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

Cu-Sn intermetallic compounds (IMC), formed at the interface between the solder and the copper substrate are found to play an important role in determining the thermal fatigue life of a surface mount solder joint. 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 Cu-Sn IMCs in 0805 and 1206 LCCC solder joints fabricated from an IR-reflow process are investigated in this work. Only the stable Cu₆Sn₅ η-phase intermetallic compound was observed in all as-solidified solder joints. The mean layer thickness was found to increase almost linearly with reflow time. The thickness of the interfacial IMC layer increased with increasing reflow temperature in 0805-type solder joints up to around 250°C and reached an equilibrium thickness above this temperature. Further intermetallic formation due to higher reflow temperature or longer reflow time appeared as Cu-Sn whiskers in the bulk solder of the solder joints. The copper land pad size and qualify of component lead metallization were also found to greatly affect the formation of Cu-Sn IMC in surface mount solder joints