M.N. Islam

Comparative study of the dissolution kinetics of electrolytic Ni and electroless Ni–P by the molten Sn3.5Ag0.5Cu solder alloy

Abstract Lead-free solders have high Sn content and high melting temperature, which often cause excessive interfacial reactions at the interface. Sn3.5Ag0.5Cu lead-free solder alloy has been used to identify its interfacial reactions with two-metal layer flexible substrates. In this paper we investigate the dissolution kinetics of Sn3.5Ag0.5Cu solder on electrolytic Ni/electroless NiP layer. It is found that during 1 min of reflow electroless NiP layer dissolves slightly lower than the electrolytic Ni due to the barrier layer formation between the intermetallic compounds (IMCs) and electroless NiP layer. Faster nucleation of IMCs on the electrolytic Ni layer is proposed as the main reason for higher initial dissolution. The appearance of P-rich Ni layer acts as a diffusion barrier layer between the solder and electroless NiP layer, which decreases the dissolution rate and IMCs growth rate than that of the electrolytic Ni layer, but weaken the interface and reduces the ball shear strength and reliability. After acquiring certain thickness P-rich Ni layer breaks and increases the diffusion rate of Sn and as a consequence both the IMCs growth rate and dissolution rate also increases. It is found that 3 μm thick electroless NiP layer cannot protect the Cu layer for more than 120 min at 250 °C. In electrolytic Ni shear strength does not change significantly and lower dissolution rate and more protective for Cu layer during long time molten reaction.

Interfacial reactions of Sn-Cu and Sn-Pb-Ag solder with Au/Ni during extended time reflow in ball grid array packages

Lead-free solders with high Sn content cause excessive interfacial reactions at the interface with under-bump metallization during reflow. The interface formed after reflow affects the reliability of the solder joint. For this paper, we investigated the interfacial reactions of Sn0.7Cu and Sn36Pb2Ag solder on electrolytic Ni layer for different reflow times. The traditionally used Sn36Pb2Ag solder was used as a reference. It was found that during as-reflowed, the formation of Cu-rich Sn–Cu–Ni ternary intermetallic compounds (TIMCs) at the interface of Sn0.7Cu solder with electrolytic Ni is much quicker, resulting in the entrapment of some Pb (which is present as impurity in the Sn–Cu solder) rich phase in the TIMCs. During extended time of reflow, high (>30 at.%), medium (30-15 at.%) and low (<15 at.%) Cu TIMCs formed at the interface. The amount of Cu determined the growth rate of TIMCs. Cu-rich TIMCs had higher growth rate and consumed more Ni layer. By contrast, the growth rate of the Ni–Sn binary intermetallic compounds (BIMCs) in the Sn36Pb2Ag solder joint was slower, and the Ni–Sn BIMC was more stable and adherent. The dissolution rate of electrolytic Ni layer for Sn0.7Cu solder joint was higher than the Sn36Pb2Ag solder joints. Less than 3 m of the electrolytic Ni layer was consumed during molten reaction by the higher Sn containing Sn0.7Cu solder in 180 mi na t 250 °C. The shear strength of Sn–Pb–Ag solder joints decreased within 30 mi no f reflow time from 1.938 to 1.579 kgf due to rapid formation of ternary Ni–Sn–Au compounds on the Ni–Sn BIMCs. The shear strength of Sn0.7Cu solder joint is relatively stable from 1.982 to 1.861 kgf during extended time reflow. Cu prevents the resettlement of Au at the interface. The shear strength does not depend on the thickness of intermetallic compounds (IMCs) and reflow time. Ni/Sn–Cu solder system has higher strength and can be used during prolonged reflow.