52.Reliability by John G. Webster (Editor)

By John G. Webster (Editor)

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9 for Pb–5%Sn solder TM ϭ Melting point temperature M ϭ Constant Cyclic Creep. Under test conditions, the cyclic creep component is larger than the static creep component because the cycles expected in the field lifetime are crowded into a relatively short test period. Cyclic creep is a function of the number of cycles rather than time because each cycle induces a shear ‘‘burst’’ of stress (and strain) which relaxes in a finite time to a threshold level. Cyclic creep has been studied in depth with flip chips (22).

It appears that cracks form and grow in each cycle by being fed dislocations that move in from the surrounding lattice. Thus, the intersection of moving dislocations is a principal cause of the formation of vacancies and fatigue damage. We know from the striations on the fracture surface that crack propagation takes place in each cycle. Forsyth (6) has advanced a mechanism for the formation of striations, which involves first, the extension of the crack front by simple slip on the primary glide plane and second, further extension by cross-slip as the stress increases.

It remained for Coffin and Manson to show that many polycrystalline materials, fatigued at constant plastic strain amplitude, obey the relationship, C ϭ ͙Nf и ⌬⑀p, where Nf is the number of cycles to failure, ⌬⑀p the plastic strain range, and C is a constant of the material. Apparently the reason so many materials obey an equation like Coffin’s is because of the fundamental nature of the mechanism of crack initiation and propagation. Therefore the ‘‘modified’’ fatigue model is an extension of the Coffin-Manson relationship that also enables one to relate fatigue to frequency and temperature, which are important variables in the solder fatigue encountered in electronic packages.

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