Reliability studies of surface mount solder joints - effect of Cu-Sn intermetallic compounds

Cu-Sn intermetallic compounds (IMCs), formed at the interface between the solder and the copper substrate are found to play a dominant role in determining the thermal fatigue life of surface mount solder joints fabricated from a conventional infrared reflow process. In order to predict the growth of this IMC layer during the operating life of the solder joint and its effect on the thermal fatigue life, the formation characteristics of the IMC´s in 0805 and 1206 LCCC solder joints are systematically studied in this investigation. Only the stable Cu₆ Sn₅% η-phase intermetallic compound was observed in all as-solidified solder joints as confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The mean layer thickness was found to increase almost linearly with reflow time up to about 200 s. The thickness of the interfacial IMC layer increased with increasing reflow temperature for 0805-type solder joints up to around 250°C and reached a saturated thickness of 2.5 μm beyond this temperature. Additional intermetallic formation due to higher reflow temperature or longer reflow time would appear as Cu-Sn whiskers in the bulk solder of the joint. The copper land pad size and quality of component lead metallization were also found to greatly affect the formation of Cu-Sn IMC in surface mount solder joints, and hence its reliability in terms of thermal fatigue life and mechanical properties