Sheng-lin Yu

Microstructure and creep properties of Sn-Ag-Cu lead-free solders bearing minor amounts of the rare earth cerium

Purpose – The purpose of this paper is to investigate the effects of minor addition of the rare earth (RE) element cerium, Ce, on the microstructures and creep properties of Sn‐Ag‐Cu solder alloys.Design/methodology/approach – The pure Sn, Sn‐Cu alloy, Sn‐Ag alloy and Cu‐Ce alloy were used as raw materials. Sn‐Ag‐Cu alloys with different contents of RE Ce were chosen to compare with Sn‐Ag‐Cu. The raw materials of Sn, Sn‐Cu alloy, Sn‐Ag alloy, Cu‐Ce alloy were melted in a ceramic crucible, and were melted at 550°C±1°C for 40 minutes. To homogenize the solder alloy, mechanical stirring was performed every ten minutes using a glass rod. During the melting, KC1+LiCI (1.3:1), were used over the surface of liquid solder to prevent oxidation. The melted solder was chill cast into a rod.Findings – It is found that the microstructure exhibits smaller grains and the Ag3Sn/Cu6Sn5 intermetallic compound (IMC) phases are modified in matrix with the addition of Ce. In particular, the addition of 0.03 wt.% Ce to the Sn‐...

Determination of Anand parameters for SnAgCuCe solder

A unified viscoplastic constitutive model, Anand equations, was used to represent the inelastic deformation behavior for Sn3.8Ag0.7Cu/Sn3.8Ag0.7 Cu0.03Ce solders in surface mount technology. The Anand parameters of the constitutive equations for the SnAgCu and SnAgCuCe solders were determined from separated constitutive relations and experimental results. Non-linear least-squares fitting was selected to determine the model constants. Comparisons were then made with experimental measurements of the stress–inelastic strain curves: excellent agreement was found. The model accurately predicted the overall trend of steady-state stress–strain behavior of SnAgCu and SnAgCuCe solders for the temperature ranges from −55 to 125 °C and for the strain rate range from 1% s−1 to 0.01% s−1. It is concluded that the Anand model can be applied to represent the inelastic deformation behavior of solders at high homologous temperatures and can be recommended for finite element simulation of the stress–strain response of lead-free soldered joints. Based on the Anand model, the investigations of thermo-mechanical behavior of SnAgCu and SnAgCuCe soldered joints in fine pitch quad flat package by the finite element code have been done under thermal cyclic loading, and it is found that the reliability of the SnAgCuCe soldered joints is better than that of the SnAgCu soldered joints.

Reliability study of Sn–Ag–Cu–Ce soldered joints in quad flat packages

Abstract The reliability of Sn–Ag–Cu–Ce lead-free soldered joints in quad flat packages under thermal cycling was investigated based on finite element simulation and experiments. The stress and strain response of fine pitch QFP device lead-free soldered joints were analyzed using finite element method based on Garofalo–Arrhenius model. The simulated results indicate that the creep distribution is not uniform, the heel of joints is the maximum creep strain concentrated sites. And comparisons were then made with experimental results of the cracks observed in the Sn–Ag–Cu–Ce soldered joints subjected to the temperature cycled experiment. In addition, the relative mechanical and metallurgical factors, which dominate the failure of soldered joints, were utilized to analyze the phenomena. The fracture surfaces indicate that crack initiate and propagate along the interface among bulk Cu 6 Sn 5 phases in Sn–Ag–Cu–Ce soldered joints.

Tensile strength of fine pitch QFP lead‐free soldered joints with diode laser soldering

Purpose – The purpose of this paper is to investigate the laser soldering of fine pitch quad flat package (QFP) devices using lead‐free solders and solder joint reliability during thermal cycling.Design/methodology/approach – QFP devices were selected as the test vehicles and were soldered with four alloy types, Sn37Pb, Sn3.5Ag, Sn3.8Ag0.7Cu and Sn3.8Ag0.7Cu0.03Ce. The experimental samples were QFP‐256 devices with lead‐free solder paste on the printed circuit boards. The packages were dried for 24 h at 125°C prior to reflow soldering. Soldering experiments on the QFP devices were carried out with an infrared (IR) reflow soldering oven and a diode laser (DL) soldering system. Reflow soldering was performed at peak temperatures of 210°C (SnPb), 240°C (SnAgCu and SnAgCuCe) and 250°C (SnAg), as determined on the boards. Pull testing was adopted to evaluate the tensile strength of the four solders using an STR–1000 micro‐joint strength tester.Findings – The tensile force of the QFP micro‐joints increased as l...

Laser Soldering of Fine Pitch QFP Devices Using Lead-Free Solders

In this paper, soldering experiments of fine pitch quad flat package (QFP) devices were carried out when soldered with Sn―Ag―Cu and Sn―Cu―Ni lead-free solders by means of diode laser soldering system, and compared with the experimental results soldered with Sn―Pb solders and with infrared (IR) reflow soldering method. The results indicate that under the conditions of laser continuous scanning mode and the fixed laser soldering time, an optimal power is obtained when the optimal mechanical properties of QFP microjoints are achieved. Mechanical properties of QFP microjoints soldered with laser soldering system are better than that of QFP microjoints soldered with IR soldering method. Results also indicate that adding rare earth element Ce to Sn―Ag―Cu and Sn― Cu―Ni lead-free solders improves mechanical properties of QFP microjoints, and the optimal amount of Ce is about 0.03%.

Effects of bulk Cu6Sn5 intermetallic compounds on the properties of Sn‐Ag‐Cu‐Ce soldered joints

Purpose – The purpose of this paper is to explore the formation and growth mechanism of bulk Cu6Sn5 intermetallic compounds, selecting Sn‐Ag‐Cu‐Ce solders as specimens.Design/methodology/approach – In order to further enhance the properties of SnAgCu solder, trace amount of rare earth Ce was selected as alloying addition into the alloy; in previous investigations, the enhancements include better wettability, physical properties, creep strength and tensile strength. In this paper, the microstructure of Sn‐Ag‐Cu‐Ce soldered joints and its interfacial intermetallic compounds were investigated. Moreover, different morphologies of Cu6Sn5 IMCs were enumerated and described, and Ostwald ripening theory was employed to interpret the formation mechanism of bulk Cu6Sn5 IMCs.Findings – In addition, based on finite element simulation, it is found that the von Mises stress concentrate around the bulk Cu6Sn5 IMCs inside the Sn‐Ag‐Cu‐Ce soldered joints after three thermal cycling loading (−55‐125°C). From the stress dis...