Scanning acoustic microscopy (SAM) is a non-destructive technique that uses ultrasound to examine the internal structure and surfaces of materials. The document discusses several case studies where SAM was used successfully for failure analysis, including detecting missing underfill material in flip chip devices, delamination between layers in round electronics, voids in solder mask coatings, and delamination in obsolete components. SAM provided high resolution imaging to identify these issues internally in a non-destructive manner and more efficiently than alternative destructive techniques such as cross-sectioning. The case studies demonstrate how SAM can be applied practically for reliability testing and failure analysis in microelectronics.

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Scanning Acoustic Microscopy Technique in Failure Analysis
Helle Rønsberga, Yavuz Kösea
DELTA, Venlighedsvej 4, Hoersholm, Denmark
Abstract. Scanning Acoustic Microscopy (SAM) is a non-destructive technique and a powerful tool for
investigation of plastic package moisture sensitivity, subsurface damages, coating quality for thickness and voids
non-destructively in microelectronics. In this paper SAM is used for some specific cases; (a) flip chip underfill
quality for voids/missing material fill, (b) Delamination between metal electrode and epoxy encapsulate for round
objects, (c) Voids in coating materials – here solder mask over PCBA and (d) intrusions/delamination in plastic
packages were studied by SAM technique. SAM technique was used in named studies with successfully and results
were given in the paper. Results obtained by SAM technique were verified using alternative techniques– destructive
techniques were used for this purpose such as; back side grinding for underfill voids, cross-sectioning and SEM
technique for solder mask quality for voids, dye penetrant technique combined with cross-sectioning for epoxy to
metal electrode interface delamination.
Keywords: SAM, microelectronics, obsolete, delamination, plastic package, failure analysis.
Helle Rønsberg, E-mail: HR@delta.dk, Yavuz Köse, E-mail: yak@delta.dk
1 Introduction
SAM is a non-destructive technique which can be used for internal structure of complex devices,
especially interfaces for delamination. SAM can provide a resolution down to sub-micron
thicknesses. SAM is an efficient tool for evaluating sealing quality, printed circuit boards,
underfills, BGA, SMDs, QFN, passive components, wafers.
Ultrasound waves propagate through liquids and solids. Whenever there is change in material
acoustic impedance such as at interface/boundary of internal flaws, change of material or change
of density difference in the same material, partial reflection and transmission takes place. The
amplitude, polarity and the time of flight of the reflected signal provides important information.
Characteristic acoustic impedance, Z, of a material is given by
Z = .v
Where, ρ = density of the material, and v = velocity of sound in the material.
Characteristic impedance of air is several orders of magnitude lower than that of solids and this
leads to nearly 100% (total reflection) reflected waves. This total reflection characteristics of the
air gaps/delamination is what makes SAM unique in interface quality investigations in all type of
structures. This is why in SAM system, a liquid bath – couplant – is used to transmit acoustic
waves between transducer and the sample. Water is traditionally used as a couplant due to
practical reasons. Fig. 1 illustrates the working principle of SAM technique.
Fig. 1 Basic working principle of SAM. Delamination/air gap on the left side and good adhesion interface structures
on the left are shown.
PR = │ (Z2-Z1) │/(Z2+Z1) PT = 2Z2/(Z2-Z1)

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Where PI = incident wave amplitude, PR = reflected wave amplitude and PT= transmitted wave
amplitude. As it can be seen from reflection formula, higher the characteristic impedance
difference means higher the reflection signal amplitude.
SAM is a non-destructive technique which uses ultrasound to examine surface and internal
structure of a solid material or structures e.g. an electrical component.
Schematic configuration of a scanning acoustic microscopy is shown in Figure 2, it composed of;
 A piezoelectric transducer that sends pulses of acoustic waves through liquid couplant
and into the sample. Between pulses a receiver takes the echoes reflecting from sample.
In C-SAM transmitter and the receiver is the same piezoelectric transducer which
electronically switching between two modes of pulse sending and echo receiving.
 Mechanical scanning unit which enables to focus the signals onto interest area and raster
scanning of the sample
 Control unit or PC where the scanning and the data analysis are provided by the software
system.
 Fig. 2: Schematic presentation of pulse-echo mode operated SAM instrumentation
There are two inspection modes; pulse-echo and through-transmission modes. Reflection signal
is used for pulse-echo mode imaging while transmitted signal is used for through-transmission
mode imaging.
Pulse-echo mode can determine and locate the delamination, defect and voids in bulk material
and provide high spatial resolution images. On the other hand, through-transmission images have
a less spatial resolution and cannot locate the defect position in bulk sample. Through-
transmission mode works as a complementary tool for pulse-echo mode findings and BGA
scanning.
Different modes of scanning options are available. Most used ones are;
 A-Scan: A-Scan is real time waveform of reflected signals which is shown on XY-axis.
The horizontal scale (X-axis) defines the depth within a sample while the vertical scale
(Y-axis) defines amplitude and polarity of the reflected sound.
 C-SAM: Classical SAM is primarily a reflection-based microscope that generates very
high-resolution images of a sample surface or a near surface plane but C-SAM
corresponds to bulk scan where internal investigation of beneath the sample surface is
conducted.

