1. 1
Assessment of XRF Technique as a Method to Measure Percent Ag in SnAg Solders
for Flip Chip Applications
Jennifer D Schulera
Chia-Hsin Shihb
, Charles L Arvina
, KyungMoon Kimc
, Eric Perfectoa
a
Semiconductor Research and Development Center, IBM, Hopewell Junction, New York
12533
b
STATS ChipPAC Taiwan Ltd, Hsin-Chu Hsien Taiwan, R.O.C 307
c
STATS ChipPAC Korea Ltd, Kyoungki-do, 467-814 South Korea
Pb-free SnAg solder has become the industry standard for
fabricating flip chip interconnects utilizing C4 (controlled collapse
chip connection) technology. One area of interest for
manufacturability of Pb-free solders is the ability to control and
measure the %Ag composition and its variation from wafer to
wafer, chip to chip, and C4 to C4.
There are various ways to measure solder composition. These are
divided into two categories which are invasive and non-invasive
referring to whether solder must be removed from the wafer in
order to conduct the measurement.
There are a variety of invasive methods including Atomic
Absorption (AA), Differential Scanning Calorimetry (DSC),
Inductively Coupled Plasma (ICP) and Electron Probe Micro-
Analyzer (EPMA) used with cross sections. Non-invasive methods
are limited, making the development of the non-invasive X-Ray
Fluorescence (XRF) method an important technique to determine
both the thickness and composition of C4s on wafers without
modifying the wafer.
There are many factors which can affect the accuracy of the XRF
measurements. These include bump geometry, composition, UBM
(under bump metallurgy) stack, bump spatial density, underlying
chip wiring, tool vibration and tool parameters, such as collimator
size, power levels, scan time, etc.
This paper will address the implementation issues in utilizing XRF
for Pb-free solder SnAg systems. The paper will describe:
(1) Experimental bumping variables,
(2) XRF configuration, calibration, optimized measuring
methodology and the importance of having known standards
with the same dimensions of the bumps being measured
(3) Measuring accuracy and correlation with ICP and DSC,
(4) Ag distribution study in the die and wafer level

2. 2
Sample Preparation
Several experimental bumping variables impact the XRF measurement, including
underlying metallurgy, BLM (bottom layer metallurgy) size, UBM stack, C4 height and
Ag% target (Table 1). To fully understand the various factors, two kinds of wafers were
used. One was a silicon oxide wafer which was classified as mechanically good (MG).
The other type was with underlying chip wiring and front-end processing, which was
classified as electrically good (EG). The test wafers received a sputtered seed layer, and
then received a plated UBM and SnAg solder. To understand the effect of the UBM on
the XRF reading, two UBM stacks were prepared containing a thick Cu UBM or no Cu
UBM.
Vehicle
No.
Wafer Type
BLM
(µm)
C4 Height
(µm)
UBM Stack %Ag Target in SnAg
Solder
1 MG 90 70 No Cu SnAg (0.6%Ag)
2 MG 90 70 No Cu SnAg (1.7%Ag)
3 MG 90 70 Thick Cu SnAg (0.6%Ag)
4 MG 90 70 Thick Cu SnAg (1.7%Ag)
5 EG 90 70 No Cu SnAg (0.6%Ag)
6 EG 90 70 No Cu SnAg (1.7%Ag)
7 EG 90 70 Thick Cu SnAg (0.6%Ag)
8 EG 90 70 Thick Cu SnAg (1.7%Ag)
9 MG 110 90 Thick Cu SnAg (0.6%Ag)
10 MG 110 90 Thick Cu SnAg (1.7%Ag)
11 EG 110 90 Thick Cu SnAg (0.6%Ag)
12 EG 110 90 Thick Cu SnAg (1.7%Ag)
Table 1. Test Vehicle List. MG: Mechanically Good. EG: Electrically Good.
In order to setup and calibrate the XRF settings, C4NP standards were constructed
[2]. C4NP is a solder transfer process which controls the Ag% composition to ± 0.1%Ag.
ICP was used to confirm the samples produced by C4NP (Table 2).
No.
BLM
Size
C4
Height
Wafer
Type
Known Bump
Composition
UBM
Stack
SnAg Solder
Process
A 90 70 MG 0.5%Ag Ni/SnAg C4NP
B 90 70 MG 1.8%Ag Ni/SnAg C4NP
C 110 90 MG 0.5%Ag Ni/SnAg C4NP
D 110 90 MG 1.8%Ag Ni/SnAg C4NP
Table 2. Standards For XRF setup & calibration.
Tool Configuration
Three tools were utilized to determine bump composition (Table 3). These were
the XRF, ICP-OES (optical emission spectroscopy) and DSC. Through data comparison
with ICP and DSC, a non-invasive XRF method was created and calibrated. At the same
time, procedures were created to allow for daily checks and future validation and re-
calibration of the XRF unit if readings began to drift.

