Nucleation of tin on the Cu6Sn5 layer in electronic interconnections

Abstract

A Cu6Sn5 layer is an integral part of many electronic interconnections. Here we show that, although primary Cu6Sn5 is not a potent nucleant for Sn, the Cu6Sn5 layer plays a key role in Sn nucleation and microstructure formation in solder joints. Combining thermal analysis, FIB-tomography and EBSD, we show that conical cavities are present between the scallops of the Cu6Sn5 layer that act as geometric nucleation sites for Sn, that Sn grows from the Cu6Sn5 layer, and that reproducible nucleation orientation relationships (ORs) exist between Cu6Sn5 and Sn. With these ORs, a near-random distribution of Sn orientations is predicted from joint to joint even for Cu6Sn5 layers with a strong [0001] fibre texture. It is shown that the nucleation undercooling is strongly affected by manipulation of the Cu6Sn5 layer shape, and that it is possible to prevent nucleation on the Cu6Sn5 layer by adding more potent nucleants.

Cu6Sn5 crystal growth mechanisms during solidification of electronic interconnections

“…2B and the nucleation undercooling for Sn, ∆ , was defined as the Sn-Cu6Sn5 eutectic temperature (226.9C) minus the nucleation temperature prior to recalescence. ∆ was found to be relatively high and near-constant at 19-25 K for all compositions and cooling rates studied [24]; any influence of cooling rate on this undercooling was masked by the scatter in the data.…”

Grain refinement of electronic solders: The potential of combining solute with nucleant particles

“…For example, βSn usually nucleates on the Cu6Sn5 layer in joints on Cu substrates unless heterogeneous nucleants are added [65]. In this paper, we restrict ourselves only to freestanding balls and bulk alloys (i.e.…”

“…Note that in solder joints, the liquid solder is in contact with a rough intermetallic reaction layer that significantly affects βSn nucleation [12,64,65]. For example, βSn usually nucleates on the Cu6Sn5 layer in joints on Cu substrates unless heterogeneous nucleants are added [65].…”

Anisotropic thermal expansion of Ni 3 Sn 4 , Ag 3 Sn, Cu 3 Sn, Cu 6 Sn 5 and βSn

“…The growth facets on Cu6Sn5, Ag3Sn and Ni3Sn4 are identified in Figure 10 and their captions based on EBSD analysis of IMC crystals in common solder-substrate combinations. In Figure 10 [57].…”

“…Since primary Ag3Sn form thin plates (Figure 10 (e) and (f)), this OR would minimise the in-plane mismatch for the whole plate. However since, in practice, the OR is near-random between Sn and primary intermetallics [57], it is preferable to have no primary intermetallics to ensure that the worst in-plane mismatch with Sn does not occur; and to encourage Cu6Sn5, Ag3Sn and Ni3Sn4 to grow as numerous, small, closely-spaced eutectic particles where some benefit might be achievable from CTE mismatch strengthening as is used in composite design [59].…”