DocumentCode :
2507437
Title :
Evolution of the tension/compression and shear cyclic stress-strain behavior of lead-free solder subjected to isothermal aging
Author :
Mustafa, Muhannad ; Cai, Zijie ; Roberts, Jordan C. ; Suhling, Jeffrey C. ; Lall, Pradeep
Author_Institution :
Center for Adv. Vehicle & Extreme Environ. Electron. (CAVE3), Auburn Univ., Auburn, AL, USA
fYear :
2012
fDate :
May 30 2012-June 1 2012
Firstpage :
765
Lastpage :
780
Abstract :
The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work, we have demonstrated that the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during elevated temperature aging were unexpectedly large and universally detrimental to reliability. Solder joints in electronic assemblies are typically subjected to thermal cycling, either in actual application or in accelerated life testing used for qualification. Mismatches in the thermal expansion coefficients of the assembly materials leads to the solder joints being subjected to cyclic (positive/negative) mechanical strains and stresses. This cyclic loading leads to thermomechanical fatigue damage that involves damage accumulation, crack initiation, crack propagation, and failure. While the effects of aging on solder constitutive behavior (stress-strain and creep) have been recently examined in some detail, there have been no prior studies on the effects of aging on solder failure and fatigue behavior.In our current work, we have examined effects of several parameters (aging temperature and time, testing temperature, strain/stress limits, and solder alloy composition) on the cyclic stress-strain behavior of SAC lead free solders. Both uniaxial specimens subjected to cyclic tension/compression and Iosipescu lap shear samples subjected to cyclic positive/negative shear have been studied. Samples were subjected to mechanical cycling under both strain control (constant positive and negative strain limits) and stress control (constant positive and negative stress limits). The hysteresis loop size (area) was calculated from the measured cyclic stress-strain curves for a given solder alloy and temperature. This area represents the energy dissipated per cycle, which is correlated to the damage accumulation in the specimen. In this paper, we- report on our findings on the effects of isothermal aging on the cyclic stress-strain behavior of SAC lead free solders. Prior to cyclic testing, the specimens were aged (preconditioned) at 125oC for various durations (up to one year). From the recorded cyclic stress-strain curves, we have been able to characterize and empirically model the evolution of the solder hysteresis loops with aging. Similar to solder stress-strain and creep behavior, there is a strong effect of aging on the hysteresis loop size (and thus the rate of damage accumulation) in the solder specimens. The observed degradations in the fatigue/failure behavior of the lead free solders are highly accelerated for lower silver content alloys, and in this paper we concentrate on presenting the results for SAC105 (Sn-1.0Ag-0.5Cu) lead free solder.
Keywords :
ageing; copper alloys; electronics packaging; failure analysis; fatigue; life testing; reliability; silver alloys; solders; stress-strain relations; thermal expansion; tin alloys; Iosipescu lap shear samples; SAC lead free solders; Sn-Ag-Cu; accelerated life testing; assembly materials; crack initiation; crack propagation; creep behavior; cyclic positive-negative shear; cyclic stress-strain behavior; cyclic stress-strain curves; cyclic testing; damage accumulation; electronic assembly; elevated temperature aging; failure behavior; fatigue-failure behavior; hysteresis loop size; isothermal aging; lead free solder joint microstructure; lead-free solder; mechanical cycling; mechanical response; mechanical strains; mechanical stresses; reliability; shear cyclic stress-strain behavior; solder failure; solder fatigue behavior; solder hysteresis loops; strain control; stress control; temperature 125 degC; tension-compression evolution; thermal cycling environments; thermal expansion coefficients; thermomechanical fatigue damage; Aging; Cooling; Fatigue; Metals; Strain; Stress; Temperature measurement;
fLanguage :
English
Publisher :
ieee
Conference_Titel :
Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2012 13th IEEE Intersociety Conference on
Conference_Location :
San Diego, CA
ISSN :
1087-9870
Print_ISBN :
978-1-4244-9533-7
Electronic_ISBN :
1087-9870
Type :
conf
DOI :
10.1109/ITHERM.2012.6231505
Filename :
6231505
Link To Document :
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