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March 2015 • The PCB Magazine 37 Where: Nf = Number of cycles to failure d = a material dependent constant ΔT = entire temperature cycle-range for the device m = an empirically determined constant This power-rule relationship explains the effect that temperature range has on thermal- fatigue life cycles-to- failure distribution. Gen- eral suggestions for m for ductile metal fatigue range from about 1–3 [12, 17, 18, 19] , and a critical review of a large number of papers led Blish to extract a useful set of m constants with copper listed as 5.0 [19] . The acceleration factor for the test conditions is then derived by: With low cycle fatigue (plastic strain), the acceleration factor is typically applied to the number of thermal cycles rather than the tem- perature exposure time [11] . Humidity is another commonly used accelerating variable, particu- larly for failure mechanisms involving corro- sion and certain kinds of chemical degradation; however, humidity was not considered for this model. With high humidity, we would expect to see failures such as creep corrosion, whisker growth, and conductive anodic filament shorts (CAF). Escobar, Meeker, O'Connor, and Kleyner discuss and review several humidity models [10, 11] . Combined environmental stress testing (CERT) allows for the study of the effect of an interac- tion between two or more accelerated stresses, but it is generally not possible to develop an ap- propriate prediction model under CERT [1] . The IPC-TR-579 committee concluded that PTH reliability decreases as the thickness of the PWB increases, higher laminate T g increase thermal cycle performance, and precondition- ing the PWB (simulated assembly temperatures) reduces reliability [18] . Neumann, et al. [13] , found a significant re- lationship between laminate T g and PTH ther- mal cycling reliability. Laminate T g significantly outweighed plated acid copper elongation for determining PTH reliability with the elongation varying between 15–25%. The researchers came to the same conclusion for the acid copper-plat- ed thickness; that of the laminate T g significant- ly outweighed the acid copper plated thickness for determining PTH reliability with the thick- ness varying between 20–40 micron. Thermal cycling was done both below and above lami- nate T g . Similar conclusions were demonstrated and reached showing reduced thermal cycling reli- ability of lower T g laminates vs. high T g lami- nates [20] , and reductions in reliability were seen with larger deltas between the laminate T g and peak thermal cycling temperature reached [7] . figure 5a: Thermal stressed, pad rotation is evident. b: pTh stress axes. c: Thermal induced stresses. Feature RELIABILITy TESTING AND STATISTICS continues