Jue Li

Evolution of microstructure and failure mechanism of lead-free solder interconnections in power cycling and thermal shock tests.

Failure mechanisms of lead-free solder interconnections in power cycling and thermal shock tests have been investigated in this work. Even though there are some characteristic differences between the two tests, the failures in both cases were induced by recrystallization-assisted crack nucleation and propagation. The three major differences between the tests were: (i) minimum temperature during power cycling was considerably higher in comparison to thermal shock, (ii) the current flow in the power cycling test resulted in electromigration, and (iii) in the power cycling test heat originates locally from components themselves. These differences were also reflected in the test results in the following way: firstly, in the power cycling test the recrystallization occurred earlier than in the thermal shock test, mainly owing to the higher average temperature and secondly, the enhanced growth of intermetallic compound layer at the anode side due to the electromigration was observed during power cycling.

Reliability of Lead-Free Solder Interconnections in Thermal and Power Cycling Tests

Lead-free solder interconnection reliability of thin fine-pitch ball grid array (BGA) lead-free packages has been studied experimentally as well as with finite-element (FE) simulations. The reliability tests were composed of the thermal shock test, the local thermal cycling test (resistors embedded in the board around the package), and the power cycling test (heat generation in the die). A 3-D board-level finite-element analysis (FEA) with local models was carried out to estimate the reliability of the solder interconnections under various test conditions. Due to the transient nature of the local thermal cycling test and the power cycling test, a sequential thermal-structural coupling analysis was employed to simulate the transient temperature distribution as well as the mechanical responses. Darveauxs approach was used to predict the life time of the solder interconnections. Furthermore, the numerical results validated by the experimental results indicated that the diagonal solder interconnections beneath the die edge were the most critical ones of all the tests studied here. It has been found that the fatigue life in the power cycling test was much longer than that in the other two tests. Detailed discussions about the failure mechanism of solder interconnections as well as the microstructural observations of the primary cracks are reported in this paper.

Inhomogeneous deformation and microstructure evolution of Sn–Ag-based solder interconnects during thermal cycling and shear testing

Abstract Orientation imaging microscopy was adopted to characterize the microstructural changes in Sn–Ag-based solder interconnects during thermal cycling and shear testing. The deformation and microstructure evolution of Sn–Ag-based solder interconnects are inhomogeneous, depending on the orientations of β-Sn grains in the as-solidified microstructure. Recovery or recrystallization can take place even under pure shear stress at room temperature, and it tends to occur at high-angle grain boundaries in multi-grained solder interconnects, while it localizes in near-interface region in solder interconnects with only one grain inside. During thermal cycling, the hardness of recrystallized microstructure decreased significantly due to the segregation of Ag3Sn IMC particles towards the newly-formed recrystallized boundaries, increasing the ease of localized deformation in this weakened microstructure. As a consequence, cracks were propagated intergranularly in the recrystallized microstructure.

Shock impact reliability characterization of a handheld product in accelerated tests and use environment

Abstract The effect of mechanical shock impacts is a key factor in the reliability of modern handheld products. Due to differences in product enclosures, impact orientations, strike surfaces and mountings of component boards, the loading conditions induced in a true product drop differ from those encountered in standardized board-level tests. In order to better understand the correlation between board-level drop testing and actual drops of a complete device, series of board and product-level drop tests were conducted using specialized test boards. The mechanical shock impact response of the commercial handheld device component board was characterized with the help of acoustic excitation laser vibrometry and finite element analysis. The results were used to design the mechanically compatible specialized test board for both 4-point supported board-level and unsupported product-level drop tests. Special care was taken to ensure that the vibration behavior of the test board accurately represented the vibration behavior of the commercial component board. Additional board-level drop tests were conducted using a JEDEC JESD22-B111 compliant component board for comparison. The drop test results showed that, even though the test board design and supporting method have a marked influence on the strain conditions and lifetime of solder interconnections, the primary failure mode and mechanism under the product-level drop tests is comparable to that typically encountered in the standard JEDEC JESD22-B111 board-level drop tests. More detailed analyses suggest that the comparability of the shock impact loading conditions affecting solder interconnections can be characterized using three metrics: (1) the maximum component board strain rate, (2) the maximum board strain amplitude and (3) the damping of the component board.

