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Reliability Impact of Defects in Lead-Free BGA Solder Joints - A Systematic Study
Keywords: Lead Free Solder, Solder Defects, Reliability Test
In high volume manufacturing environments, design rules and process variables must be tightly controlled to ensure high quality production yields. However, in spite of various controls, defects in solder joints can arise due to deviations in PCB quality, assembly process, and handling. Poorly formed solder joints are insidious latent defects that will escape functional tests and can pass undetected in most in-line visual and x-ray inspection steps, only to cause early field reliability problems. In this study, a systematic DOE approach has been adopted to study the long term reliability impact of lead free solder joint defects. A wide variety of solder joint defects will be intentionally manufactured under controlled conditions. The test vehicle is a daisy chain MiniDIMM memory module with 16 layers and 125 mil thickness. The cards will be populated with 0.8mm pitch 60-ball daisy chain BGA components assembled in both single sided and double-sided configurations. In addition to completely normal test cards, various solder joint defects will be selectively introduced during PCB design, PCB fabrication, paste printing, component placement, reflow, cleaning, and handling stages. Each type of defect will be manufactured at two levels of severity under tightly controlled conditions. The advantage of this novel approach is that the specific modules and BGA pad locations with certain types of defects are known in advance. All modules will be subjected to thermal cycling from 0 to 100C and the daisy chain resistances will be continuously monitored during the test. While all efforts must be made to eliminate such defects, the results from this study will provide quantitative reliability data to understand the impact of process excursions to long term reliability. The results will also provide guidance on critical manufacturing parameters and processing windows for high reliability applications.
Anurag Bansal, Manufacturing Engineer
Cisco Systems, Inc.
San Jose, CA

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