Ag3Sn plate formation in the solidification of near-ternary eutectic Sn-Ag-Cu

Abstract

a b cNear-ternary eutectic Sn-Ag-Cu alloys are leading lead-free candidate solders for various applications. These alloys yield three phases upon solidifi cation: β-Sn, Ag 3 Sn, and Cu 6 Sn 5 . Large, plate-like, pro-eutectic Ag 3 Sn structures can grow rapidly within the liquid phase, potentially adversely affecting the mechanical behavior and reducing the fatigue life of solder joints. This article reports on the formation of such plates in Sn-Ag-Cu solder balls and joints and demonstrates how large Ag 3 Sn plate formation can be minimized.

The Microstructure, Thermal Fatigue, and Failure Analysis of Near-Ternary Eutectic Sn-Ag-Cu Solder Joints

“…The growth of large Ag 3 Sn plates in Sn-Ag-Cu alloys has been extensively studied in terms of cooling rate, [8][9][10] alloy content, 8,9) and minor alloying elements. 11,12) The cooling rate during the reflow of Sn-Ag-Cu alloys was found to be a critical factor in controlling the formation of large Ag 3 Sn plates in SAC joints.…”

“…11,12) The cooling rate during the reflow of Sn-Ag-Cu alloys was found to be a critical factor in controlling the formation of large Ag 3 Sn plates in SAC joints. [8][9][10] At a high-cooling rate, such as 1.5 C/s or higher, the formation of large Ag 3 Sn plates can be kinetically suppressed during a reflow process. However, providing a high-cooling rate is not always practical, especially, in the case of large thermal mass chip carriers.…”

“…9,10) It was found that large Ag 3 Sn plate formation was substantially reduced in alloys having a Ag content less than 3 mass%, even under extremely slow cooling conditions, such as 0.02 C/s. 9) In addition, the Cu content in Sn-Ag-Cu alloys was found to have a less significant effect than Ag content on the formation of large Ag 3 Sn plates. But, lowering the Cu content was also found to be beneficial in reducing the so-called pasty range (the temperature range between liquidus and solidus temperature) of the molten solder, resulting in fewer joint defects.…”

“…9,14) Although tin pest formation has not been reported with near-ternary Sn-Ag-Cu alloys, it would be prudent to modify the alloy composition to retard or prevent the formation of tin pest.…”

“…9,15) One example of a modified alloy composition is Sn-2.3Ag-0.5Cu-0.2Bi (mass%). A small addition of Bi content in the alloy can reduce the possibility of tin transformation as claimed in the literature.…”