Metallurgical considerations for accelerated testing of soft solder joints

The occurrence of new packages as well as the ongoing miniaturisation in SMT make the evaluation of the reliability of solder joints an permanent task. Accelerated testing, especially passive thermal cycling, is a important tool to evaluate the lifetime of solder joints. However, tin-lead solder behaves viscoplastically even at ambient temperature because of its low melting point and therefore the temperatures 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 mandatory to take the metallurgical behaviour of tin-lead solder into account when accelerated tests are to be performed. However, many accelerated test performed in industry do not at all care for this fact: Temperature shock chambers are used in order to shorten the test time activating deformation mechanisms that do not occur in reality. 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, which temperatures and temperature exchange rates occur in the solder joints.