Effect of 0.5 wt% Cu in Sn-3.5%Ag solder to retard interfacial reactions with the electroless Ni-P metallization for BGA solder joints application

Among the most advanced microelectronic packages, ball-grid-array (BGA) technology are expected to have increasing applications because of their higher input-output connection density achieved through area-array solder joints. In this study, the role of 0.5 wt % Cu in the interfacial reaction between the Sn-3. 5%Ag solder and the electroless Ni-P metallization on the BGA substrate was investigated. Sn-3.5%Ag and Sn-3.5% Ag-0.5% Cu solders were reflowed on the electroless Ni-P layer at the peak temperature of 240 degrees C and the duration above 220 degrees C was 0.5 min. After reflowing, the samples were aged at 150 degrees C temperature for different times ranging from 24 h to 400 h. It was found that Cu addition retards the reaction rate with the electroless Ni-P layer significantly-especially during reflow soldering. However, among different layers formed by interfacial reactions, the P-rich Ni layer grew at a slower rate when Cu was present in the solder. Higher solder reaction rate for the Cu free alloys was explained in term of higher Ni dissolution and rapid formation of Ni3SR4. Due to a higher reaction rate, the amorphous electroless Ni-P layer was transformed to the crystalline phases of Ni-P compounds rapidly. The presence of 0.5 wt% Cu in the Sn-3.5%Ag solder alloy reduces Ni dissolutions in the liquid solder and changes the driving force of interfacial reactions forming (Cu,Ni)(6)Sn-5 instead of Ni3Sn4. The growth of (Cu,Ni)(6)Sn-5 requires less Ni compared to the growth of Ni3Sn4 and thus reduces the consumption of the electroless Ni-P as well as the growth of the P-rich crystalline Ni layer. Retardation of the interfacial reactions, in particular, a thin P-rich crystalline layer reduces the chance of solder joint failure, thus improves the reliability of solder joint with electroless Ni-P metallization. Results from other concurrent investigators were also compared and discussed to understand the findings from this study.