EVALUATION OF DEFECTS IN LEAD-FREE BGA SOLDER JOINTS UNDER RANDOM VIBRATION STRESS
Authors: Anurag Bansal, Kuo-Chuan Liu, and Jie Xue Company: Cisco Systems, Inc. Date Published: 10/24/2010
Abstract: Lead-Free assembly with fine pitch BGA components can be prone to latent defects at the solder joint level. These defects could arise due to design, variations in the quality of incoming components or PCB, assembly process, shipping or handling. Latent solder joint defects are often not detected in the end of line inspections and electrical functional testing, and they pose a serious reliability risk of early life failures in the end use application. In this study, a systematic Design of Experiments (DOE) has been conducted to evaluate the performance of latent defects in an accelerated random vibration environment. Lead-free MiniDIMM modules with 0.8 mm pitch daisy chain PBGAs have been assembled with a variety of solder joint defects that were intentionally introduced during PCB design, PCB fabrication, assembly, and handling. The advantage of this novel approach is that the nature of defects and the specific location of the defects were known in advance. HALT tests were performed on a limited sample of daisy chain modules and it was found that the test conditions were not sufficiently aggressive to cause failures in the fine-pitch BGA joints. An EMS shaker was subsequently used to perform random vibration tests. A stepped test was performed in a relative narrow frequency range that was centered at the natural frequency of the MiniDIMM modules. Based on the time to failure, the relative severity of various types of latent defects was evaluated in a random vibration environment. This study builds upon our prior work wherein the long term reliability impact of various defects was evaluated in a thermal cycling environment.
Key words: Lead Free, BGA, Defect, HALT, Vibration