The objective of this review is to study interfacial reactions between pure Sn or Sn-rich solders, and common base metals used in Pb-free electronics production. In particular, the reasons leading to the observed interfacial reactions products and their metallurgical evolution have been analyzed. Results presented in the literature have been critically evaluated with the help of combined thermodynamic–kinetic approach based on the concept of local equilibrium and microstructural knowledge. The following conclusions have been reached: Firstly, the formations of intermetallic compounds in solid/liquid reaction couples are primarily controlled by the dissolution processes of base metals. Other factors that need be considered are the thermodynamic driving force for the formation of intermetallic compounds, their structures and concentration profiles in liquid. Secondly, annealing of solder interconnections in solid state can drastically change the microstructures formed in the solid/liquid reactions, especially if only one of the components in the solder takes part in the interfacial reactions. Thirdly, additional elements can have three major effects on the binary reactions between a base metal and Sn: (i) they can increase or decrease the reaction/growth rates, (ii) the additives can change the physical properties of the phases formed, and (iii) they can form additional reaction products or displace the binary equilibrium phases by forming new reaction products. Finally, if the local stable or metastable equilibrium is established at the interface, stability information together with kinetic considerations can provide a feasible approach to analyze interfacial reactions, which can have significant impact on the reliability of soldered electronics assemblies.

Interfacial reactions between lead-free solders and common base materials

The interfacial reactions between Sn-based solders and two common substrate materials, Cu and Ni, are the focuses of this paper. The reactions between Sn-based solders and Cu have been studied for several decades, and currently there are still many un-resolved issues. The reactions between Sn-based solders and Ni are equally challenging. Recent studies further pointed out that Cu and Ni interacted strongly when they were both present in the same solder joint. While this cross-interaction introduces complications, it offers opportunities for designing better solder joints. In this study, the Ni effect on the reactions between solders and Cu is discussed first. The presence of Ni can in fact reduce the growth rate of Cu3Sn. Excessive Cu3Sn growth can lead to the formation of Kirkendall voids, which is a leading factor responsible for poor drop test performance. The Cu effect on the reactions between solders and Ni is then covered in detail. The knowledge gained from the Cu and Ni effects is applied to explain the recently discovered intermetallic massive spalling, a process that can severely weaken a solder joint. It is pointed out that the massive spalling was caused by the shifting of the equilibrium phase as more and more Cu was extracted out of the solder by the growing intermetallic. Lastly, the problems and opportunities brought on by the cross-interaction of Cu and Ni across a solder joint is presented.

Interfacial reaction issues for lead-free electronic solders

Effect of thermal aging on the interfacial structure of SnAgCu solder joints on Cu.

Interfacial intermetallic growth and shear strength of lead-free composite solder joints

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that large degradations occur in the material properties (stiffness and strength) and creep behavior of Sn-Ag-Cu (SAC) lead free solders during aging. These effects are universally detrimental to reliability and are exacerbated as the aging temperature and aging time increases. Conversely, changes due to aging have been shown to be relatively small in conventional Sn-Pb solders. Aging effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. In the current investigation, we have extended our previous studies to include a full test matrix of aging temperatures and SAC lead free solder alloys. In an attempt to reduce the aging induced degradation of the material behavior of SAC solders, we are also exploring various doped SAC-X alloys. These materials are SAC solders that have been modified by the addition of small percentages of one or more additional elements (X). Using dopants (e.g. Bi, In, Ni, La, Mg, Mn, Ce, Co, Ti, Zn, etc.) has become widespread to enhance shock/drop reliability, wetting, and other properties; and we have extended this approach to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on solder samples that were aged for various durations (0–6 months) at room temperature (25 °C), and several elevated temperatures (50, 75, 100, and 125 °C). Four “standard” SAC alloys have been examined in this work including SAC105, SAC205, SAC305, and SAC405. This selection has allowed us to explore the effects of silver content on aging behavior (we have examined SACN05 with N= 1%, 2%, 3%, and 4% silver; with all alloys containing 0.5% copper). The doped SAC solder materials being considered in our ongoing studies include SAC0307-X, SAC105-X, and SAC305-X. In this work, we will concentrate on presenting the results for SAC0307-X (SAC-X), where X is 0.1%Bi. This alloy has been proposed as a lower cost SAC variation suitable for enhancing drop reliability. For all of the solders, variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and aging temperature. Our findings show that the doped SAC-X alloy illustrates reduced degradations with aging for all of the aging temperatures considered. The stress-strain and creep mechanical properties of SAC-X are better than those of SAC105 after short durations of aging, and approach those of SAC205 with longer aging times. After long term aging, the SAC-X alloy was found to have more stable behavior than all of the standard SACN05 alloys. Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes.

