Thermal mechanical fatigue

Jeng, S.M., Yang, J.M. and Aksoy, S. (1992). Damage mechanisms of SCS-6/Ti-6Al-4V composites under thermal-mechanical fatigue. Mater. Sci. Eng. A 156, 117-124. 

Gao, M., Dunfee, W., Miller, C., Wei, R. P., and Wei, W., Thermal Fatigue Testing System for the Study of Gamma Titanium Aluminides in Gaseous Environments, in Thermal-Mechanical Fatigue Behavior of Materials, Vol. 2, ASTM STP 1263, M. J. Verrilli and M. G. Castelli, eds., American Society for Testing and Materials, West Conshohocken, PA (1996), 174-186. 

Strictly from an assembly process point of view, the mixing of Pb-free and traditional Sn-Pb solder can be beneficial. The Sn-Pb solder can improve the wetting and spreading performance of the Pb-free solder by two phenomena. First, Pb contamination lowers the molten solder surface tension of the solder. Second, the Pb contamination reduces the melting temperature of the Pb-free alloy. However, concerns are raised by the mixing of Sn-Pb and Pb-free solders and its effect on the long-term reliability of interconnections under thermal-mechanical fatigue environments. 

The accepted method of nondestructive testing used to control the underfill process is SAM. The thin layer allows this technique to detect voids in the underfill material, which when located near the solder interconnections can be responsible for a significant loss of thermal mechanical fatigue reliability. X-ray techniques can be used to monitor the density of the underfill material, specifically, the distribution of filler material within the layer under the die. Density variations can indicate a larger distribution of underfill mechanical and physical properties, which may affect long-term reliability performance of the solder joints. Quantitative image analysis can be coupled into SAM and x-ray analysis data to provide valuable process control tools for the factory floor. 

Fig. 1 Optical micrographs that illustrate damage to surface mount solder joints due to (a) mechanical shock and (b to d) fatigue. Note the coarsening of the Pb-rich phase in latter case that is the signature of thermal mechanical fatigue in Sn-Pb solders.

Solder interconnects are most often exposed to TMF while in service. Thermal mechanical fatigue takes place when temperature fluctuations combine with thermal expansion mismatch between different substrate materials, the global mismatch, or between the solder and the substrate materials, the local mismatch, to generate a cycle strain in the interconnections. The complicating factor of TMF, when compared to isothermal fatigue, is that the temperature changes concurrently with the strain cycle so that the entire strain event (As) does not occur at the same... 

Thermal mechanical fatigue is a low-cycle fatigue process that is described, explicitly, by the temperature limits (Fig. 10b). The cycle is characterized by (a) relatively slow ramp rates between the maximum and minimum temperatures and (b) hold times at the minimum and maximum temperatures. Both parameters determine the cycle frequency. It has been demonstrated with the Pb-bearing solders that the ramp... 

WSRC, DPST-87-886 (ECS-LOCA-5), "Thermal/Mechanical Fatigue."... 

Failures will occur preferentially in the Pb n solder under slow strain rate load conditions. As such, an intermetallic compound layer has minimal impact on failures that occur due to time-dependent deformation (creep) and low-cycle fatigue (e.g., thermal mechanical fatigue). The failure path occurs in the solder, near to one of the intermetallic compound layers. Cracking in the solder is preempted when one of the interfaces associated with the intermetallic compound layer is inherently weak, such as in the case of the AuSr /Au structure (Fig. 14). 

The term thermal mechanical fatigue (TMF) was introduced to describe this temperature cycle plus thermal expansion mismatch degradation mode in surface mount solder joints. This phase emphasizes that the source of the cyclic deformation as being temperature variations rather than applied cyclic loads (e.g., vibration), thereby distinguishing TMF from mechanical fatigue. 

Vianco, P.T. Burchett, S.N. Neilsen, M.K. Rejent, J.A. Erear, D.R. Coarsening of the Sn-Pb solder microstructure in constitutive model-based predictions of solder joint thermal mechanical fatigue. J. Electron. Mater. 1999, 28, 1288-1294. 

It is important to note that a major role of solder in area array solder joints at the chip and package level is to accommodate plastic strain without cracking that is, the solder is a plastic strain absorber. This implies that weak solders are potentially good candidates for this application. It has been demonstrated that solders with low shear strengths exhibit the greatest thermal cycling lifetimes [124], and this explains the lower thermal mechanical fatigue resistance of tin-based solders compared to lead-based solders (Table 5). 

Thermal Mechanical Fatigue and Fatigue Initiated from Wear Cracking... 

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