Ni Solder Joints Research Articles

Creep Properties of Sn-1.0Ag-0.5Cu Lead-Free Solder with Ni Addition

In this work, tensile creep tests for Sn-1.0Ag-0.5Cu-0.02Ni solder have been conducted at various temperatures and stress levels to determine its creep properties. The effects of stress level and temperature on creep strain rate were investigated. Creep constitutive models (such as the simple power-law model, hyperbolic sine model, double power-law model, and exponential model) have been reviewed, and the material constants of each model have been determined based on experimental results. The stress exponent and creep activation energy have been studied and compared with other researchers’ results. These four creep constitutive models established in this paper were then implemented into a user-defined subroutine in the ANSYS™ finite-element analysis software to investigate the creep behavior of Sn-1.0Ag-0.5Cu-0.02Ni solder joints of thin fine-pitch ball grid array (TFBGA) packages for the purpose of model comparison and application. Similar simulation results of creep strain and creep strain energy density were achieved when using the different creep constitutive models, indicating that the creep models are consistent and accurate.

Microstructure of interfacial reaction layer in Sn–Ag–Cu/electroless Ni (P) solder joint

This study examined the interfacial reaction in Sn-3.5Ag-0.7Cu/electroless Ni (P) solder joints using various TEM techniques. TEM confirmed that three types of intermetallic compounds (Ag3Sn, (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4) formed in the solder joints. In addition, interfacial reaction layers between the IMCs and the electroless Ni (P) are composed of two reaction layers (ternary and P-rich Ni layers). The ternary layer is composed of orthorhombic Ni2SnP phase and the P-rich Ni layer is dominantly composed of Ni3P. Furthermore, Kirkendall voids were clearly observed in the ternary layer and P-rich Ni layer. The Sn has diffused preferentially along the grain boundaries in the (Ni,Cu)3Sn4 IMCs.

In situ TEM characterization of interfacial reaction in Sn–3.5Ag/electroless Ni(P) solder joint

This study examined the interfacial reaction in Sn–3.5Ag/electroless Ni(P) solder joints using in situ transmission electron microscopy (TEM). In situ TEM confirmed that the thickness of the ternary layer increased by approximately 70 nm. A new interfacial reaction layer (Ni3P) formed in the P-rich Ni layer/Ni(P) interface. Several Ni–P compounds that formed in the reflow process changed into the stable Ni3P phase. This suggests that the P-rich Ni layer acts as a diffusion barrier layer between the intermetallic compounds and the Ni(P) layer during the annealing process.

Characterization of ternary Ni 2SnP layer in Sn–3.5Ag–0.7Cu/electroless Ni (P) solder joint

This study examined the ternary Ni2SnP layer in Sn–3.5Ag–0.7Cu/electroless Ni (P) solder joints using in situ transmission electron microscopy (TEM). TEM confirmed the formation of an Ni2P phase underneath the ternary Ni2SnP after reflowing. We can directly observe the Sn diffused into Ni2P to form Ni2SnP during annealing in TEM. These results suggested that the ternary Ni2SnP forms through the diffusion of Sn into Ni2P. In addition, small Kirkendall voids were newly formed in the ternary Ni2SnP layer after annealing in TEM.

Interfacial reactions between Sn-2.5Ag-2.0Ni solder and electroless Ni(P) deposited on SiC p/Al composites

A novel Sn-2.5Ag-2.0Ni alloy was used for soldering SiC p/Al composites substrate deposited with electroless Ni(5%P) (mass fraction) and Ni(10%P) (mass fraction) layers. It is observed that variation of P contents in the electroless Ni(P) layer results in different types of microstructures of SnAgNi/Ni(P) solder joint. The morphology of Ni 3Sn 4 intermetallic compounds (IMCs) formed between the solder and Ni(10%P) layer is observed to be needle-like and this shape provides high speed diffusion channels for Ni to diffuse into solder that culminates in high growth rate of Ni 3Sn 4. The diffusion of Ni into solder furthermore results in the formation of Kirkendall voids at the interface of Ni(P) layer and SiC p/Al composites substrate. It is observed that solder reliability is degraded by the formation of Ni 2SnP, P rich Ni layer and Kirkendall voids. The compact Ni 3Sn 4 IMC layer in Ni(5%P) solder joint prevents Ni element from diffusing into solder, resulting in a low growth rate of Ni 3Sn 4 layer. Meanwhile, the formation of Ni 2SnP that significantly affects the reliability of solder joints is suppressed by the low P content Ni(5%P) layer. Thus, shear strength of Ni(5%P) solder joint is concluded to be higher than that of Ni(10%P) solder joint. Growth of Ni 3Sn 4 IMC layer and formation of crack are accounted to be the major sources of the failure of Ni(5%P) solder joint.

