Microstructural Investigation of Sn‐Ag and Sn‐Ag Solder Joints*

Abstract

Detailed studies to characterise the coarsening behaviour of eutectic Sn‐Ag and near‐eutectic Sn‐Pb‐Ag solder joints were carried out on samples reflow soldered and solidified at various cooling rates. Light and scanning electron microscopy as well as EDS were used to study the microstructural evolution, while microhardness measurements were used to monitor the change in the mechanical properties. Samples consisting of copper substrates and solder paste were reflow soldered about 30 °C above their melting points and then solidified at cooling rates ranging from furnace cooling to rates associated with water quenching. Analysis of some of these samples showed that increasing the cooling rate increased the quantity (volume fraction) of primary Sn‐dendrites, decreased the (EQ) intermetallic phase in the bulk solder, and resulted in finer microstructures with higher hardness. The microstructural evaluation involved characterisation of bulk intermetallica and dendrite/eutectic ratios. Subsequent isothermal annealing of these reflow soldered joints at 125 °C for times between 0.25 h and 8 days resulted in an initially fairly rapid decrease in hardness to a given level for each alloy and each cooling rate.

Intermetallic formation in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with immersion Ag surface finish

“…15 The dissolved Cu also caused the formation of Cu 6 Sn 5 intermetallics in the Sn-3.5Ag solder matrix, which could be suppressed by increasing the cooling rate. 16 Anderson et al revealed that the rapidly solidified Sn-3.6Ag-1.0Cu/Cu solder joints possessed a cellular/dendritic microstructure decorated with fine intermetallic phases of Cu 6 Sn 5 and Ag 3 Sn. 5 The addition of the Co element into Sn-3.6Ag-1.0Cu solder was also reported by Anderson et al to show a beneficial effect on its shear strength induced by refining the joint microstructure.…”

Lead‐free and No‐clean Soldering for Automotive Electronics*

“…It is believed that excessive intermetallics formation will have a detrimental effect on the solder interconnect reliability. Work is in progress to study the intermetallic formation and growth (under isothermal and continuous conditions) in the solid as well as liquid state of the solder 456, 151617…”

The scaling effect on microstructure and creep properties of Sn-based solders

“…Beside the formation of rodlike intermetallics Ag3Sn intermatallics precipitate in the Sn dendrites because the maximum solubility of ag in Sn drops from a maximum of O,OSwt% at 221°C to 0,004wt% at room temperature [2]. The size of Ag3Sn precipitates should be smaller than 200 nm [3]. Intermetallics offj-Cu6Sns grow in rounded, hexagonal or trapezoidal shape [4,5].…”