Reduction of lead free solder aging effects using doped SAC alloys

Intermetallic compound layer growth at the interface between Sn-Cu-Ni solder and Cu substrate

Growth kinetics of intermetallic compound (IMC) layers formed between the Sn-3.5Ag-5Bi solder and the Cu and electroless Ni-P substrates were investigated at temperatures ranging from 70°C to 200°C for 0–60 days. With the solder joints between the Sn-Ag-Bi solder and Cu substrates, the IMC layer consisted of two phases: the Cu6Sn5 (η phase) adjacent to the solder and the Cu3Sn (e phase) adjacent to the Cu substrate. In the case of the electroless Ni-P substrate, the IMC formed at the interface was mainly Ni3Sn4, and a P-rich Ni (Ni3P) layer was also observed as a by-product of the Ni-Sn reaction, which was between the Ni3Sn4 IMC and the electroless Ni-P deposit layer. With all the intermetallic layers, time exponent (n) was approximately 0.5, suggesting a diffusion-controlled mechanism over the temperature range studied. The interface between electroless Ni-P and Ni3P was planar, and the time exponent for the Ni3P layer growth was also 0.5. The Ni3P layer thickness reached about 2.5 µm after 60 days of aging at 170°C. The activation energies for the growth of the total Cu-Sn compound layer (Cu6Sn5 + Cu3Sn) and the Ni3Sn4 IMC were 88.6 kJ/mol and 52.85 kJ/mol, respectively.

Growth of an intermetallic compound layer with Sn-3.5Ag-5Bi on Cu and Ni-P/Cu during aging treatment

The ball shear strength of BGA solder joints during isothermal aging was studied with Sn–35Ag–075Cu solder on three different pads (Cu, electroless Ni–P/Cu, immersion Au/Ni–P/Cu) at temperature between 70 and 170°C for times ranging from 1 to 100 days The reliability of solder ball attachment was characterized by mechanical ball shear tests As a whole, the shear strength of BGA joints decreased with increasing temperature and time The shear strength for both the immersion Au/Ni–P/Cu and electroless Ni–P/Cu pads was consistently higher than that of the Cu pad for all isothermal aging conditions The fracture surface showed various characteristics depending on aging temperature, time, and the types of BGA pad The P-rich Ni layer formed at the interface between (Cu, Ni)6Sn5 and Ni–P deposits layer after aging, but fracture at this interface was not the dominant site for immersion Au/Ni–P/Cu and electroless Ni–P/Cu pad

Effect of Isothermal Aging on Ball Shear Strength in BGA Joints with Sn-3.5Ag-0.75Cu Solder

The ball shear strength of BGA solder joints during isothermal aging was studied with Sn-3.5Ag-0.75Cu solder on three different pads (Cu, electroless Ni-P/Cu, immersion Au/Ni-P/Cu) at temperature between 70 and 170°C for times ranging from 1 to 100 days. The reliability of solder ball attachment was characterized by mechanical ball shear tests. As a whole, the shear strength of BGA joints decreased with increasing temperature and time. The shear strength for both the immersion Au/Ni-P/Cu and electroless Ni-P/Cu pads was consistently higher than that of the Cu pad for all isothermal aging conditions. The fracture surface showed various characteristics depending on aging temperature, time, and the types of BGA pad. The P-rich Ni layer formed at the interface between (Cu, Ni) 6 Sn 5 and Ni-P deposits layer after aging, but fracture at this interface was not the dominant site for immersion Au/Ni-P/Cu and electroless Ni-P/Cu pad.

Effect of isothermal aging on ball shear strength in BGA joints with Sn-3.5Ag-0.75Cu solder : Lead-free electronics packaging

The growth kinetics of intermetallic compound layers formed between eutectic Sn–58Bi solder and (Cu, electroless Ni–P/Cu) substrate were investigated at temperature between 70 and 120◦C for 1 to 60 days. The layer growth of intermetallic compound in the couple of the Sn–58Bi/Cu and Sn–58Bi/electroless Ni–P system satisfied the parabolic law at given temperature range. As a whole, because the values of time exponent (n) have approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by diffusion mechanism over the temperature range studied. The apparent activation energies of Cu6Sn5 and Ni3Sn4 intermetallic compound in the couple of the Sn–58Bi/Cu and Sn–58Bi/electroless Ni–P were 127.9 and 81.6 kJ/mol, respectively.

Chang-Bae Lee

Papers

Intermetallic compound layer growth at the interface between Sn-Cu-Ni solder and Cu substrate

Growth of an intermetallic compound layer with Sn-3.5Ag-5Bi on Cu and Ni-P/Cu during aging treatment

Effect of Isothermal Aging on Ball Shear Strength in BGA Joints with Sn-3.5Ag-0.75Cu Solder

Effect of isothermal aging on ball shear strength in BGA joints with Sn-3.5Ag-0.75Cu solder : Lead-free electronics packaging

Interfacial Reactions Between Sn-58 mass%Bi Eutectic Solder and (Cu, Electroless Ni-P/Cu) Substrate