Prepared by Amir Baranzahi, ITN, Campus Norrköping, Linköping University, March 2004
C-SAM®….. How it works…..
The C-SAM®, or C-Mode Scanning Acoustic Microscope, is a very high frequency pulse-echo
ultrasonic microscope that generates images by mechanically scanning a transducer over a sample. A
highly focused beam of ultrasound, generated by an acoustic lens, is brought to the sample by a
coupling medium, usually water or an inert fluid. The transducer alternately acts as both sender and
receiver of the pulses. After the pulse enters the sample return echoes arise
from internal interfaces. The return times of the pulses are a function of the distances from the
interfaces and the transducer. An oscilloscope display of the echo pattern, known as an A-Scan,
clearly shows these echoes and their time-distance relationships from the sample surface. This
provides a basis for investigating anomalies at specific depth levels. An electronic gate “opens” for a
defined duration allowing only the information from a specific level to be imaged while excluding all
other echoes. The gated echo magnitude brightness modulates an image monitor that is synchronized
with the transducer position. In this way reflection mode images are produced.
Transmission mode images, also known as “Thru-scans”, are made by placing a second
transducer on the opposite side of the sample to receive ultrasound pulses that have passed
through the entire sample. Internal features and defects will cause the ultrasound to be spatially
altered and the image to have different brightness levels in the affected areas.
In C-SAM® the image contrast changes relative to the background brightness constitute the
important information. Voids, cracks, disbonds and delaminations provide high contrast and are
easily distinguished. More detailed information on resolution and different imaging modes is
available.
Combined with the ability to isolate and “focus” at specific levels, C-SAM® is a powerful tool for
locating and analyzing the nature of defects.
Resolution
The resolution in a C-SAM® acoustic microscope is governed by several factors:
1. The ultrasound frequency
2. The spot size of the transducer.
3. The number of pixels in the image.
The ultrasound frequency imposes a theoretical upper limit to the resolution in any sample and that
limit is one half wavelength of sound in the material being probed. The wavelength is calculated by
dividing the intrinsic velocity of sound propagation by the ultrasound frequency. Most transducers
are not designed to achieve this theoretical limit because of the short focal lengths required and
limited sample penetration.
The spot size of the transducer is determined by the ultrasound frequency plus the lens design. In
general lenses that have the widest angular collection of range have the best resolution (smallest spot
size). It is possible to achieve the same spot size with different ultrasonic frequencies and the choice
of frequency is determined by penetration and layer separation requirements of the analysis.
As with any digital image, the number of data points (pixels) determines whether all available sample
detail is acquired. If, in the scanning of the sample the data points are too far apart, relative to the
spot size, tiny features may be missed in the image. If the data points are too close together the image
may appear fuzzy – however, there will be no loss of detail.
Prepared by Amir Baranzahi, ITN, Campus Norrköping, Linköping University, March 2004
Ultrasound Scanning
SONAR
MEDICAL Acoustic
Microscopy
Training
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6RQL[,QF7KH\ may not be copied, reproduced,
modified, published, uploaded, posted, transmitted,
University of California Medical Center or distributed in any way, without prior written
San Francisco, California permission from Sonix.
What
Whatare
areUltrasonic
UltrasonicWaves?
Waves?
8700 Morrissette Drive
Ultrasonic
Ultrasonicwaves
wavesrefer
referto
tosound
soundwaves
wavesabove
above20
20kHz
kHz Springfield, VA 22152
(not
(notaudible
audibleto
tothe
thehuman
humanear)ear) tel: 703-440-0222
fax: 703-440-9512
Copyright Sonix, Inc
4 e-mail: info@sonix.com
Transducer Beam Profile A-SCAN
Transducer
Depth of Field Initial Pulse
The purple region Front surface
is referred to as the
focal area or depth
Interface of Sample
of field of the
transducer beam. interest
Back surface
Copyright Sonix, Inc Copyright Sonix, Inc
15 24
Digital Oscilloscope Time Of Flight
1
X2
Initial pulse 1st Echo X1
2nd Echo 3rd Echo
Time of Flight images provide a
relative depth within a sample.
Structures which appear white or
light gray are closer to the surface
The peak signal for location 1 occurs at
of the sample.
