Abstract
Sn/3.5Ag eutectic solder is one of the most elemental materials among the various Sn/Ag or Sn/Ag/Cu lead-free solders. In order to evaluate the thermal fatigue crack initiation by microstructural observation of the solder, a series of thermal cycle tests was carried out by using some PCBs on which resistance chips were mounted. Consequently, noting that dispersing Ag_3Sn phase grows, the microstructural evolution is quantified by phase growth parameter S that is defined by average phase size to the 4th power d^4. Additionally, a power law relation exists between S and number of cycles N and enables us to evaluate the thermal fatigue crack initiation of the solder joints.
