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

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

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. 

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

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

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

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

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

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

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

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.

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

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

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

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

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)

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

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

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

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

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

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

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

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

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.

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.

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