1. Analysis of solder joint edge effect in acoustic micro imaging of microelectronic packages
General Engineering Research Institute
Electronic and Ultrasonic Engineering
Chean Lee
Dr. Guangming Zhang
Prof. Dave Harvey
27 February 2013

2. Outline
Acoustic Microscopy of
Microelectronic Packages
Objectives
Finite Element Modelling
Results
Post Processing
Results and Discussion
Conclusion

3. Acoustic Microscopy of Microelectronic Packages
Non-Destructive technique
Sensitive to voids, delaminations and cracks
Detects sub-micron discontinuities within material and interconnects
Technique characterized by spot size, focal depth, focal length & frequency
Study microstructures of specimen
X-Ray
AMI
Unreflowed Solder Bump, AMI presents better contrast of defect

4. Acoustic Microscopy of Microelectronic Packages
Edge Effect occurring at solder bump and die edge
Manifest as dark circles around the bump
Acoustic information loss, obscure minor cracks or voids.
Characterization of bond quality is inexplicit
Edge Effect seen around the flip chip solder bumps and the edges of the flip chips

5. Objectives The physical mechanism of edge effect is not clear.
Scatter theory is widely accepted
The acoustic energy is scattered by the physical geometry of the interface
This study aims to test scatter theory
Utilizing ANSYS APDL
2D Cross-Section Acoustic Simulation
Propose characterization methodology

6. Finite Element Modelling : Basics
Two things to understand about finite element modelling.
Nodes and elements.
The nodes make up a web of mesh that contain the structural properties.
Elements are formed when the web establishes a relationship with neighbouring nodes via vectors.
Basic Equation
The stiffness matrix defines the geometric and material properties between nodes. In its simplest form is derived from
Fx = kxyuy Fx = Force in direction X
kxy = Stiffness coefficient of x-y
uy = Displacement in Y
In short, Finite Elements can be said to be giant molecules that make an object. The finer the molecules the more accurate the results but require more computing power. This is where number of Element Per Wavelength (EPW) becomes important to determine how fine the mesh should be.
(Usually 5 EPW for draft simulations and 20 EPW for fine detailed simulations.)
The real explanation is much more complicated but this is brief rough description.
F=KU only represent the relationship between nodes for one direction. To adequately describe it, a matrix is build using this equation in all directions. Hence the “stiffness matrix”. This is the very basic equation for Finite Element Analysis.

7. Finite Element Modelling : Acoustic Wave Equation
Fluid Element propagation equation
C = speed of sound =
ρo= mean fluid density
K = bulk modulus of fluid
P = acoustic pressure
t = time
This equation neglects viscous dissipation. Therefore represents a lossless wave equation for sound in fluids.
For Fluid-Structure Interactions , the transient dynamic equilibrium equation below is considered simultaneously with the above acoustic wave equation
1. Wave propagation equation
2. Infomation (energy) transfer equation (when the simulation data transfer from fluid to solid medium)
[M] = Structural Mass Matrix
[C] = Structural Damping Matrix
[K ] = Structural Stiffness matrix
{ϋ} = nodal acceleration vector
{ύ} = nodal velocity vector
{U} = nodal displacement vector
{Fa} = applied load vector
The equation is employed using the generalized-α method.

8. Finite Element Modelling : General Simulation Model
Transducer is represented as a curve to produce a focused effect.
Transducer parameters
used in this experiment
(Sonoscan)
Diameter of transducer = 4750µm
Focal Length = 9500µm
Frequency = 230MHz
Wavelength in water = 7.5µm
If 20 Element per Wavelength,
then total elements = 120million elements (4750µm X 9500µm)
Hence, computational load is impractical.
The rule of thumb is that 15 to 20 element per wavelength to accurately represent the waveform. Anymore and it is a waste of resources, any less and it will compromise the fidelity of the simulation. 5 is good for draft simulations.

9. Finite Element Modelling: Simulation Model
Top half of the solder bump
Water medium
Virtual transducer
Absorbing boundaries
Under bump metallization
Solder bump 140µm diameter
SiO2 thickness 33µm
Overall width 187µm
230MHz acoustic pulse
Reducing the model is normal practice but the tricky part was the create an accurate scaled down virtual transducer.
Virtual transducer is placed very close to structure
Ansys guideline: Distance of absorbing boundary = 0.2Lambda from any structure.
Thickness of SiO2 layer is calibrated to account for compressional focus
Compressed focal point incidents on the Under Bump Metalization

10. Finite Element Modelling: Virtual Transducer
2 steps for rescaling of the transducer to a virtual microscale transducer.
Step 1, decide on the radius of the microscale transducer
Snell’s Law has to be considered and is given as
V1 = Acoustic Velocity of Material 1
V2 = Acoustic Velocity of Material 2
θ1 = Incident Angle
θ2 = Refraction Angle
The results show the focal point that occurs after the magnifying effect (compressed focus)
Simulation is carried out to measure performance of the virtual transducer inside SiO2.
Results show the focal point at 38µm and focal depth of 22µm inside SiO2
Thickness of the SiO2 layer of the model is then adjusted accordingly.

