This document discusses using X-ray fluorescence (XRF) as a non-invasive method to measure the percentage of silver (%Ag) in tin-silver (SnAg) solders used in flip chip applications. It describes how XRF was calibrated and optimized through comparisons with invasive techniques like inductively coupled plasma (ICP) and differential scanning calorimetry (DSC). Factors that impacted XRF measurement accuracy included solder bump geometry, collection time, and background noise. Flattening solder bumps improved XRF reliability and consistency. With proper calibration and accounting for limitations, XRF was found to be an excellent non-invasive method for measuring solder composition variability across wafers, chips and individual solder

1. © 2009 IBM Corporation
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

2. © 2009 IBM Corporation
2
Introduction
X-Ray Fluorescence (XRF) is a non-invasive method of measuring Ag%
composition in Pb-free solders.
1. Sample preparation - Experimental bumping variables
2. Tool configuration - XRF configuration, calibration, optimized measuring
methodology and the importance of having known standards with the same
dimensions of the bumps being measured
3. XRF recipe setting - Measuring accuracy and correlation with ICP and DSC
4. Data interpretation - Ag distribution study in the die and wafer level

3. © 2009 IBM Corporation
3
Why XRF?
One area of interest for Pb-free
solder manufacturing is the ability
to control and measure the %Ag
composition and its variation from
wafer to wafer, chip to chip, and C4
to C4.
Invasive Non-Invasive
Atomic Absorption (AA) X-Ray Fluorescence
(XRF)
Differential Scanning
Calorimetry (DSC)
Inductively Coupled
Plasma (ICP)
Electron Probe Micro-
Analyzer (EPMA)
Methods to Measure Solder CompositionWafer to Wafer Chip to Chip C4 to C4
Ag% Composition Control
Pb-free SnAg solder has
become the industry standard for
fabricating flip chip interconnects
utilizing C4 (controlled collapse
chip connection) technology.

4. © 2009 IBM Corporation
4
BumpVariablesWaferVariables
Underlying Metallurgy
Bottom Layer Metallurgy
(BLM) Size
UBM Stack
C4 Height
Ag% Target
Mechanically Good (MG)
Electrically Good (EG)
90 um
110 um
Thick Cu
No Cu
70 um
90 um
0.6%
1.7%
Sample Preparation – Test Vehicle List
VariablesImpactingXRFMeasurement

5. © 2009 IBM Corporation
5
Tool Configuration
# BLM
Size
C4
Height
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
C4NP is a solder
transfer process
which controls the
Ag% composition to
± 0.1%Ag.
XRF Calibration Standards
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
Verification Methods
Through data comparison with ICP
and DSC, a non-invasive XRF
method was created and
calibrated.

6. © 2009 IBM Corporation
6
Mass Transport Effect
 The need for the correct geometry comes from the learning that the %Ag is mass transport
controlled. The concentration of Ag in a larger 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.
C
Site A has a largest height controlled by
diffusion plating compared to B or C
Site A has the lowest Ag%
Diffusion Controlled Plating Convection Controlled Plating
10um
Site C has the smallest height controlled by
diffusion plating. Compared to A or B
Site C has the highest Ag%

7. © 2009 IBM Corporation
7
Optimized collection time creates a
stronger XRF feedback generating
more stable readings.
From C. Shih
XRF Performance by Collection Time

8. © 2009 IBM Corporation
8
Process time 180sec on C4NP
Standards
20 XRF Readings on
Randomly C4s
# BLM
Size
C4
Hei
ght
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%
Remeasure on flattened C4NP Standards
Process time 180sec on flattened
C4NP standards
20 XRF Readings on
Randomly C4s
# BLM
Size
C4
Hei
ght
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%
XRF inspection on C4NP Standards
 Tool issues such as stage movement accuracy and laser alignment were investigated and
then eliminated as possible causes.
The X-ray spot size is 40µm -> a C4 geometry issue was hypothesized.
Reliability dramatically improved after the flattening, proving that C4 geometry impacts the
intensity of the signal. Flattening is recommended if the bump height is less than 70um
What caused the instability in A and B?
Bump Geometry

9. © 2009 IBM Corporation
9
FlattenedUn-Flattened
SEM photos comparing flattened and un-flattened C4’s
The spot size of the X-ray
opening is 40 µm.
The detector is collecting
photoelectrons from any region
that is excited by the X-rays.
Flattening reduces the chance
of exciting larger regions which
would cause artificially higher
readings and thus a larger
standard deviation.
From S. McLaughlin

10. © 2009 IBM Corporation
10
The spectrum data shows a stronger
feedback of X-ray
fluorescence radiation on larger C4s.
Ag%
Un-reflowed Bump
From C. Shih
90um, 1.8% 70um, 1.8%

11. © 2009 IBM Corporation
11 From C. Shih
Wafer 1
-thick Cu
Wafer 2
-thick Cu
Wafer 3
-thin Cu
Comparison Post Reflow

12. © 2009 IBM Corporation
12 From C. Shih
Wafer 1
-thick Cu
Wafer 2
-thick Cu
Comparison Post Flattening
Wafer 3
-thin Cu

13. © 2009 IBM Corporation
13
XRF readings on MG wafers are comparable with ICP data (within 0.2%Ag), but the XRF
readings are not reliable on EG wafers due to high background noise
To alleviate this, the XRF collection time was shortened to mitigate primary X-ray penetration.
Collection Time on EG Wafers
From C. Shih
EG/MG Spectrum Comparison

14. © 2009 IBM Corporation
14 From C. Shih
EG spectrum pre-flattening EG spectrum post-flattening

15. © 2009 IBM Corporation
15
Conclusion
XRF is a popular, non-invasive inspection method for bump composition.
Several challenges were encountered when establishing this technique.
 Balancing X-ray power voltage and current settings to obtain suitable X-ray penetrating ability.
 Mitigating background noise, especially from EG chips
 Finding an appropriate collection time
 Characterizing 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.

16. © 2009 IBM Corporation
16
References
Acknowledgements
J. Sylvestre, et al.,”The Impact of Process Parameters on the Fracture of Brittle Structures During
Chip Joining on Organic Laminates,” 2008 ECTC.
E. Perfecto, et. al, “C4NP Technology: Present and Future,” 2008 IMAPS Device Packaging
Conference.
B.. Beckhoff B. Kanngießer N. Langhoff R.Wedell H.Wolff (Eds.) in Handbook of Practical X-Ray
Fluorescence Analysis 2006.
Special thanks go to Chia-Hsin Shih and KyungMoon Kim from STATSChipPAC as well as Charles
Arvin and Eric Perfecto from IBM for their guidance and support. Thanks also go to John Pennacchia,
Stephen McLaughlin and the entire C4 team.