3. 3
Tool Description
XRF
Anode material is made of
tungsten or molybdenum
ICP-OES
Invasive method to get C4
composition on global zone
DSC
Invasive method to get C4
composition on global zone
Table 3. Bump Composition Inspection Tool Information.
It is important to understand the general workings of the XRF unit in order to
write the XRF recipe. The X-ray tube generates the primary X-radiation (primary
radiation). The electrically heated cathode emits electrons. Accelerated by the applied
high voltage to very high speeds, the electrons bombard the anode material. This
generates the primary X-radiation. The primary filter optimizes the energy distribution of
the primary X-radiation. The shutter serves as a safety device and closes the access of
the primary X-radiation to the measurement chamber, if needed. Within the recipe, it
requires one to pick the type of filter, the accelerating voltage, the power, etc. Each one
of these values were chosen based upon the type of structure to be analyzed. It was
extremely important to know how each impacted the accuracy of the final reading. In
many cases, it was unknown how each new structure would react. Thus, much trial and
error was required initially until the final recipe could be determined. In all cases, it was
paramount that standards of the correct size and geometry were utilized.
XRF Recipe Setting
The Si PIN Diode detector with the advanced de-convolution software makes it
possible to reduce the number of standards required. However, even with this advance, it
was important to establish the recipe correctly and have at least two standards of the
correct geometry and structure. The need for the correct geometry comes from the
learning that the %Ag is mass transport controlled. Thus, the smaller the opening in the
photoresist, the %Ag in the final deposit is reduced. In addition, for a constant plating
rate, as the plating gets closer to the top of the photoresist, the %Ag in the deposited
SnAg increases. This means that the concentration of Ag in a test structure on the edge
of a wafer may have a different and most likely higher concentration of Ag than the Ag in
an actual C4.
Establishing the recipe first required defining the structure to be measured. This
included listing the various stacks of layers if they could contribute to the intensity of a
given element and if the layer was a single element or a combination. Initial calibration
was conducted in accordance with the recommended procedure in the tool manual using
the C4NP standards. Based on experimental data, the initial recipe process time was set
to180s (Figure 1). Unstable readings were detected on the C4NP standards A and B with
70µm C4 height (Table 4). As a result, standards C and D were used for calibration and
further investigations were conducted to determine the root cause of the unstable readings
on standards A and B.
Tool issues such as stage movement accuracy and laser alignment were
investigated and then eliminated as possible causes. The next item looked at was the
impact of the sample itself. The X-ray spot size of 40µm was an appropriate dimension
to measure standards A and B. Thus, a C4 geometry issue was hypothesized. To prove

