This paper presents the low cycle fatigue properties of two kind of solder materials and evaluation method of thermal fatigue life for solder joints. Thermal stress at the solder joint of semiconductor devices subjected to thermal cycle is discussed. Thermal fatigue test results of semiconductor devices are compared with the predicted life obtained from the low cycle fatigue strength of solder.
The results obtained are as follows:
(1) The mechanical shear properties depend on the test temperature and strain rate for two solder materials (60 Pb-40 Sn and 88 Pb-10 Sn-2 Ag). The low cycle fatigue strength of these solder materials can be expressed by equation (1). This equation (1) is valid for the test temperature range from 20°C to 135°C and the hold time up to one hour.
ΔγT·Nf0.85=14.0 (1)
where ΔγT and Nf represent the total strain amplitude and the number of cycles to failure, respectively.
(2) By taking the nonlinear stress-strain properties into account, a theoretical analysis was made to obtain the thermal shear strain in the solder joint. The effects of major structural dimensional factors on shear strain were clarified.
(3) Thermal fatigue life tests of semiconductor devices were conducted and the resistance change method was adopted to detect the fatigue failure of devices. The predicted life was found to be in a rather safety side compared with the measured life.
