Flip chip on board assemblies have been widely used in high reliability product applications such as automotive, aerospace and defence industries. However, flip chip solder interconnections are often the weakest link in terms of product reliability. This is mainly due to the high CTE (coefficient of thermal expansion) mismatch deformation between the silicon die and organic substrate during extreme environmental operation that leads to fatigue failures. Thus, this thesis aims to develop a non-destructive and automated monitoring system using ultrasound technology to assess solder joint through-life performance during thermal excursions. The capability and feasibility of using ultrasound technology to achieve non-destructive monitoring was investigated and discussed. An automated ultrasonic inspection and monitoring system on flip chip solder joints was designed to solve the existing issues in capturing the fatigue failures during thermal cycling. Acoustic Micro Irnaging (AMI) techniques have been used along with the accelerated thermal cycling (ATC) test to inspect solder joints due to their strong capability to detect discontinuities within materials and interconnections. However, this approach has not previously been used as an effective tool in monitoring solder joints through life performance throughout environmental testing. The research regime proposed in this thesis was to track the condition of solder bonds monitored through ultrasound images from birth to failure, and to see how their reliability was affected by joint position. This work included accelerated thermal cycling tests to deliberately generate fatigue defects on custom designed test boards, and perform AMI scanning every 8 test cycles over a total period of 96 test cycles. A robust image segmentation and feature extraction system was designed and implemented on the ultrasound images to extract the image features such as histogram difference, bonding area and intensity level that represent the quality of the joint. Through data analysis and classification, the failure occurrence cycles were measured. Finally, a measure of solder joint fatigue life distribution was plotted and compared to a Finite Element prediction. The results have showed that corner joints have the lowest levels of reliability whereas joints closer to the neutral point in the centre of a chip have higher reliability. These results are in accord with the FE prediction and cross-sectional analyses. This matches flip chip failure theory where corner joints are the most critical joints during thermal excursions. AMI monitoring has displayed the defect occurrences in a more realistic manner which encounters real life conditions as compared to FE prediction. This capability allows AMI to non-destructively monitor end user products through life and to be able to reflect real 'in-field' reliability. The monitoring techniques were systematically investigated and verified in this thesis. The results have highlighted the potential of AMI in the non-destructive evaluation of solder joints throughout environmental tests.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:555848 |
| Date | January 2012 |
| Creators | Yang, Swee How |
| Publisher | Liverpool John Moores University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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