4. 4
this, the C4s from standards A and B were stamped to make them flat on the top surface
to alleviate X-ray scattering effects. Reliability dramatically improved after the
flattening, proving that C4 geometry impacts the intensity of the signal. (Table 5, Figure
2, Figure 3).
Figure 1. Longer recipe process time creates a stronger XRF feedback generating
more stable readings. The condition with recipe process time 1000 sec is best. However,
180 sec produces a comparable standard deviation and significantly reduces the data
collection time from 17 min to 3 min.
Process time 180sec on C4NP Standards 20 XRF Readings on Randomly C4s
No. BLM Size C4 Height Ag% Target Mean Std Dev Range
A 90 70 0.5% 0.66% 0.44% 1.84%
B 90 70 1.8% 1.77% 0.16% 0.53%
C 110 90 0.5% 0.48% 0.06% 0.19%
D 110 90 1.8% 1.79% 0.05% 0.21%
Table 4. XRF inspection on C4NP Standards
Process time 180sec on flattened C4NP
standards
20 XRF Readings on Randomly C4s
No. BLM Size C4 Height Ag% Target Mean Std Dev Range
A 90 70 0.5% 0.50% 0.05% 0.20%
B 90 70 1.8% 1.89% 0.06% 0.27%
Table 5. XRF inspection on flattened C4NP Standards
The spot size of the X-ray opening is 40 µm. However, this is not the same as for
the detector. Thus, the detector is collecting photoelectrons from any region that is
excited by the X-rays. Thus, by stamping, it reduces the chance of exciting larger regions
which would cause artificially higher readings and thus a larger standard deviation. This

5. 5
concept is further elaborated in the next few sections by also taking into account the
background noise.
Un-stamped Flattened
Figure 2. The SEM photos for the comparison between un-stamped and flattened C4s.
Figure 3. The spectrum data shows a stronger feedback of X-ray fluorescence radiation
on larger C4s.

6. 6
The XRF region of interest was restricted to read the X-ray fluorescence range of
Kα, Kβ of Sn and Ag only in order to more accurately measure alternate UBM stacks
since there is a high risk of spectrum overlay among Lα, Lβof Sn, Ag and other materials
in the UBM stack. Voltage and current must also be balanced to keep the linear range on
the detector since the voltage was needed to cause the electron transitions and current was
directly proportional to intensity. If at all possible, it is best to choose conditions that can
cover all analysis instead of changing them since changing tube conditions affects overall
X-ray stability which in turn would affect each calibration. Voltage also affected the
depth of penetration of X-rays. With most XRF spectrometers, X-rays penetrate deep
into the sample to generate X-ray fluorescence. The fluorescent X-rays coming out of a
sample depend on the energy of the incoming X-ray and the travel distance to the
detector. Often X-rays are self-absorbed within the samples and never reach the detector.
As energy increases, the penetration is deeper into the sample. However, there is a point
known as infinite thickness for each element where the photoelectron intensity leaving
the sample does not change with thickness. This is considered a bulk type analysis. High
power at 50kV was used to excite the electrons on K layer to enhance the peaks of Kα, β
of Sn and Ag (Figure 4).
Figure 4. Generation of X-ray fluorescence radiation.
Since flattened C4 tops provide a stronger XRF signal feedback, measurements were
attempted prior to reflow. Unfortunately, the diffusion limited plating properties of Ag%
were highlighted in this study. Ag deposition at the initialization of the stack is
haphazard, but gradually the Ag% increases within the deposit from bottom to top. The
highest Ag% was found on the top of the C4 column. The XRF penetration may not be
deep enough to reach the bottom of solder column. This generated inconsistently high
Ag% readings relative to homogeneous reflowed bumps.
Results
Data Interpretation
Examining the XRF data (Table 6), the flattened C4 top method was necessary to get
stable and exact readings on samples with low C4 height <=70 µm. XRF readings on MG
wafers are comparable with ICP data (within 0.2%Ag), but the XRF readings are not
reliable on EG wafers. According to the fluorescence spectrum, high background noise is
found on some C4 positions with underlying chip wiring on EG wafers. Rebuilt C4NP
EG chips confirmed the background noise issue. Underlying chip wiring caused high
background noise in XRF spectrum (Figure 5, Figure6). To alleviate this, the XRF
collection time was shortened to mitigate primary X-ray penetration (Table 7). If the

