Eutectic Pb-Sn solder has been widely used as a soldering material between electronic components and PCB(Printed Circuit Boards). Eutectic Sn-Ag solder is substituting Pb-Sn for the reason that it isn’t harmful to the human body and is suitable for higher temperature applications and has better mechanical properties. In spite of good mechanical properties of eutectic Sn-Ag alloy, another alternatives with better reliability are required for solder joints that can withstand much higher service temperature and thermomechanical fatigue due to CTE(Coefficient of Thermal Expansion) mismatch between components and substrates. One approach to improve the reliability is composite solder which consists of solder matrix and intermetallic reinforcements.
Sn-6.9Cu-2.9Ag alloy has a primary $Cu_6Sn_5$ intermetallic compound in the form of dendrite in cast strips. After rolling the strips, the intermetallics were crushed from dendrites into fine particles and redistributed uniformly in the solder matrix. As the rolled strips became thinner, the average size of the crushed particles reached the critical size which wouldn’t be changed any more. The critical size was nearly the same as the average width of intermetallic dendrite trunks.
To apply composite solder to the real BGA package, the stamped segments of rolled strip reinforced with intermetallic particles were converted into spherical solder balls in hot oil and subsequently reflowed on Cu pads. The crushed intermeallic particles didn’t melt and remained in solid sate during reflow soldering process due to their high melting temperature. The particle coarsening and gravitational segregation were observed during reflow soldering.
In wetting reaction, Cu_6Sn_5 intermetallic compound(IMC) between composite solder and Cu pad was formed in scallop-type morphology, while the IMC between Sn-3.5Ag and Cu pad was formed in column-type morphology. After aging the two reflowed solder balls, the IMC thickness of composite solder is much smaller than that of Sn-3.5Ag. It was found that the presence of intermetallic particles retarded the growth of IMC layer during aging.
