1. Electronic and Ultrasonic Engineering Group
Solder Joint Reliability Assessed by Acoustic Imaging during Accelerated Thermal Cycling
Ryan Yang
05/03/2010
G M Zhang, D M Harvey
Electronic and Ultrasonic Engineering Group
GERI

2. Presentation Outline Introduction Accelerated Thermal Cycling
Acoustic Micro Imaging
Feature Extraction & Blob analysis
Test and Analysis
Future Work & Conclusion

3. Introduction
Solder joint reliability is a primary concern in the assembly of all Electronic components and products.
The importance of solder joint reliability became more emphasized in recent years as a result of three factors:
1) The shift from leaded to lead-free solders in semiconductor industry.
2) Shrinking in die size as well as solder balls dimension.
3) The emergence of fine-pitched area array packages that employ hundreds of solder joints for electrical connection.
Environmental friendly
Electronics manufacturers are willing to make the transition to Pb-free solders
-but are concerned with the reliability implications of such a transition. 
-Thus, companies are ensuring that all the necessary modeling and reliability tests are conducted on their products. 

4. Introduction “How long will this component last?”
Solder joints reliability is the ability of solder joints to remain in conformance to their mechanical and electrical specifications over a given period of time, under a specified set of operational conditions.
It is a measure of the likelihood that a solder joint will not fail throughout its intended operating life, subject to various thermo-mechanical stresses that it encounters in its operation.
It is extremely important especially in automotive, avionics and defence industries since the electronics operate in harsh environments.
Solder joint interconnect serve two purposes:
To form electrical connection between component and substrate
To form mechanical bond that hold the component to the substrate

5. Introduction
Solder joints failures can be attributed to three core factors:
Fracture – Tensile rupture/fracture through mechanical overloading
Drop, Force fitting, Collision,
Creep – Long Lasting permanent loading
Board Warping, Weight, Pressure
Cyclic Fatigue – Cyclic loading
Thermal Cycling, Power Cycling, Vibration

6. Figure 1: CTE mismatch producing cycling stress
Accelerated Thermal Cycling
Most fatigue failures are attributed to the thermo-mechanical stresses in the solder joints caused by Coefficient of Thermal Expansion (CTE) mismatch.
Plastic Deformation
Figure 1: CTE mismatch producing cycling stress
CTE -- Materials expand because an increase in temperature leads to greater thermal vibration of the atoms in a material, and hence to an increase in the average separation distance of adjacent atoms.
This CTE mismatch imposes cyclic strain (fatigue) to the joints.
Due to viscoplastic nature of plastic, large creep and plastic deformations accumulate in the solder joints lead it to thermal fatigue failures.
The creep-fatigue mechanism involves crack initiation and crack growth until complete rupture of the solder connection
Imposes Cyclic Strain
Crack Initiation and Growth

7. Figure 2: Illustration of the Thermal Cycle
Accelerated Thermal Cycling
The reliability assessment of solder joints under creep-fatigue effect can be carried out by Accelerated Thermal cycling (ATC) tests.
Temperature
Time
-40C
+125C
Hot Dwell
Tmin
Cold Dwell
Tmax
Ramp Rates
Figure 2: Illustration of the Thermal Cycle

8. Accelerated Thermal Cycling
ATC test result will be used to:
Estimate field product reliability
Provide data for Numerical (FEA) model
Verify and validate the FEA model
Conventional Inspection Techniques:
Electrical Resistance Measurement
Physical Micro-sectioning
Destructive
Not allowed for detailed study
Destroy Evidence

9. Acoustic Micro Imaging
Acoustic Micro Imaging (AMI)
Non-destructive inspection
Makes use of the properties of ultrasonic waves which range from 5MHz to 500MHz
Reflected, refracted or absorbed with respect to the differences between acoustic impedances
(your going to get asked questions about ultrasound. Give a clearer description of what is going on. Remember it is the impedance of the materials which determine how much energy is absorbed. High freq…better resolution less penetration. Low freq the opposite is true.
Figure 3: Reaction of ultrasound wave in an object

10. Acoustic Micro Imaging
Face down Chip
Metalized Pads
Underfilled
Test Board
Connectors
Solder balls
Describe solder joint behavior
Figure 4: C-scan image of the Flip chip substrate-bump interface

11. Solder joint Detection
Feature Extraction
Solder joint Detection
Analysis / Inspection
Feature Extraction
Gradient-based Hough Transform
Blob Analysis
Tagging and Labelling
Mask Solder joint region
Extract Masked Solder joint features
Intensity
Gradient
Area
Centroid
Number of Joints
Histogram
Intensity ratio
Rule-based analysis
Template-based analysis
A group of pixels organized into a structure is commonly called a blob. Area of touching pixels with the same logical state.

12. Feature Extraction Gradient-Based Hough Transform
Figure 5: 2D and 3D view of Accumulation Array from Circular Hough Transform

13. Figure 6: Raw image with circle detected
Feature Extraction
Gradient-Based Hough Transform
Figure 6: Raw image with circle detected

14. Figure 7: Image of all solder joints with tag and label
Feature Extraction
Tagging and Labelling
Figure 7: Image of all solder joints with tag and label

15. Figure 8: Image of all solder joints cover with mask
Feature Extraction
Mask solder joints region
Figure 8: Image of all solder joints cover with mask

16. Figure 7: Photograph of completed circuit board (front and back)
Test and Analysis
Test Board
Figure 7: Photograph of completed circuit board (front and back)

17. Figure 8: Thermal Profile of the circuit board under test
Test and Analysis
Thermal Profile
Figure 8: Thermal Profile of the circuit board under test

18. Test and Analysis R0M2 Figure 9a: Flip Chip U23 before thermal cycling
Figure 9b: Flip Chip U23 after 500 cycles thermal cycling

19. Figure 10: Histogram of Flip Chip U23 solder joint R0M 2
Test and Analysis
Figure 10: Histogram of Flip Chip U23 solder joint R0M 2

20. Figure 10: Comparison of good and bad joints by ratio of bright pixels
Test and Analysis
Figure 10: Comparison of good and bad joints by ratio of bright pixels

21. Test and Analysis
Figure 11: Microscope image of flip chip and PCB with solder joints left on the PCB board after thermal cycling

22. Test and Analysis
Figure 12: Magnified Microscope image of flip chip and PCB with solder joints left on the PCB board after thermal cycling

23. Figure 9a: Test Board 04_04, Flip Chip U26, Solder Joint BBM 19
Results and Analysis
Figure 9a: Test Board 04_04, Flip Chip U26, Solder Joint BBM 19

24. Results and Analysis
Figure 10: Comparison of BOTTOM-BOTTOM joints by ratio of bright pixels

25. Future Work More data analysis will be carried out, such as:
Template-based analysis
Rule-based analysis
Automated feature extraction
Defect classification
Large amount of sample image and data will be tested using above analysis system.
The system will be used in recording the variation of image parameters during the thermal cycling test.
Physical cross section analysis and SEM analysis will be used to verify the results.

26. Conclusion An Accelerated thermal cycling test has been carried out.
A feature extraction and blob analysis system has been discussed and results of the inspection are shown.
Accelerated thermal cycling test is one of the key tools for evaluating solder joints reliability.
Inspecting the samples is time consuming and mostly dependant on the operator’s experiences.
A robust feature extraction and data analysis system is vital to provide more consistent and accurate analysis as well as achieve high quality inspection.

27. Thank You for your Attention!