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

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

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Introduction

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.

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

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

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

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

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

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

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

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

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

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

Acoustic Micro Imaging a 230MHz transducer with 0.375 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

Acoustic Micro Imaging Histogram of bump before and after ATC test

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

Feature Extraction Gradient based Circular Hough transform

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.

Feature Extraction Radial Gradient based Region Growing

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

Result and Discussion Mean Intensity VS Area Plot

Result and Discussion Similar Characteristic Plot

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.

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.

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.

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

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

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.

Prediction and Monitoring Comparison of Prediction and Monitoring Result

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.

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