1. Through-Life Non-Destructive Monitoring of Solder joints using Ultrasound
Ryan S.H. Yang
04/11/2011
Supervisors:
David M. Harvey
Guang-Ming Zhang

2. Presentation Outline Introduction Reliability Assessment
Acoustic Micro-Imaging
Experimental Procedure
Feature Extraction
Results and Discussion
Conclusion

3. Introduction 1910 Ford Model T
How car electronics and reliability have moved on since 1908
Ford Model T

4. Introduction 2011 Ferrari Enzo
How car electronics and reliability have moved on since 1908
Ferrari Enzo

5. Introduction

6. Introduction Required Operation Temperature
“How long will this component last?”
The reliability of the electronic that operate in harsh environment become a major concern.
Among the reliability concern, solder joint reliability is the most critical issue, in many cases they are the weakest link in terms of product reliability.

7. Figure 2: Thermal mechanical expansion
Introduction
Thermo-mechanical stresses in the solder joints caused by Coefficient of Thermal Expansion (CTE) mismatch.
Imposes shear stress and Cyclic Strain b a
cooling
heating
CTE a < CTE b
During the cooling and heating, the silicon and PCB expand and contrast at different rate, hence generate plastic deformation.
Crack Initiation and Growth
Figure 2: Thermal mechanical expansion

8. Reliability Assessment
Solder joint fatigue failure could occur after thousands of cycles, which could mean 10 or 20 years of usage.
Accelerated Thermal Cycling (ATC) is a test that mimic the field service condition, and accelerated it with larger temperature range to reduce the test time.
ATC test is commonly used to generate rapid ageing of the solder joint.
Acceleration Factor (Coffin Manson equation):
In order to evaluate the reliability of the solder joints, ATC are used.
Number of cycle to fail in field to number of cycle to fail in test

9. Reliability Assessment Thermal cycling test chamber and test profile
During the ATC, we need tools to monitor the performance of the solder joints
Thermal cycling test chamber and test profile

10. Reliability Assessment
Common Solder joint failure monitoring
Electrical Testing.
The electrical continuity is monitored during the thermal cycling test.
Whenever a fracture occurs in a solder joint, the resistance reading will increased dramatically, thus indicating a failure.
Spikes higher than 300 ohms (IPC-SM-785 standards, IPC, 1992)
Electrical indication of failure can be intermittent as the fractured solder joint is still in contact to the substrate.
We need some robust technique!!
However, the electrical indications of failure can be intermittent as the fractured solder joint is still in contact to the substrate

11. Acoustic Micro Imaging Reaction of ultrasound wave in an object
Acoustic Micro Imaging (AMI)
Able to solder joint cracked during the thermal cycling test
Non-destructive inspection
Reflected, refracted or absorbed with respect to the differences between acoustic impedances
Reaction of ultrasound wave in an object

12. Acoustic Micro Imaging
Output (A-scan & Image)
Input
Scan motion
Ultrasound wave
De-ionised water
Flip chip
Bump to Board
Interface
Chip to Bump
Front surface
A-scan

13. Acoustic Micro Imaging
Face down Chip
Metalized Pads
Underfilled
Test Board
Connectors
Solder balls
Figure 4: C-scan image of chip-to-bump interface

14. Acoustic Micro Imaging
Typical resolution:
250 microns for 10MHz
75 microns for 30MHz
25 microns for 100MHz
10 microns for 230MHz
Factors affect the resolution in the acoustic image:
Frequency
Focal Length
Fluid Path
Signal Strength

15. Test Board Organic FR4 test board of 0.8mm thickness
14 flip chips on both sides of the board
Die Thickness = 725μm
die size = 3948μm × 8898μm
109 solder bumps
Ball height = 125μm

16. Accelerated Thermal Cycling Accelerated Thermal Cycling Test Profile
Accelerated Thermal cycling (ATC) test was carried out for 96 cycles. Test boards were investigated every 8 cycles by performing AMI imaging.
Accelerated Thermal Cycling Test Profile