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 B-Scan: B-Scan corresponds to imaginary cross-sectioning of the sample where the
sample is scanned cross-sectional.
 Tomographic Acoustic Micrography Imaging (TAMITM
- SONIX): TAMI slices the bulk
material into micrometer thick layers and keeps the focus automatically at each layer.
TAMI enables to examine the whole sample by one scan in minutes.
Spatial resolution in SAM increases with increasing frequency but higher frequency means lover
depth information at the same time. The tradeoff between a low- and a high-frequency transducer
is in the depth of penetration and resolution.
The most important feature of SAM technique is that it is non-destructive & non-invasive.
Case stories
Having described the principles of the scanning acoustic microscope we will now present some
cases which are studied by SAM technique. Here four distinct cases which demonstrate how
SAM technique is practically and effectively detect the failures. Due to confidential nature of the
subjects in case stories, limited explanation was given about their details.
1 - Voids/missing underfill material in Flip Chip devices
Company A encounters short failures in their flip chip devices mounted on PCBA. Customer
believes that the voids/ missing underfill material between the solder balls result in short
connection during the reflow process. X-Ray technique was used for investigation. X-ray
detected existing short connections but not voids.
SAM technique was used for non-destructive analysis of the devices. SAM analysis was
conducted through the epoxy mold side. Due to the two sided and populated PCBA structure,
scanning through the substrate side was not effective enough. A demonstrative SAM micrograph
on a virgin sample is shown in Fig. 3. Rectangular white texture, between the solder balls in the
rows, shows missing underfill material.
Fig.3 SAM micrograph of an original sample. White rectangular texture between solder balls imply missing
underfill material.
In SAM analysis un-curved and plane surfaces helps for achieving best results by keeping the
surface at the focus and preventing diffraction of the signals during x-y plane raster scanning. In
case of here, Flip Chip devices, the surface was rough and curved. High frequencies, such as
100MHz and UHF (Ultra High Frequency) transducers were useless due to surface roughness
caused diffraction event.
For sharp images, destructively removal of the top epoxy mold was conducted by gentle
grinding. Exposing the flip chip device back side enabled high frequency transducer usage and

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better quality imaging. In Fig. 4, UHF transducer used TAMI mode SAM micrographs for a
sample is given. Images form a single scan thanks to TAMI mode scanning.
Fig. 4 UHF TAMI images for Flip Chip device. A) Die to underfill interface area, B) bulk of underfill, C) underfill
to substrate interface area.
In order to proof the accuracy of the SAM results, grinding from epoxy mold side down to
die/underfill interface were done by further grinding and then images were taken under
stereomicroscope. SAM image and corresponding stereo microscope image of the same device
were given in Fig 5. It is demonstrated that SAM could detect the underfill voids non-
destructively.
Fig. 5 On the right, optical micrograph of the Flip Chip device after removing epoxy mold and silicon die by
grinding, on the right SAM image of the same device before silicon die removal.
2 – Delamination study in round objects
Company B faces field-failed product returns. Reason for failure was attributed to delamination
between epoxy encapsulate and metal deposited ceramic electrode. Device has a round structure
and length of a couple of tens of millimeter, while having circumference around 10 mm.
Destructive analysis techniques, such as; cross-sectioning was used successfully for detection of
delamination but customer B wanted a non-destructive analysis option.
Even though SAM is conventionally used for flat surfaced-objects, usage for round object also is
getting spread out. A narrow scan line approximately 0.5 mm wide, along the device was
executed at several locations by turning the device using a 50 MHz transducer. Resulting SAM
micrograph is given in Fig.6. Device in image was expected to have partial delamination by
customer prior to SAM analysis in DELTA.

