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Single shear lap creep specimens with a 1 mm2 cross sectional area (similar in size to small lead-free solder joints used in electronic packaging and jointing) between thin copper strips were developed and fabricated using lead-free solder (Sn-3.5Ag) to quantify their creep strain with in situ micro electronic-resistance measurement. Where the solder joints' micro electronic-resistance is in situ measured by an electronic testing system (tailor-made for the micro electronic-resistance and stress measurement) and recorded by a PC via serial port, then all data of micro electronic-resistance and elapsing time are formed in curves. They are used to describe the solder joints' micro electronic-resistance and electronic- resistance strain varied with time. Most of curves can reveal the continual development of damage and fracture mechanisms which are consistent with observations generated by literatures. The quantitative relationship between electronic-resistance strain and mechanical-creep strain was proved theoretically using a mathematic model. These mean that the in-situ micro electronic-resistance measurement can be used as an alternative quantification of creep strain in small lead-free solder joints. Thus, provide an alternative and simplified evaluation method about the reliability of a solder joint.