11. Finite Element Modelling: Virtual Transducer
Step 2, calculate the chord to accurately apply excitation loads given as
Transducer C0
Where R0
Chord = Chord of the micro transducer curve
R = Radius of microscale transducer
C0 = lens diameter of sonoscan transducer
R0 = focal lenght of sonoscan transducer
The curve of the transducer that is modelled is actually longer then the excitation arc, so the chord had to be determined to get an accurate representation.
Microscale Transducer
Chord R

12. Finite Element Modelling: Execution
Acoustic Pulse generated from 1nm displacement vectored perpendicular to the transducer curve. Length of excitation arc is inferred from the chord length.
Each iteration: Transducer position moved 0.5µm
Scan area: centre to edge of solder bump (70µm, half of model)
The chord is determined as specified in the last slide.
Transducer moves from centre to the edge, 70um is the position of the edge assuming the centre axis is 0um
Start of Scan
X=0µm
End of Scan
X=70µm

13. Results : Video Acoustic pulse at 35µm off-central axis of solder ball
This is the result when the transducer is positioned 35um off the central axis, a.k.a iteration no 70
Acoustic pulse at 35µm off-central axis of solder ball
Each iteration produces an A-Scan information. This is combined to create a B-Scan

14. Results : B-Scan Image
Each iteration produces an A-Scan information. This is combined to create a B-Scan
Main Bang
(Initial Acoustic Pulse)
Water to SiO2 interface
This is only half of the model cross-section, from the centre of the solder bump to the edge. Since the geometry is symmetrical, this data can be mirrored to make a complete cross-section.
SiO2 to Under Bump Metallization Interface
A-Scan
B-Scan

15. Post Processing: C-Line
Novel method:
C-Line, to characterize edge effect
Obtained by finding the peak of the gated data at every x-axis position.
Edge Effect Occurring Between 30µm to 50µm
Intensity begins to decline around 25µm

16. Post Processing: C-Scan Reconstruct
Rotate the C-Line data around a central axis
A C-Scan image of that layer can be constructed.
This C-Scan would represent an symmetrical solder bump.
Comparison between measured C-Scan image and simulated version of the same solder bump

17. Post Processing: C-Scan Reconstruct
Measured (Averaged)
Measured
Simulated

18. Results and Discussion: Scatter Theory
Portions of the acoustic energy is
loss as the scan progresses closer to the edge.
Results show that the edge effect occurs long before the acoustic pulse nears the beginning of the edge.
The pulse a.k.a spot size of the setup is 5µm wide
Results shown in slide: “Post Processing: C-Line”
The beginning of the edge is filled with water, which has a higher impedance mismatch. Reflection should be stronger.

19. Result and Discussion: Measured vs Simulated C-Line
Results shown in slide: “Post Processing: C-Line”

20. Result and Discussion: Measured vs Simulated C-Line
Hypothesis
Side lobe effect
Phase
Scatter
Modal change
Results shown in slide: “Post Processing: C-Line”

21. Results and Discussion: Initial Evaluation
Side Lobe
Side lobes are secondary acoustic emissions.
Could contribute measurable effect on overall average intensity detected by receiving transducer
Offset at 35µm
The vector of side lobe should allow it to reflect strongly from the water interface and into the transducer causing a strong signal. But overall intensity shows a decline.
Results shown in slide: “Post Processing: C-Line”
X-Axis = 35µm, where the intensity has already declined significantly
Phase of the side lobe doesn’t show. Decline of intensity starting too early.

22. Conclusion
Scatter theory does not have straight forward relationship with Edge effect Phenomena
Novel method of characterizing Edge Effect proposed
Results show that 60% of structural information is obscured.
Work raises more questions about edge effect mechanism

23. Further Work Employing C-Line to evaluate defect detect mechanism
Crack propagation
Voids
Minimum detectable defect
Hypothesis testing
Side Lobe
Mode Changes
Etc
Predict effective spot size inside medium