is relatively straightforward [6]. This is because the reflections
from each interface can be clearly separated in the time domain
(A-Scan). The reflective inspection mode is time based. A reflection from the top of the package returns earlier than a reflection from a layer within the package. The time base is used
to separate layers from the package. For example, as shown in
Fig. 5(b), the reflection at the water and package interface is followed by the package and die surface interface, and then by the
die and die bottom interface. The thickness of either layer can
be determined by measuring the time lag between the two reflections if the velocity of the ultrasound wave in either region
is known.
After the standoff height measurement and data acquisition
using the SAM, samples were sectioned and polished. In order
to validate the results obtained by the acoustic microscopy,
Scanning Electron Microscope (SEM) was employed to obtain
correlated destructive data. The standoff height of the flip chip
assemblies was measured directly from the magnified images
(320 ) of the sectioned samples.
III. RESULTS
Flip chip on flex assemblies samples had three interfaces in
this study. For samples with underfill (either with or without
filler particles)/without underfill, the following interfaces are
present:
1) water and die surface interface;
2) die bottom and underfill interface (for samples with underfill) or die bottom and water interface (for samples
without underfill;
3) underfill and flex substrate interface.
The representative A-scan signals of these interface and the corresponding C-scan images are shown in Figs. 6 and 7 respectively.
Measurements on the samples (6 nos.) without underfill on
the five positions of each chip were performed. By means of the
measurement of the time lag between the representative signals,
the time for the ultrasonic wave to travel to-and-from the chip
thickness and standoff height can be determined (Fig. 8). The
standoff height can be calculated by
SOH
TANG et al.: NONDESTRUCTIVE METHODOLOGY FOR STANDOFF HEIGHT MEASUREMENT OF FLIP CHIP ON FLEX (FCOF)
165
Fig. 6. A-scan waveform shown in the oscilloscope: 1) water and die surface
interface, 2) die bottom and water interface, and 3) water and substrate surface
interface.
Fig. 8.
Measurement of time lapse for standoff height measurement.
TABLE I
STANDOFF HEIGHT OF SAMPLE 1 AT THE FIVE MEASUREMENT POSITIONS
(a)
(b)
Fig. 7. (a) C-scan image of the die bottom. (b) C-scan image of the substrate
surface.
where
SOH standoff height;
speed of ultrasonic wave in water (samples without underfill) or underfill (samples with underfill);
1370 m/s (for water);
time lag between two interfaces in an A-scan image.
The chip thickness of the samples is also calculated for the
sake of comparison. The calculated standoff height of the samples without underfill was shown in Table I and Fig. 9.
As shown in Fig. 9, we see that the height at the five positions
of the sample is not the same, suggesting that there is an intrinsic
error (measurement error) or that the bottom of the chip is not
perfectly planar. Whether the variation is due to an intrinsic error
or imperfect plane, another experiment, the scanning electron
microscopy (SEM), was performed to verify the cause of this
difference.
After the completion of the acoustic microscopy, two samples
were sectioned and polished. Direct standoff height measurement was performed using the SEM on the magnified images,
as shown in Figs. 10 and 11. The results of standoff height measurement by scanning acoustic microscope (SAM) and scanning
electron microscope (SEM) are compared in order to check the
validity of the data obtained by SAM (A-scan). The comparison
of standoff height measurement is shown in Table II and plotted
in Fig. 12.
As shown in Table II, we find that the maximum deviation
between standoff height measured by SAM and SEM for sample
1 is only 0.5 m, which is only a 1.36% deviation.
Moreover, from Fig. 10, we see that the trend of the standoff
heights of each sample measured by SEM is same as the trend by
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IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 24, NO. 2, MAY 2001
Fig. 9. Standoff height of the six samples.
SAM. Since the data and trend of the two experiments are comparable, it suggests that the deviation of standoff height within
each sample is due to the imperfect plane of the sample, and the
intrinsic error is very small.
After our investigation on samples without underfill, we have
performed the same measurement on the samples with underfill (with and without filler particles). We find that three issues arise during data acquisition by the scanning acoustic microscopy when samples with underfill were scanned. First, due
to the presence of underfill, it is difficult to identify the representative signal of the substrate for standoff height calculation.
Second, a reference speed of ultrasonic wave travelling in the
underfill material must be known before standoff height measurement. Third, due to the irregular density of the filler particles and the density of the underfill material, and hence the
speed of the ultrasonic wave varies in different locations, i.e.,
the reference speed of the ultrasonic wave also varies, so calculated standoff height also varies. Therefore, if samples with underfill (without filler particles) are under investigation, a control
experiment must be run to determine the reference speed of the
underfill. [Applying underfill to flip chip assembly with known
standoff height, then by using the method described (Standoff
height measurement by SAM), reference speed of the underfill
can be calculated.] However, for underfill with filler particles,
due to the irregular density of the filler particles, it is quite difficult to determine an accurate speed for standoff height calculation.
Fig. 10.
2
SEM image—magnification of sectioned sample (20 ).
IV. CONCLUSION
A nondestructive technique using the SAM is demonstrated.
The method, by means of the implementation of the pulse separation technique, time difference of the representative signals
2
Fig. 11. SEM image—magnification of sectioned sample (320 ).
