Heterogeneous nucleation of Cu6Sn5 in Sn–Cu–Al solders

Abstract

Cu6Sn5 is a common intermetallic in Pb-free soldering. We explore the influence of dilute aluminium additions on the heterogeneous nucleation and grain refinement of primary Cu6Sn5 in Sn-4Cu-xAl solders. For cooling rates relevant to soldering, it is found that 0.02 and 0.2wt%Al cause significant grain refinement of Cu6Sn5 with 0.2wt%Al increasing the number of Cu6Sn5 grains per unit area by a factor of ~8. Grain refinement is shown to be due to heterogeneous nucleation of Cu6Sn5 on either delta-Cu33Al17 or gamma1-Cu9Al4, coupled with significant constitutional supercooling ahead of growing Cu6Sn5 crystals. Reproducible orientation relationships are measured between the Cu-Al intermetallics and Cu6Sn5, with a planar lattice mismatch of less than 2.5%. The role of potent nuclei and solutal growth restriction are discussed with reference to the grain refinement of structural casting alloys.

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

“…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]. These two Al-Cu intermetallics have very similar crystal structure [7,9,10] and it was shown that both can form interfaces with only ~2.5% lattice mismatch to the first order and second order prism planes of Cu6Sn5 (i.e.…”

“…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.…”

“…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]. These two Al-Cu intermetallics have very similar crystal structure [7,9,10] and it was shown that both can form interfaces with only ~2.5% lattice mismatch to the first order and second order prism planes of Cu6Sn5 (i.e. the {11 ̅ 00} and {112 ̅ 0}-type planes of Cu6Sn5) [7].…”

“…These two Al-Cu intermetallics have very similar crystal structure [7,9,10] and it was shown that both can form interfaces with only ~2.5% lattice mismatch to the first order and second order prism planes of Cu6Sn5 (i.e. the {11 ̅ 00} and {112 ̅ 0}-type planes of Cu6Sn5) [7].…”

“…The crystallographic study on the nucleation of Cu6Sn5 on -Cu33Al17 or 1-Cu9Al4 particles was performed on large (0.5g) samples of Sn-4Cu-xAl and did not involve a solder substrate [7]. Common solders such as Sn-0.7Cu and Sn-3Ag-0.5Cu are 3 hypoeutectic (i.e.…”