Creep strain is probably the most important time-dependent damage accrual factor affecting solder joint reliability. Under typical multi-hour loading conditions, creep-induced strain is a complex function of solder metallurgical structure, solder temperature, loading time per cycle, the applied stress, and the spring constant of the combined part/lead/board system. The complex system level creep-fatigue interactions involved in electronic part solder joints are shown to be a strong function of the relative stiffness ratio κ, which is the ratio of the stiffness of the combined solder-lead system to the stiffness of the solder element by itself. For a leadless chip package, κ is close to unity. For a compliant leaded package, κ is typically in the 0.01 to 0.0001 range. Important environmental stress dependencies, including the effects of operating temperature, displacement amplitude due to thermal and mechanical cycling, and cyclic frequency of loading are investigated for different levels of k. Understanding the sensitivity of solder strain range to relative stiffness and these key environmental parameters is important to understanding the behavior of alternative packaging concepts and to achieving robust electronic packaging designs and testiong approaches.

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