TANG et al.: NONDESTRUCTIVE METHODOLOGY FOR STANDOFF HEIGHT MEASUREMENT OF FLIP CHIP ON FLEX (FCOF)
Fig. 12.
167
Comparison of standoff height measurement by SAM and SEM.
TABLE II
COMPARISON OF STANDOFF HEIGHT MEASUREMENT BY SAM AND SEM
of the die bottom and water interface and water and substrate
surface interface from the A-scan image can be found. Then,
the corresponding standoff height can be calculated. When comparing the results obtained by SAM with the traditional destructive measurement method, for an average standoff height of 37.1
m, the maximum deviation between the two methods is only
0.5 m, which is a 1.36% deviation. Moreover, the trends of
standoff height of each sample measured by SAM and SEM
compromise with each other, which suggests that the method
under our study yields reliable results. Our research results may
contribute to the industry a more efficient method of nondestructive standoff height measurement.
ACKNOWLEDGMENT
The authors would like to thank Dr. H. Wang and H. Leung,
SAE Magnetics (H. K.), Ltd., for providing the samples and
their valuable discussion.
REFERENCES
[1] “Flip chip technology and markets worldwide,” TechSearch International, Inc., Ind. Rep., Mar. 1997.
[2] E. J. Jan Vardaman and T. Goodman, “Flip chip market trends and infrastructure limitation,” in Proc. 1997 IEMT/IMC, 1997, p. 37.
[3] K. F. Becker, F. Ansorge, and E. Zakel, “Acoustic microscopy investigation on BGA and BGA packages,” in Proc. IAMIS 97, Anaheim, CA.
[4] S. Canumalla, G. A. Gordon, and R. N. Pangborn, “In situ measurement
of Young’s modulus of an embedded inclusion by acoustic microscopy,”
ASME Trans. J. Eng. Mater. Technol., vol. 119, no. 2, pp. 143–147, 1997.
[5] J. Sigmund and M. Kearney, “TAMI analysis of flip chip packages,” Adv.
Packag., July/August 1998.
[6] S. Canmalla and L. W. Kessler, “Toward a nondestructive procedure
for characterization of molding compounds,” in Proc. IEEE 35th IRPS,
Denver, CO, Apr. 1997, pp. 149–155.
C. W. Tang received the B.Sc. degree in mechanical
engineering (with first class honors) and the M.Sc.
degree (with distinction) from the University of Hong
Kong, and is currently pursuing the M.Phil. degree
in advanced packaging of flip chip assemblies at the
City University of Hong Kong.
His research interests are in advanced electronics
manufacturing technology and reliability issues of
no-flow underfill and anisotropic conductive film
(ACF) of flip chip assemblies.
Y. C. Chan received the B.Sc. degree in electrical engineering, the M.Sc. degree
in materials science, and the Ph.D. degree in electrical engineering, all from the
Imperial College of Science and Technology, University of London, London,
U.K., in 1977, 1978, and 1983, respectively.
He joined the Advanced Technology Department, Fairchild Semiconductor,
Mountain View, CA, as a Senior Engineer, and worked on integrated circuits
technology. In 1985, he was appointed to a Lectureship in electronics at the Chinese University of Hong Kong. Between 1987 and 1991, he worked in various
senior operations and engineering management functions in electronics manufacturing (including SAE Magnetics (HK) Ltd. and Seagate Technology). He set
up the Failure Analysis and Reliability Engineering Laboratory for SMT PCB
in Seagate Technology (Singapore). He joined the City Polytechnic of Hong
Kong (now City University of Hong Kong) as a Senior Lecturer in electronic
engineering in 1991. He is currently Professor in the Department of Electronic
Engineering and Director of EPA Centre. He has authored or co-authored over
100 technical publications in refereed journals and conference proceedings. His
current technical interests include advanced electronics packaging and assemblies, failure analysis, and reliability engineering.
168
K. C. Hung received the B.Sc. degree in applied
physics from the City Polytechnic of Hong Kong in
1993 and the Ph.D. degree in physics and materials
science from the City University of Hong Kong in
1998.
He currently works in the Department of Electronic Engineering, City University of Hong Kong,
as a Research Fellow. He is in charge of several
industrial collaborative research projects focusing on
conductive adhesives and lead-free solder materials.
He has authored or co-authored over 40 technical
publications in refereed journals. His current research interests include
advanced electronics packaging technology, lead-free soldering, reliability
engineering, failure analysis, and nondestructive testing.
IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 24, NO. 2, MAY 2001
D. P. Webb received the B.Sc. degree in mathematical physics (with honors)
from the University of Manchester Institute of Science and Technology
(UMIST), Manchester, U.K., in 1988, the M.Sc. degree in amorphous and
microcrystalline electronic materials from the University of Dundee, U.K., in
1990, and the Ph.D. degree from the University of Abertay Dundee in 1994.
He took up a position in the Department of Electronic Engineering at City
University of Hong Kong in 1995 and is currently a Research Fellow. Projects
at City University have included improvement of the wear properties of organic
photoreceptor layers in xerography, and evaluation of a new fabrication technique for amorphous silicon. He is currently Principal Investigator on a City
University Strategic Research Grant to study charge transport in organic electroluminescent device materials.
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