Thermomechanical fatigue, solders

D. Frear, Thermomechanical Fatigue in Solder Materials, Solder Mechanics A State of the Art Assessment, 1991, p 191-237... 

Thermomechanical fatigue of solders may be further complicated because the compositions and volume fractions of the phases present vary with temperature. In contrast to Sn-Pb solders, this is likely to only be a minor problem with Sn-Ag eutectic solder because the solubility of silver in solid tin is very small at all temperatures. This is also true for Sn-Cu-based solders. With Sn-Zn-based solders, the effect of temperature on the microstructure is minimal because the maximum solubility of Zn in Sn is only 0.6 at.% at the eutectic temperature. Although the phase... 

In general, it appears that while SMD pads tend to improve the resistance of solder joints to mechanical shock loading conditions, they also tend to degrade the thermomechanical fatigue life of the joints. Therefore, when the choice of pad construction is made, the impact on both thermomechanical fatigue and shock conditions should be characterized. (Details of typical shock testing conditions are outhned in Chap. 59). 

In addition, this chapter outlines the role of numerical analysis techniques in anatyzing and improving the reliability of solder joint interconnects. Detailed procedures illustrating the use of finite element analysis (FEA) in estimating the thermomechanical fatigue Ufe of solder joint interconnects and monotonic bend testing are also discussed. 

Bartelo, J., Thermomechanical Fatigue Behavior of Selected Lead-Free Solders, IPC SMEMA Council, APEX, San Diego, January 14-18,2001, LF2-2. 

For rehability life prediction of solder interconnects through creep fatigue interactions under thermomechanical loading conditions, time and path dependent creep models are needed. A modified Coffin-Mason type equation is typically assumed to relate the number of cycles to failure to the creep strain energy density. Work is ongoing to evaluate the materials constants in the Coffin-Mason equation for SAC (Ref 106-107). 

J. Bartelo, S.R. Cain, D. Caletka, K. Dar-bha, T. Gosselin, D.W. Henderson, D. King, K. Knadle, A. Sarkhel, G. Thiel, and C. Woychik, Thermomechanical Fatigue Behavior of Selected Lead-Free Solders, IPC SMEMA Council Proc. APEX, paper LF2-2, 2001... 

Under field-use conditions or under accelerated thermal cycling qualification conditions, the solder joints experience cyclic thermomechanical loads due to GTE mismatch or thermal gradients among various parts of a packaging assembly and failure due to such thermomechanical loads. In addition to thermomechanical fatigue loads, the solder joints experience stresses due to mechanical loads such as vibration, shock, etc. However, such mechanically-induced failures and other chemically or electrically-induced failures are not the focus of this chapter. 

Guo, Q. Cutiongco, E. Keer, L. Eine, M. Thermomechanical fatigue life prediction of 63Sn 37Pb solder. J. Electron. Packag. 1992, 114, 145 151. 

FIG. 22 Cycles-to-failure for thermomechanical (TMF) 25-80°C and isothermal fatigue at 80°C in 96.5Pb-3.5Sn solder vs. cycle period. The TMF eifect is negative for periods less than 60 sec but positive at longer cycle periods. (From Ref. 20.)... 

FIG. 23 Cycles-to-thermomechanical-fatigue failure (20-80 °C) plotted against total strain range for Sn-Ag and Sn-Zn eutectic solders. The ramp time was 50 sec. The mechanical and thermal cycles were in phase. The plot also allows a comparison to be made between the 50% reduction from and abrupt load drop failure criterion. (From Ref. 7.)... 

Palmer, M.A. Redmond, P.E. Messier, R.W. Thermomechanical fatigue testing and analysis of solder alloys. J. Electron. Packag. 2000, 122, 48-54. 

Solders with intentionally incorporated reinforcements are termed composite solders. The composite approach to solders was developed mainly to improve service-temperature capabilities. This improvement results from a stabilized fine-grained nticrostructure and an increased uniformity in solder-joint deformation. These factors serve to improve the overall mechanical properties of a solder joint, especially creep and thermomechanical fatigue resistance. An important additional feature is that the reinforcements do not alter the melting point of the solder matrix, but effectively increase the service-temperature capabilities of the base solder materials. 

Stress relaxation describes the time-dependent deformation behavior of a material under constant displacement, which is evaluated by measuring the stress reduction with time. The evaluation of the stress relaxation process is necessary in order to assess the eflect of hold periods in the thermomechanical fatigue behavior of solder joints. Understanding of the stress relaxation mechanisms is important because the deformation processes involved in stress relaxation are fairly representative of the stress history experienced by solder joints at the temperature extremes of thermal cycling experienced in service. Thus the stress relaxation behavior may be a better... 

In addition to these studies, which focused on implementation issues, primarily with respect to manufacturing and reliability, other studies yielded materials property data for lead-free solders that give insight into phase transformations, solderability, and thermomechanical fatigue (TMF) resistance. For example, over 20 universities in the United States, Canada, and the... 

Accelerated Thermal Cycle Testing. The mechanical property data was included as part of the quantitative down-selection process based on a thermomechanical fatigue (TMF) test the test vehicle consisted of a 441/0 leadless ceramic chip carrier (LCCCs-44) with only corner leads soldered to provide conditions for high stress. Test parts were thermally cycled between —55 and + 125 ° C at the rate of 24 cycles per day. The TMF data was expressed as the number of cycles-to-failure for an alloy expressed as a percentage of the cycles-to-failure for eutectic Sn-Pb solder. 

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