7. 7
SnAg thickness was sufficient (in this study, 90 µm), good reliability was found on XRF
readings for both 0.5% and 1.8% Ag.
Wafer
Type
BLM
(µm)
BH
(µm)
UBM Stack
(µm)
%Ag
Target
Unstamped
or Stamped
XRF Reading
(%)
ICP
(%)
Mean σ
1 MG 90 70 No Cu
Plated
0.6%Ag
Unstamped 0.78 0.45
0.63
Stamped 0.66 0.05
2 MG 90 70 No Cu
Plated
1.7%Ag
Unstamped 1.83 0.22
1.72
Stamped 1.71 0.06
3 MG 90 70 Thick Cu
Plated
0.6%Ag
Unstamped 0.84 0.24
0.73
Stamped 0.74 0.03
4 MG 90 70 Thick Cu
Plated
1.7%Ag
Unstamped 1.93 0.32
1.71
Stamped 1.73 0.05
5 EG 90 70 No Cu
Plated
0.6%Ag
Unstamped 1.04 0.37
0.65
Stamped 0.80 0.22
6 EG 90 70 No Cu
Plated
1.7%Ag
Unstamped 2.23 0.61
1.77
Stamped 1.97 0.32
7 EG 90 70 Thick Cu
Plated
0.6%Ag
Unstamped 1.22 0.56
0.76
Stamped 0.97 0.19
8 EG 90 70 Thick Cu
Plated
1.7%Ag
Unstamped 2.33 0.71
1.65
Stamped 1.96 0.16
9 MG 110 90 Thick Cu
Plated
0.6%Ag
Unstamped 0.44 0.04
0.46
Stamped 0.45 0.05
10 MG 110 90 Thick Cu
Plated
1.7%Ag
Unstamped 1.56 0.04
1.52
Stamped 1.52 0.04
11 EG 110 90 Thick Cu
Plated
0.6%Ag
Unstamped 2.11 0.70
0.41
Stamped 0.63 0.30
12 EG 110 90 Thick Cu
Plated
1.7%Ag
Unstamped 2.06 0.40
1.50
Stamped 1.60 0.20
Table 6. Experiments. Sample size: 20 random C4s/1 chip/Test Vehicle on plated chips.
Wafer
Type
BLM
(µm)
BH
(µm)
UBM
Stack
(µm)
%Ag
Target
Process
Time
XRF Reading (%)
Mean σ
Range
13 EG 90 70 2Ni
C4NP
0.5%Ag
30 sec 0.66 0.32 1.10
180 sec 1.29 0.42 0.99
14 EG 90 70 2Ni
C4NP
1.8%Ag
30 sec 2.23 0.24 1.05
180 sec 2.00 0.12 0.90
15 EG 110 90 2Ni
Plated
0.5%Ag
30 sec 0.53 0.07 0.24
180 sec 0.69 0.28 0.92
16 EG 110 90 2Ni
Plated
1.8%Ag
30 sec 1.90 0.06 0.24
180 sec 1.90 0.06 0.17
Table 7. Experiments. Sample size: 20 random C4s/1 chip/Test Vehicle on C4NP EG
chips.

8. 8
Figure 5. High background spectrum noises are found on EG chips.
Figure 6. Illustration explaining spectrum performance between EG and MG chips and
why high Ag% is detected on EG chips.
Prior to flattening C4 Post flattening C4
Figure 7. Flattened C4 can help obtain a stronger XRF peak. The spectrum post flattening
is better than an unstamped C4
EG, 70 µm C4, 0.5%Ag, C4NP
MG, 70 µm C4, 0.5%Ag, C4NP
ROI 644~920

