Finite element modeling and experimental validation of conventional and high speed shear testing in Pb-free environment

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Title: Finite element modeling and experimental validation of conventional and high speed shear testing in Pb-free environment
Author: Ajmera, Abhinav; Ramkumar, S. Manian; Liu, Ti
Abstract: With the extensive use of Pb-free solder in electronic assemblies, there is a growing concern about the reliability of the solder joint. The integrity of the intermetallics, formed during the reflow process, at the component–pad interface is one of the critical determinants of joint reliability. Studies indicate that the brittle fracture of intermetallic compound (IMC) at the component-pad interface makes Pb-free solder joints more vulnerable to failure. Pb-free alloys with a high content of Sn and high reflow temperatures; experience accelerated and thicker intermetallic formation at the interface [1]. The brittle intermetallics are susceptible to fracture during a major stress event, such as drop, over the entire life cycle of the joint [2]. This necessitates the investigation of possible approaches to predict and detect brittle fracture. The tests that are available currently do little in this regard as they are incapable of consistently applying the force to demonstrate brittle fracture [3]. The low speed shear test has not been successful in generating bond failures, as the failure typically happens in the bulk of the solder joint. However, a high speed shear test may succeed in demonstrating the brittle fracture as they replicate high strain rate events [3]. The need to identify the capability of high speed shear, to reveal brittle fracture failures, was the driving force behind this study. Shearing of 0603 resistors mounted on a PCB with Pb-free solder was considered as the test process setup for modeling and experimentation. Finite Element Analysis (FEA) was employed to replicate the process of shearing the solder joint. A comparison was drawn between the FEA results and the results obtained through the actual lab testing. The FEA results for the low speed shear test suggest that the failure would have a very high probability of occurrence in the bulk of the solder with the shear force ranging from 20- 28 N. This range, predicted by FEA, was found to be 20% lower than the actual test results. Furthermore, FEA provides reasonable assurance about the capability of high speed shear to demonstrate brittle fracture at the intermetallic. Application of force in the range of 56-65 N would be required for this failure mode at the interface. Moreover, the results establish Finite Element Analysis as a reasonable approach to illustrate the changing stress patterns that a component undergoes when subjected to shear. On the whole, the results of the analysis substantiate high speed (high force) shear being a promising test to predict and detect the brittle fracture failure of the Pb-free interconnects.
Description: ©2008 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE."Finite element modeling and experimental validation of conventional and high speed shear testing in Pb-free environment," Proceedings of the 58th Electronic Components & Technology Conference. Held in Orlando, Florida: 27-30 May 2008. ©2008 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.
Record URI: http://hdl.handle.net/1850/8375
Date: 2008

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