14.2 microseconds (light gray) while the
The 1st set of echoes is the Multiple Echoes Structures which appear darker peak signal for location 2 occurs at 14.6
area of interest, gate placement shades of gray or black are deeper microseconds (dark gray).
will be on this group. Copyright Sonix, Inc within sample. Copyright Sonix, Inc
38 44
Prepared by Amir Baranzahi, ITN, Campus Norrköping, Linköping University, March 2004
Transducers Typical Transducer Selection
Sample Application Transducer
High Frequency Low Frequency
Short Focus Long Focus T/X Receiver 10 MHz w/0.75” focus
PLCC, QFP, PQFP 15 MHz w/0.5” focus
1.1. Higher 1.1. Lower
Lowerresolution
Higherresolution
resolution resolution Power Pak 15 MHz w/0.5” focus
2.2. Shorter 2.2. Longer
Longerfocal
focallengths
Shorterfocal
focallengths
lengths lengths BGA Top 50-75 MHz w/12mm focus
3.3. Less 3.3. Greater
Greaterpenetration
Lesspenetration
penetration penetration Capacitors 75 MHz w/12mm focus
(Thinner (Thicker
(Thickerpackages)
(Thinnerpackages)
packages) packages)
TSOP 75 MHz w/12mm focus
General Flip Chip Underfill 110 MHz w/8mm focus
Generalrules:
rules:
• •Ultra Flip Chip Interconnect UHF w/ 5.9 mm focus
UltraHigh
HighFrequency
Frequency(200+
(200+MHz)
MHz)forforflip
flipchips
chipsand
andwafers.
wafers.
• •High
HighFrequency
Frequency(50-75
(50-75MHz)
MHz)for
forthin
thinplastic
plasticpackages.
packages.(110MHz-UHF)
(110MHz-UHF)for
for
flip chips. Bonded Wafer 110 MHz w/8mm focus
flip chips.
• •Low
LowFrequency
Frequency(15
(15MHz)
MHz)for
forthicker
thickerplastic
plasticpackages.
packages. Bonded Wafer UHF w/ 5.9 mm focus
Copyright Sonix, Inc Copyright Sonix, Inc
14 16
Acoustic Properties
Material D ensity LongitudinalWave Acoustic Impedance
3 2 6
(g/cm ) Velocity (m/s) (kg/m s) (x10 )
0
Water (20 C) 1.00 1483 1.48
0
Alcohol (20 C) 0.79 1168 0.92
0
Air (20 C ) 0.00 344 0.00
S ilicon 2.33 8600 20.04
Gold 19.3 3240 62.53
C opper 8.90 4700 41.83
Aluminum 2.70 6260 16.90
E poxy Resin 1.20 2600 3.12
R esin (for IC pkg) 1.72 3930 6.76
Glass (Quartz) 2.70 5570 15.04
Alumina (AL2O3) 3.80 10410 39.56
Copyright Sonix, Inc
18
Examples
Sample & Method - A surface acoustic scan was
performed of the entire MCM for reference. A
surface scan uses only the first return echo, with
minimal penetration into the sample. Individual
components were then scanned at the die attach
depth using a 230 MHz transducer.
Result - All three scanned components showed
massive voids (red) within the die attach material.
Prepared by Amir Baranzahi, ITN, Campus Norrköping, Linköping University, March 2004
Flip Chip: Disbonded bumps, halo defects (C-SAM®)
Sample & Method -A Flip Chip package was imaged
from the top side (through the back of the die) at the high
acoustic frequency of 230 MHz using high-resolution
1024 x 960 pixel scanning. Gating was on the interface
between the die face and the underfill material.
Result - ince gating is one the interface between the die
face and the underfill, it also includes the bonding of the
solder bumps to the bond pads on the die face. Red
bumps are those where bonding absent or incomplete. In
addition, next to some bumps (both bonded and
disbonded) are small red halo defects, vertical
delaminations in the underfill caused by failure of the
fluid underfill to thoroughly wet the bump.
PBGA: Delaminations and Popcorn Crack (C-SAM®) using
THRU-Scan ™Mode
-
Sample & Method - These 4 plastic BGA packages were imaged
with C-SAM at 15 MHz. The THRU-Scan technique was used, which
places a second transducer beneath the sample to collect ultrasound
pulsed by the topside transducer. The resulting image, shows internal
defects at any depth within the sample.
Result - Note the excellent transmission through the BGA package,
and the appearance of the solder bumps. Two of the packages (upper
left and lower right) have significant areas where the molding
compound is delaminated from the substrate. The package at lower
left has a large popcorn crack. Popcorn cracks are typically caused by
the expansion of retained humidity during the reflow process.
Flip Chip: Solder ball disbonds and missing (void) underfill
(C-SAM®) Källa: www.sonoscan.com
ple & Method - Part of a Flip Chip development project, this :www.sonix.com
age was imaged by C-SAM ®through the substrate side. Gating
on the interface between the cured underfill and the substrate,
ncludes the bonding of the solder balls to their pads.
Result -Red areas are large voids in the underfill. In addition,
the area of bond of the solder balls (white) is in many instances
incomplete, indicating that these solder balls are only partially
bonded.
