Low-Temperature Embrittlement of Lead-Free Solders in Joint Level Impact Testing

The low-temperature embrittlement of tin-based (Sn- based) lead-free (Pb-free) solder alloys has been previously reported for bulk specimens tested in impact by means of a conventional Charpy V-notch test setup. The brittle failure of Sn-based Pb-free solders at low temperatures (i.e. below -30degC) is of great concern for electronic assemblies that operate at harsh conditions, because it can lead to occurrences of early failures. A miniaturized impact test setup has been devised to perform a scaled-down version of the conventional Charpy impact test on specimens, which lie in the size range of 300 mum to 3 mm. In this work, the impact tests have been carried out in the temperature range between 23 degC and -110degC. The specimens consisted of SuperBGA^reg (SBGA^reg) substrates bumped with 4 different solder alloys: Sn3%Ag0.5%Cu (SAC 305), Sn4%Ag0.5%Cu (SAC 405), Sn3.5%Ag, and Sn37%Pb. The energy absorbed during the fracture of the solder joints was seen to decrease as a function of temperature for the SAC 305 and SAC 405 alloys, while for the Sn37%Pb and Sn3.5%Ag this trend was less pronounced. The ductile-to-brittle transition in the fracture behavior of Sn- based Pb-free solders was confirmed by scanning electron microscopy (SEM) investigations. These investigations showed the progressive bulk embrittlement of the Sn matrix of SAC 305 and SAC 405 solder alloys. The onset of the Sn embrittlement was observed between -40degC and -50degC leading to the complete brittle fracture of the solder joint at around -70degC.