Thermal Fatigue Strength Estimation of Solder Joints of Surface Mount IC Packages

Long‐term fatigue life estimation for solder joints of surface mount IC packages is studied through elasto‐plastic stress analysis and temperature cycling tests. Strain on the solder joint induced by thermal expansion mismatch between package and substrate has been analysed by considering elasto‐plastic behaviour of the solder and by treating leads as rigid frames. Validity of the analysis has been confirmed by stiffness measurement of the soldered leads. Dynamic shear stress‐strain relationships of type 60Sn/40Pb solder are obtained as a function of temperature and frequency using hollow solder specimens of 15 mm in diameter and hollow solder joint specimens with the same diameter in the temperature range of −60°C to 150°C with frequencies of 0.01 Hz and 0.3 Hz. Fatigue tests are carried out for the solder specimens and the solder joint specimens under shear strain control and for the solder joints of the real IC packages under displacement control. All fatigue tests are conducted at room temperature with a frequency of 1 Hz. Fatigue test data of solder, solder joint and the solder joints of real IC packages fall in the same scatter band in the stra'un‐cycles to failure diagram. A fatigue life estimation model for solder joints of surface mount IC packages is proposed, which is derived by combining the strain calculated by the elasto‐plastic analysis and the fatigue data. To apply the proposed model to IC packages, the temperature cycling test between −55°C and +150°C is performed for two IC packages with different lead designs mounted on two different substrates (ceramics and glass‐epoxy). It is found that the fatigue life of solder joints by the temperature cycling test can be estimated by the proposed fatigue life estimation model. The proposed method is viable because it has sufficient accuracy with a cost of less than 1/100 when compared with the finite element method.