Sn-2.5Ag-0.7Cu-0.1Re-xNi Lead-Free Solder Alloy and its Creep Properties of Solder Joints

The effects of Ni on the properties of the Sn-2.5Ag-0.7Cu-0.1Re solder alloy and its creep properties of solder joints are researched. The results show that with adding 0.05wt% Ni in the Sn-2.5Ag-0.7Cu-0.1Re solder alloy, the elongation can be sharply improved without decreasing its tensile strength and it is 1.4 times higher than that of the commercial Sn-3.8Ag-0.7Cu solder alloy. Accordingly the creep rupture life of Sn-2.5Ag-0.7Cu-0.1Re-0.05Ni solder joints is the longest, which is 13.3 times longer than that of Sn-2.5Ag-0.7Cu-0.1Re and is also longer than that of the commercial Sn-3.8Ag-0.7Cu solder alloy. In the same environmental conditions, the creep rupture life of Sn-2.5Ag-0.7Cu-0.1Re-0.05Ni solder joints can sharply decrease with increasing the temperature and stress.

Microstructure Evolution of Sn-2.5Ag-2.0Ni Solder Joint with Various Ni Platings on SiC<sub>p</sub>/Al Composites

A novel Sn-2.5Ag-2.0Ni alloy has been developed for soldering of SiCp/Al composites substrate with various types of Ni coatings. An investigation about electroplated Ni layer, electroless Ni-5 wt.% P, Ni-10wt.% P and Ni-B layers has been carried out. It is found that the solder joints possess a single intermetallic compound (IMC) Ni3Sn4, which coarsens with an increase in aging time. The formation of Ni2SnP has been observed to significantly affect the reliability of the solder joints. But the formation of Ni2SnP can be suppressed by lowering the P contents in as-deposited Ni coatings. It has been also found that the thermal stresses generated in solder joint increases with the decrease of P contents in Ni-P layer. Furthermore, the concentration of thermal stresses in the electroplated Ni solder joint is found to be higher than that in other three electroless Ni layers. Out of four as-deposited Ni coatings, the Ni-B layer exhibits good wettability with solder and low IMC growth rate during aging. Also, the shear strength of solder joint decreases with an increase in aging time and Ni-B solder joint demonstrates the highest shear strength after long term aging.

Effect of various Ni plating layers and aging on microstructure and shear strength of Sn–2.5Ag–2.0Ni solder joint

Sn–2.5Ag–2.0Ni alloy is used to solder silicon carbide reinforced aluminum matrix (SiC p/Al) composites substrate with various Ni coatings. An experimental study is carried out to assess the effects of various Ni coatings on the microstructure and reliability of solder joint. Three electroless Ni layers, including Ni–5 wt.%P, Ni–10 wt.%P and Ni–B, are investigated. The electroless Ni–B layer is found to have good wettability with solder and gives the lowest growth rate of Ni 3Sn 4 IMCs during aging. The formation of Ni 2SnP, which significantly affects the reliability of solder joints, is suppressed by using the low P content Ni–5 wt.%P layer. The electroless Ni–B layer has given the highest shear strength of solder joint while Ni–5 wt.%P layer yields the higher shear strength as compared to Ni–10 wt.%P layer. Growth of Ni 3Sn 4 IMCs and formation of crack are observed to be the major sources of failure for the solder joint with Ni–5 wt.%P, whereas, the failure of Ni–10 wt.%P solder joint is accounted for the formation of Ni 2SnP, P-rich Ni layer and Kirkendall voids after long term aging.

Electromigration in Sn3.0Ag0.5Cu flip chip solder joint

Electromigration in Sn30Ag05Cu flip chip solder joints was investigated under a current density of the order of 104 A/cm2. The transfer of atoms or vacancies and the formation of Joule heating induced by electromigration were studies in terms of microstructural evolution. The formation of pancake-type void at the interface between solder and intermetallic compound was dominated by current concentration and the directional transfer of vacancies induced by electromigration. Series of voids were also found at the interface between Ni（P） finishes and solder joint. The area of voids at substrate side are less than that at chip side. The flux of Cu atoms in the pad has the same direction as the electron flux. The transfer of Cu atoms from pad to solder joint led to the formation of large areas of intermetallic compound in solder joint, and the amount of intermetallic compound increased along the direction of electron flux.

Coupling effects at Cu(Ni)–SnAgCu–Cu(Ni) sandwich solder joint during isothermal aging

Interfacial reactions and consumption of metallization during liquid reaction and isothermal aging at 175 °C for Cu–SnAgCu–Cu, Ni–SnAgCu–Ni and Cu–SnAgCu–Ni solder joints were investigated. Interfacial microstructures were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). Optical microscopy (OM) was employed to measure the consumed thickness of metallization layer. A coupling effect occurs between Cu/SnAgCu and Ni/SnAgCu interfaces at Cu–SnAgCu–Ni solder joint. The inter-diffusion of Cu and Ni atoms contributes to the formation of ternary (Cu,Ni)6Sn5 IMCs at both interfaces. The Ni(P) layer in Ni–Ni solder joint is fully depleted and changes to Ni3P after aging 500 h. However, due to the coupling effect, the consumption of Ni(P) layer is limited and the Cu consumption is accelerated compared to Cu–Cu and Ni–Ni solder joint, which prevents the Ni(P) layer from being depleted and improves the reliability.

