SMTA International Conference Proceedings


INVESTIGATION OF IMPACT RELIABILITY OF Pb-FREE SOLDER INTERCONNECTS IN BALL GRID ARRAY ASSEMBLIES

Authors: Fahad Mirza, Saurabh Athavale, Krishnan Vishwanathan, Amol Kane, Mike DiPietro, Eric Cotts, James Pitarresi, and Daryl Santos, Ph.D.
Company: Integrated Electronics Engineering Center, Binghamton University
Date Published: 10/11/2007   Conference: SMTA International


Abstract: Board level solder joint reliability performance during impact loading is a critical issue in the electronics industry. The growth in the use of handheld consumer electronic products such as cell phones, digital cameras and PDAs has pressed the manufacturers to address their shock reliability issue. Also, the gradual transition to Pb-free semiconductor products has led to the urgency in reliability assessment of Pb-free solder joints. Due to absence of Pb, these solders are very brittle and more prone to failure against shock than Sn-Pb joints which are softer, thereby making impact reliability assessment for Pb-free a very significant concern. This paper deals with the investigation of the dynamic response of the Pb-free test vehicles (TVs) to various mechanical testing protocols such as drop shock, ball shear, and die shear. Statistical impact reliability evaluation of the 2nd level solder interconnects is completed and various modes of failure are analyzed and discussed. The PCB is designed as per JEDEC JESD22-B111 which is in conjunction with the bend test JEDEC standard and assembled with Ball Grid Array (CABGA) components with a soak reflow profile for Pb-free solder. The Test Vehicle (TV) is then subjected to published JEDEC standard impacts (input accelerations) of 1500-g, 2000-g and 2900-g with half sine pulses of 0.5, 0.4, 0.3-ms time period, thereby inducing different velocity changes (delta v), hence providing different energies for each drop scenario. A comparative study of the number of cycles to failure and failure modes has been demonstrated with regards to location of components on the board, input accelerations and the orientation of the TV during drop.

Furthermore, a comprehensive study of the effect of high temperature aging on the reliability of Pb-free assemblies in high strain rate environments is also completed. Both quantitative and qualitative comparisons between the nonaged (0 hours, fresh TVs) and aged (upto 1000 hours aging at 150°C) assemblies are presented in order to account for the product end use conditions. It was observed that aging not only lowered the impact reliability but also caused the change in the failure mechanism of the solder joint. The failure mode transitioned from pad cratering (board side) for 0 hours aged TVs to Intermetalic layer crack (IMC failure at component side joint) for 500 and 1000 hours aged TVs.

Key words: Pad cratering, impact reliability, high temperature aging



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