Metallurgical considerations for accelerated testing of soft solder joints

The development of new packages as well as the ongoing miniaturization in surface mount technology (SMT) make the evaluation of the reliability of solder joints a permanent task. Accelerated testing, especially passive thermal cycling, is a important tool to assess the lifetime of solder joints. However, tin-lead solder behaves viscoplastically even at ambient temperature because of its low melting point and therefore the temperature extremes and the temperature swing of the tests performed as well as the temperature change rate are very important parameters for testing. Different deformation rates cause different deformation mechanisms to occur. Therefore it is crucial to take the metallurgical behavior of tin-lead solder into account when accelerated tests are to be performed. However, many accelerated test designs in industry do not at all care for this fact. (1) Temperature shock chambers are used in order to shorten the test time activating deformation mechanisms that do not occur in reality. (2) Test chambers are overloaded, test specimen with high mass are tested or the specimen are placed with varying orientations to the air stream of the chamber not caring about temperatures and temperature gradients occurring in the solder joints.