9. 9
Dissolved Cu% in SnAg Solder
In order to determine how much Cu is dissolved into the bulk solder, two techniques,
DSC and XRF are used. In order to obtain the melting point of the bulk solder, it must be
separated above the Cu intermetallics (IMC). DSC was then conducted to obtain the
solidus temperature, liquidus temperature along with the melting and solidification
behaviors. By knowing the liquidus temperature and the Ag% obtained from XRF, a
SnAgCu phase diagram could then be used to obtain the Cu%. (Figure 8).
The phase diagram in Figure 8 is an equilibrium diagram. However, DSC is
collected at various rates. Three different scanning rates were used to determine the
impact of the melting point for each alloy as a function of scan rates shown in Figure 9.
Figure 10 shows how to relate the DSC scan to the phase diagram. As was expected, the
solidus temperature was independent of the scanning rate. However, the liquidus
temperature was dependent. In order to use the phase diagram, one needs to determine
the melting point at equilibrium. It can be seen in Figure 10 that the temperature of the
final melt is reduced as the scan rate is lowered. The reason for this is not allowing
enough time at each temperature step for equilibrium to occur. However, the step change
in temperature as a function of scan rate is being reduced. The tool available for this
work could only go down to 5o
C / sec. Determining the final melt temperature and each
of the scan rates of 20, 10 and 5 o
C / min, it was possible to obtain the following
relationship for this particular structure, y = 0.1422 x + 229.75. Utilizing this
relationship and extrapolating to near zero showed that only a further 0.7 C reduction
should occur from the measured value at 5o
C / sec. As such, all samples were run at 5o
C
/ sec and 0.7 degree C was subtracted from the measured value. It was also observed that
there are two peaks in Figure 8 and 9g. This was due to the various phases within the
alloy. The first peak is the transformation of the Ag3Sn / beta Sn interface from solid to
liquid which is the eutectic composition and melts first. The second peak is the
transformation of the remaining material. As the %Ag increases, the relative values of
the first peak to second peak change. In the case of the 1.7% Ag alloys, the second peak
is just a plateau.

10. 10
Figure 8. Based on the DSC data and ternary phase diagram, the rough dissolved Cu%
range in bulk solder could be determined
Figure 9. The DSC curves under different heating rates. (5,10,20 degree C/min)

11. 11
Conclusions
XRF is a popular, non-invasive inspection method for bump composition. Several
challenges were encountered when establishing this technique. A key item needed to
implement this method was how to balance X-ray power voltage and current settings to
obtain suitable X-ray penetrating ability. If the X-ray penetration was too high,
background noise was intensified in the presence of underlying wiring of EG chips. If
the penetration was too low, unstable readings were encountered and there was low
accuracy. There existed a permanent background noise baseline creating a tool ability
limitation for very low Ag% detection (less than 0.2% Ag). Through XRF, ICP, DSC
comparison, XRF recipes can be developed as an excellent monitor for non-invasive
bump composition evaluations. As in all methods, XRF has a unique set of challenges
and limitations which can be overcome through proper calibration and verification.
As the packaging trend moves toward finer pitched products, studies of small
diameter C4s will become pervasive. If XRF is preferred to measure the bump
composition of smaller C4s, geometry will become a key issue. C4 flattening helps
mitigate geometry issues on smaller C4s, but may be insufficient for micro-bump
features. Future work will involve how to define C4 geometry and the possible creation
of composition test sites which will need to have extensive correlation established.
References
1. J. Sylvestre, et al.,”The Impact of Process Parameters on the Fracture of Brittle
Structures During Chip Joining on Organic Laminates,” 2008 ECTC.
2. E. Perfecto, et. al, “C4NP Technology: Present and Future,” 2008 IMAPS Device
Packaging Conference.
3. B.. Beckhoff B. Kanngießer N. Langhoff R.Wedell H.Wolff (Eds.) in Handbook
of Practical X-Ray Fluorescence Analysis 2006.
Liquidus Line
Solidus
0.6%
Ag
1.7%
Ag
Figure 10. By rotating the DSC curves 90o
it is possible to see how they relate
to the phase diagram. Note that as the Ag% increases, the size of the first peak
increases and the size of the second reduces.
BA