Metastable eutectic in Pb-free joints between Sn-3.5Ag and Ni-based substrates

Microstructure development in solder joints between Sn – 3.5Ag and Ni-based substrates has been widely reported. However, in the present study we illustrate a new phenomenon: that during soldering of Sn – 3.5Ag to Ni or ENIG (electroless nickel, immersion gold), the bulk solder solidi fi es to contain a metastable eutectic consisting of β Sn þ Ag 3 Sn þ NiSn 4 instead of the β Sn þ Ag 3 Sn þ Ni 3 Sn 4 , expected of equilibrium solidi fi cation. It is shown that metastable NiSn 4 coarsens and then decomposes into Ni 3 Sn 4 and β Sn during aging at 150 and 200 1 C and that coarsened NiSn 4 particles deteriorate impact shear properties

The Influence of Cu on Metastable NiSn4 in Sn-3.5Ag-xCu/ENIG Joints

We have investigated the effect of small amounts of Cu on suppression of metastable bSn-NiSn4 eutectic growth in solder joints between Sn-3.5Ag-xCu solders and Ni-based substrates. For Sn-3.5Ag/electroless nickel immersion gold (ENIG) and Sn-3.5Ag/Ni solder joints we showed that the eutectic mixture contains bSn, Ag3Sn, and metastable NiSn4. It was found that addition of only 0.005 wt.% Cu to Sn-3.5Ag-xCu/ENIG or Sn-3.5Ag-xCu/Ni joints promoted formation of a stable bSn-Ni3Sn4 eutectic and that both Ni3Sn4 and NiSn4 occur in the eutectic at this Cu level. We also showed that for complete prevention of formation of metastable NiSn4 during eutectic solidification of the solder joint, addition of at least 0.3 wt.% Cu was required

Heterogeneous nucleation of βSn on NiSn4, PdSn4 and PtSn4

During Pb-free soldering, βSn often requires a high nucleation undercooling and there is an ongoing effort to develop nucleation catalysts. It is shown here that NiSn4, PdSn4 and PtSn4 are heterogeneous nucleants for βSn, reducing the nucleation undercooling to ∼4 K when these intermetallics are present either in the bulk solder or as the interfacial layer. Nucleation catalysis occurs by βSn nucleating on the (0 0 8) facet of XSn4 crystals with an orientation relationship (OR) (1 0 0)Sn||(0 0 8)XSn4 and [0 0 1]Sn||[1 0 0]XSn4 where there is a planar lattice match of ∼5%. This OR is also the origin of well-aligned lamellar βSn–XSn4 eutectic morphologies, even though the eutectics contain less than 2 vol.% of faceted NiSn4, PdSn4 or PtSn4

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

We show that dilute Al additions can control the size of primary Cu6Sn5 rods in Sn-0.7Cu/Cu ball grid array joints. In Sn-0.7Cu-0.05Al/Cu joints, the number of primary Cu6Sn5 per mm2 is 7 times higher and the mean threedimensional length of rods is 4 times smaller than in Al-free Sn-0.7Cu/Cu joints, while the area fraction of primary Cu6Sn5 is similar. It is shown that epitaxial nucleation of primary Cu6Sn5 occurs on d-Cu33Al17 or c1-Cu9Al4 particles, which are stable in the Sn-0.7Cu-0.05Al melt during holding at 250C. The observed facet relationships agree well with previously determined orientation relationships between d-Cu33Al17 and Cu6Sn5 in hypereutectic SnCu-Al alloys and result in a good lattice match with<2.5% lattice mismatch on two different interfacial planes

Nucleation and growth of tin in Pb-free solder joints

The solidification of Pb-free solder joints is overviewed with a focus on the formation of the βSn grain structure and grain orientations. Three solders commonly used in electronics manufacturing, Sn-3Ag-0.5Cu, Sn-3.5Ag, and Sn-0.7Cu-0.05Ni, are used as case studies to demonstrate that (I) growth competition between primary dendrites and eutectic fronts during growth in undercooled melts is important in Pb-free solders and (II) a metastable eutectic containing NiSn4 forms in Sn-3.5Ag/Ni joints. Additionally, it is shown that the substrate (metallization) has a strong influence on the nucleation and growth of tin. We identify Co, Pd, and Pt substrates as having the potential to control solidification and microstructure formation. In the case of Pd and Pt substrates, βSn is shown to nucleate on the PtSn4 or PdSn4 intermetallic compound (IMC) reaction layer at relatively low undercooling of ~4 K, even for small solder ball diameters down to <200 μm