Improvement on Thermal Fatigue Properties of Sn-1.2Ag-0.5Cu Flip Chip Interconnects by Nickel Addition

The thermal fatigue properties of Sn-1.2Ag-0.5Cu (in mass%) flip chip interconnect were improved by a small amount of nickel addition. The thermal fatigue resistance of Sn-xAg-0.5Cu flip chip interconnects was enhanced byaddition of 0.05 mass%Ni, and Sn-1.2Ag-0.5Cu-0.05Ni had longer thermal fatigue life than Sn-1.2Ag-0.5Cu. Cracks developed near solder/chip interface for all the bumps tested. This crack propagation is mainly governed by the nature of the solders themselves because a strain concentrated area was similar for all the tested alloys independent of the chemical contents. From the microstructural observation, fracture in Sn-1.2Ag-0.5Cu-0.05Ni due to thermal strain was a mixed mode, both transgranular and intergranular. From SEM and TEM analyses, fine Ag 3 Sn and (Cu,Ni) 6 Sn 5 formed network around Sn grains in the initial microstructure of Sn-1.2Ag-0.5Cu-0.05Ni solder. Sn-1.2Ag-0.5Cu-0.05Ni solder joint suppressed coarsening of Sn grains even after thermal fatigue test. Namely, thermal fatigue properties of the Sn-1.2Ag-0.5Cu-0.05Ni solder joint is correlated to its microstructure, and the joint had longer fatigue life in spite of its lower silver content of 1.2 mass% due to both fine Sn matrix in the initial state and suppression of Sn grain coarsening even after thermal fatigue test.

Evaluation of creep behavior of near-eutectic Sn–Ag solders containing small amount of alloy additions

Understanding and quantification of creep behavior of lead-free solder joints are essential for lifetime prediction of electronic systems. This is especially true for circuits with surface mount and chip components that are subjected to severe environments and higher temperatures. Creep deformation behavior of Sn–4Ag–0.5Cu, Sn–3.5Ag–0.5Ni and Sn–2Ag–1Cu–1Ni solder alloys was determined at room temperature (25 °C) and at elevated temperature (85 °C) using miniature single shear lap joint specimens that are comparable in size to actual solder joints used in electronic packaging. Various creep parameters such as global creep strain, secondary creep rates as well as the strain for the onset of tertiary creep in the solder joint were determined. The effects of Cu and Ni alloy additions on the creep properties of eutectic Sn–3.5Ag solder joints were studied by comparing with the creep deformation behavior of eutectic Sn–3.5Ag solder joints that were used as the baseline. General findings in this study revealed that the creep resistance of Sn–4Ag–0.5Cu solder joints is comparable to but slightly higher than that of eutectic Sn–3.5Ag solder joints at both room and elevated testing temperatures, particularly at lower stresses. The Sn–3.5Ag–0.5Ni solder joints have comparable creep resistance to Sn–4Ag–0.5Cu and eutectic Sn–3.5Ag solder joints at 85 °C, but much better creep resistance at room temperature. The Sn–2Ag–1Cu–1Ni solder joints were two orders of magnitude less creep resistant than solder joints made with other solder materials at 85 °C. However, the shear strains for the onset of tertiary creep in Sn–2Ag–1Cu–1Ni solder joints were found to be the highest at 85 °C. Microstructural analysis showed significant creep deformation along Sn grain boundaries.

Interfacial Microstructure and Joint Strength of Sn–3.5Ag–X (X = Cu, In, Ni) Solder Joint

Interfacial phase and microstructure, solder hardness, and joint strength of Sn–3.5Ag–X (X = Cu, In, Ni; compositions are all in wt% unless specified otherwise) solder alloys were investigated. Considering the melting behavior and the mechanical properties, five compositions of Sn–3.5Ag–X solder alloys were selected. To examine the joint characteristics, they were soldered on under bump metallurgy isothermally at 250 °C for 60 s. Aging and thermal cycling (T/C) were also performed on the solder joint. The interfacial microstructure of the joint was observed by scanning electron microscopy. X-ray diffraction and energy dispersive x-ray analyses were made toidentify the type of solder phase and to measure compositions. Excessive growth of an interfacial intermetallic layer in the Sn–3.5Ag–6.5 In solder joint led to a brittle fracture. In the other four solder joints, ductile fractures occurred through the solder region and the solder hardness was closely related with the joint strength.