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Fig.6 SAM micrograph (A) of a partial delamination expected device with corresponding B-scan (cross-sectional
scan), Phase Gate mode image of corresponding scan micrograph (B) is shown.
As it can be seen from above Fig. 6, different modes of scanning possibilities were used and they
show consensus about the delaminated and the not-delaminated regions. It is important to
confirm the results with different mode of scanning in SAM, this enables the operator for correct
interpretation of the results before making the final conclusion.
In order to verify SAM results, dye penetrant test was used to highlight the delaminated areas
and observe visually. Devices were grinded in both ends until the border of the interest area.
Prior to grinding, devices were epoxy molded to prevent any artificially-induced failures during
grinding and handling. Resulting stereomicroscope image is given in Fig 7 for partially
delaminated device. Yellow line on the image shows the scan line along the device.
Fig.7 Optical micrograph of partially delamination expected device, yellow line shows a demonstrative scan line
along the device. Red are at the scan start shows the delaminated part of the interface in consensus with SAM results
given in Fig 6.
3 – Coating quality – voids in solder mask study
SAM technique is often used for coating quality studies such as for thickness of coating
measurement, intrusions of contaminants, delamination and trapped air bubbles/voids.
Customer C would like to investigate the quality of the solder mask for voids which works as
moisture trap and in longer term causes moisture related corrosion failures in PCBA assemblies.
In Fig 8, SAM micrograph is given with cross-sectional SEM (Scanning Electron Microscopy)
image of the same sample. SEM image was obtained after destructive cross-sectioning the
sample and observing in SEM. For the purpose of the analysis, SAM detected all voids in one

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scan for minutes. In the case of alternative, her SEM; cross-sectioning a random location, SEM
imaging for voids and then interpolating the result for all surface area. SAM provides this type
analysis – voids in coating – for transparent and opaque coatings non-destructively.
Fig.8 SAM micrograph of PCBA (A), and SEM image of cross-sectional analysis of PCBA (B). SAM shows total
void population in one scan in seconds. Alternatively SEM is used combined with cross-sectioning and interpolating
result to whole surface area of the PCBA.
4 – Counterfeit screening – Obsolete components
Due to rapid change in technology, components such as ICs are becoming outdated and their
production is being stopped. But due to marked needs for existing devices which uses those
outdated components, obsolete component market, so-called “grey-marked” emerges. Often re-
cycled, scrapped or counterfeit components are being refurbished, reworked and send into
marked illegally.
Company D uses obsolete components which were obtained from grey-marked. Components are
tested for functionality after procurement. Even though components passed functional tests,
customer demanded SAM analysis for device internal structures. SAM scanning details with
obtained results were given in Fig 9.
Fig.9 SAM micrographs for three obsolete SMDs. Device top and rear side SAM micrographs were given together,
Phase gate mode is used for highlighting delaminated interfaces for easy understanding of the scan results. Rear side
micrographs were provided after removal from PCBA.

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In Fig 9, component B has delamination in bond area, red colored bond areas therefore it is
critical and field failure in short time may occur. Evaluation of how critical an observed
delamination is carried out based on standards such as IPC/JEDEC J-STD-020D standard
“Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices
“ and DELTA’s
Conclusion
SAM is a non-destructive technique in failure analysis area and here is presented its versatile
usage for different purposes. Compared to alternative methods for same purposes, SAM is
practical and less time consuming. Here in this paper successful SAM analysis of different
subjects and failures were demonstrated for;
 Missing underfill materials in Flip Chip ICs
 Delamination between epoxy shell and metal electrode for round objects
 Coating quality of solder mask for air bubbles – for transparent and opaque coatings
 Counterfeit scanning for obsolete components
Compared to presented alternative test methods for the same purpose, SAM technique offers
non-destructive, practical, reliable and less time consuming option for users.
Acknowledgement
This article is issued under the project “Physics of Failure based reliable product development”
which is carried out by DELTA Denmark.