Effect of Ni on the Formation and Growth of Primary Cu6Sn5 Intermetallics in Sn-0.7 wt.%Cu Solder Pastes on Cu Substrates During the Soldering Process

This paper investigates the effect of 0.05 wt.% Ni on the formation and growth of primary Cu6Sn5 in Sn-0.7 wt.%Cu solder paste soldered on a Cu substrate, using a real-time synchrotron imaging technique. It was found that small additions of Ni significantly alter the formation and growth of the primary Cu6Sn5 intermetallics, making them small. In contrast, without Ni, primary Cu6Sn5 intermetallics tend to continue growth throughout solidification and end up much larger and coarser. The primary effect of the Ni addition appears to be in promoting the nucleation of a larger amount of small Cu6Sn5. The results provide direct evidence of the sequence of events in the reaction of Ni-containing Sn-0.7 wt.%Cu solder paste with a Cu substrate, and in particular the formation and growth of the primary Cu6Sn5 intermetallic

Time-lapse imaging of Ag3Sn thermal coarsening in Sn-3Ag-0.5Cu solder joints

The coarsening of Ag3Sn particles occurs during the operation of joints and plays an important role in failure. Here, Ag3Sn coarsening is studied at 125°C in the eutectic regions of Sn-3Ag-0.5Cu/Cu solder joints by SEM-based time-lapse imaging. Using multi-step thresholding segmentation and image analysis, it is shown that coalescence of Ag3Sn particles is an important ripening process in addition to LSW-like Ostwald ripening. About 10% of the initial Ag3Sn particles coalesced during ageing, coalescence occurred uniformly across eutectic regions, and the scaled size distribution histograms contained large particles that can be best fit by the Takajo model of coalescence ripening. Similar macroscopic coarsening kinetics were measured between the surface and bulk Ag3Sn particles. Tracking of individual surface particles showed an interplay between the growth/shrinkage and coalescence of Ag3Sn

Harnessing heterogeneous nucleation to control tin orientations in electronic interconnections

While many aspects of electronics manufacturing are controlled with great precision, the nucleation of tin in solder joints is currently left to chance. This leads to a widely varying melt undercooling and different crystal orientations in each joint which results in a different resistance to electromigration, thermomechanical fatigue and other failure modes in each joint. Here we identify a family of nucleants for tin, prove their effectiveness using a novel droplet solidification technique, and demonstrate an approach to incorporate the nucleants into solder joints to control the orientation of the tin nucleation event. With this approach, it is possible to change tin nucleation from a stochastic to a deterministic process, and to generate single crystal joints with their c-axis orientation tailored to best combat a selected failure mode

Microstructure and damage evolution during thermal cycling of Sn-Ag-Cu solders containing antimony

Antimony is attracting interest as an addition to Pb-free solders to improve thermal cycling performance in harsher conditions. Here, we investigate microstructure evolution and failure in harsh accelerated thermal cycling (ATC) of a Sn-3.8Ag-0.9Cu solder with 5.5 wt.% antimony as the major addition in two ball grid array (BGA) packages. SbSn particles are shown to precipitate on both Cu6Sn5 and as cuboids in β-Sn, with reproducible orientation relationships and a good lattice match. Similar to Sn-Ag-Cu solders, the microstructure and damage evolution were generally localised in the β-Sn near the component side where localised β-Sn misorientations and subgrains, accelerated SbSn and Ag3Sn particle coarsening, and β-Sn recrystallisation occurred. Cracks grew along the network of recrystallised grain boundaries to failure. The improved ATC performance is mostly attributed to SbSn solid-state precipitation within β-Sn dendrites, which supplements the Ag3Sn that formed in a eutectic reaction between β-Sn dendrites, providing populations of strengthening particles in both the dendritic and eutectic β-Sn