17. Acoustic Micro Imaging
a 230MHz transducer with inch focal length was used.
Solder joints appear as a black ring with a slight grey area in the middle of the joints. The grey area indicates the connection between the chip and bump.
Unfortunately, due to the intrinsic properties of ultrasound, when the waves strike the edge of a material the signal is scattered causing degradation of information. The loss of return signal shows little information and appears as black in the image.
This phenomenon is known as an ‘edge effect’.
The higher intensity level is generated by the cracks in between the chip and bump.
Due to larger acoustic impedance mismatch, most of the signals are reflected back to the transducer and consequently produce higher intensities.
Figure 5: C-scan image of bump before and after test

18. Acoustic Micro Imaging Histogram of bump before and after ATC test

19. Feature Extraction Gradient based Circular Hough Transform
Input image, Io(x,y)
Find image gradient field
Thresholding
Circular Hough transform
Region growing
Radial gradient measurement
Multiply with Constraint function
Feature Extraction
INTENSITY, AREA AND HISTOGRAM extracted for each solder joint
ROIs Defined
Tagging and Labelling
Gradient based Circular Hough Transform
Radial Gradient based Region Growing

20. Feature Extraction
Gradient based Circular Hough transform

21. Feature Extraction Radial Gradient based Region Growing
Let H be the set of all unallocated pixels
Let N be the set of 8-connected neighbours pixels
Grow the seed pixels by the following rules
Repeat the growing until the all the neighbour pixels have been grown, i.e.

22. Feature Extraction
Radial Gradient based Region Growing

23. Result and Discussion
Performance Analysis:- Error Ratio & Area Similarity

24. Result and Discussion
Mean Intensity VS Area Plot

25. Result and Discussion
Similar Characteristic Plot

26. Histogram Distance VS Thermal Cycles
Result and Discussion
Fails at 24 cycles
The slope indicate some changes and show interesting behaviour. But we haven't got the result yet because the sectioning is still going on. We are exploring the feasibility of through life monitoring by AMI in this paper.
Histogram Distance VS Thermal Cycles
A sharp jump in histogram distance value after certain cycles indicates a severe failure of a solder joint.

27. Mean intensity VS Thermal Cycles
Result and Discussion
Fails at 24 cycles
The slope indicate some changes and show interesting behaviour. But we haven't got the result yet because the sectioning is still going on. We are exploring the feasibility of through life monitoring by AMI in this paper.
Mean intensity VS Thermal Cycles
A sharp jump in mean intensity value after certain cycles indicates a severe failure of a solder joint.

28. Result and Discussion Fails at 24 cycles
The slope indicate some changes and show interesting behaviour. But we haven't got the result yet because the sectioning is still going on. We are exploring the feasibility of through life monitoring by AMI in this paper.
Area VS Thermal Cycles
A sharp jump in area value after certain cycles indicates a severe failure of a solder joint.

29. Figure 11: AMI Monitoring plot for Bottom Row
Result and Discussion
Figure 11: AMI Monitoring plot for Bottom Row

30. Figure 12: 3D Plot for AMI Monitoring
Result and Discussion
Figure 12: 3D Plot for AMI Monitoring

31. Result and Discussion
The creep energy dissipation for individual bumps of specific solder volume over a single thermal cycle were solved using ABACUS FE simulation software.

32. Prediction and Monitoring
Comparison of Prediction and Monitoring Result

33. Conclusion
The failure distribution pattern over a short number of cycles was able to be tracked and monitored by an AMI inspection technique.
The reliability distribution pattern measured from AMI provides evidence of strong correlation between FE prediction and accelerated test results.
A robust monitoring technique in solder joint through lifetime performance is one of the key factors to ensure high quality electronics products.

34. Thank You for your Attention !!
1910 – 2011 – 2110?!
Thank You for your Attention !!
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