Nucleation and Growth of Cu-Al Intermetallics in Al-Modified Sn-Cu and Sn-Ag-Cu Lead-Free Solder Alloys

Controlling Bulk Cu6Sn5 Nucleation in Sn0.7Cu/Cu Joints with Al Micro-alloying

“…It has also been found that Al additions have a substantial influence on Cu6Sn5 in bulk solders [2,4,5,7,8]. Reeve et al [5] and Boesenberg et al [4] found that Cu6Sn5 often shared an interface with δ-Cu33Al17 in Sn-3.5Ag-0.95Cu, Kantarcıoğlu and Kalay [2] observed that 0.05 wt% Al additions reduce the size of Cu6Sn5 in Sn-3.5Ag-0.9Cu/Cu joints, and McDonald et al [8] and Xian et al [7] reported that Al micro-alloying decreased the size of primary Cu6Sn5 in hypereutectic Sn-4Cu alloys.…”

“…Reeve et al [5] and Boesenberg et al [4] found that Cu6Sn5 often shared an interface with δ-Cu33Al17 in Sn-3.5Ag-0.95Cu, Kantarcıoğlu and Kalay [2] observed that 0.05 wt% Al additions reduce the size of Cu6Sn5 in Sn-3.5Ag-0.9Cu/Cu joints, and McDonald et al [8] and Xian et al [7] reported that Al micro-alloying decreased the size of primary Cu6Sn5 in hypereutectic Sn-4Cu alloys. The grain refinement and reduced nucleation undercooling of primary Cu6Sn5 due to Al additions has been deduced to be due to heterogeneous epitaxial nucleation of Cu6Sn5 on either -Cu33Al17 or 1-Cu9Al4 [7].…”

“…In the pursuit of next-generation Pb-free solders, there is ongoing interest in microalloying solders with Al [1][2][3][4][5][6]. Potential benefits of dilute Al additions include (i) the suppression of nucleation undercooling for βSn in Sn-Ag-Cu [1,2] and Sn-Ag [3] solders, which has been linked to a reduction in primary (pro-eutectoid) Ag3Sn blade formation [4] and may have the potential to grain refine the βSn phase in solder joints [5]; (ii) the suppression of Cu6Sn5 layer growth on Cu substrates [6]; and (iii) the introduction of small, hard Cu-Al intermetallics that could be harnessed as a dispersion in the βSn matrix to improve the performance and reliability of solder joints [5].…”

Advances in Pb-free Solder Microstructure Control and Interconnect Design

“…[12][13][14][15][16][17][18][19][20] Anderson and co-workers have examined a wide range of additions to SAC alloys; Al (0.05 wt.%), Zn ( ‡0.21 wt.%), and Mn ( ‡0.1 wt.%) were each found to reduce b-Sn undercoolings to 2-4°C, and to reduce, or eliminate, the formation of Ag 3 Sn proeutectic blades. The early studies were not able to identify the cause of the observed benefits from the micro-alloying additions, but did state that the specific micro-alloying additions were chosen because of their substitutional solubility in Cu and with the goal of enhanced nucleation of primary Cu 6 Sn 5 from which the b-Sn could then nucleate.…”

“…This lack of microstructure control has motivated significant research on the effects of minor alloy additions to SAC or other binary Sn-based alloys on Sn nucleation over a number of years. [12][13][14][15][16][17][18][19][20] In contrast to circuit board assembly solders in which many high-Sn alloys are available commercially, Au-based eutectics (Au-Sn, Au-Si, Au-Ge) are the dominant Pb-free alloys used for high-temperature soldering applications, but Fig. 1 Two differing projections of the liquidus surface of the Sn-Ag-Cu ternary system.…”