On the effects of temperature on the drop reliability of electronic component boards

Abstract The effects of package temperature on failure mechanisms and lifetimes under mechanical shock loading were studied with the help of five different types of high-density packages (a WL-CSP and four CSP-BGAs) assembled on both double-layer and multi-layer FR4 boards. The localized heating of the packages by means of integrated heating elements was utilized in order to produce similar hot spots to those occurring in products in service. The results showed that the temperature can have a significant effect on the lifetimes of component boards under mechanical shock loading but that the effect varied according to the structures of the component boards. The average number of drops to failure of the WL-CSP component boards increased significantly with an increase in the temperature of the package, while the average number of drops to failure of the CSP-BGA component boards generally decreased. On the other hand, the drop reliability of one out of four CSP-BGA component board types was insensitive to temperature. The failure modes and mechanisms were clarified with the help of physical failure analyses that revealed different failure modes in the component boards. Furthermore, depending on the component board type, the primary failure mode may change with temperature from that identified at room temperature. Particular attention was paid to the nucleation and propagation of cracks at different test temperatures. Computational case studies were designed in order to identify the significance of a change in temperature on three factors: (a) the stiffness of the PWB; (b) the strength and elastic modulus of the solder, and (c) the thermomechanical loads. The influences of each factor on the strains and stresses in the proximity of the solder interconnections were evaluated by means of the finite element method. The results of the statistical and physical failure analyses were rationalized with the help of the results from the finite element analyses. They showed that the effects of a change in temperature on the lifetimes of the component boards under mechanical shock loading can be explained by changes in the nucleation site and/or the propagation of cracks. The results presented in this paper point out that single-load reliability tests can form an incomplete understanding of the failure mechanisms in real service environments and modifications to the currently employed reliability test standards that are needed.

Localized recrystallization and cracking behavior of lead-free solder interconnections under thermal cycling

The failure mechanism of lead-free solder interconnections under thermal cycling has been studied by cross-polarized light microscopy, scanning electronic microscopy (SEM), and nanoindentation test. From the results of finite element modeling (FEM), it was found that the critical solder interconnection was located at the chip corner, and the stress was concentrated at the outer neck region beneath the ball grid arrays (BGA) component. The FEM results were in good agreement with the experimental observation. Two failure modes of the interconnections were identified: one is the intergranular or transgranular cracking through many small equiaxed recrystallized grains and the other is the transgranular cracking in few large irregularly shaped recrystallized grains. The results show that the localized recrystallization makes the Ag3Sn intermetallic compounds (IMC) coalesce and distribute sparsely, which leads to the degradation of the recrystallized microstructure and easy propagation of the cracks.

Microstructural Evolution and Mechanical Properties of Au-20wt.%Sn|Ni Interconnection

In this paper, the microstructural evolution and properties of Au-20wt.%Sn|Ni reaction couples were investigated from two perspectives: (1) by analyzing the microstructure of the as-soldered and aged samples, as well as (2) by measuring the mechanical properties of the intermetallic compounds formed within the reaction zone. The evolution of interfacial reaction products for both the as-soldered and aged interconnections was rationalized by using the experimental results in combination with assessed thermodynamic data from the Au-Ni-Sn system. Moreover, nanoindentation tests were implemented to measure the indentation modulus and hardness of the compounds formed at the interface. It was found that aging had a negligible influence on the elastic modulus and hardness of AuSn and Au5Sn, while the solubility of the third element significantly changed the indentation modulus and hardness of the intermetallic compounds.

Reliability assessment of a MEMS microphone under mixed flowing gas environment and shock impact loading

In this work the reliability of a Micro-Electro-Mechanical Systems (MEMS) microphone is studied through two accelerated life tests, mixed flowing gas (MFG) testing and shock impact testing. The objective is to identify the associated failure mechanisms and improve the reliability of MEMS devices. Failure analyses are carried out by using various tools, such as optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS). Finite element analysis is also conducted to study the complex contact behaviors among the MEMS elements during shock impact testing. The predicted failure sites are in agreement with the experimental findings.

The Effects of Concurrent Power and Vibration Loads on the Reliability of Board-Level Interconnections in Power Electronic Assemblies

The effect of concurrent vibration and electrical power loads on the solder interconnections of a surface-mount power transistor package has been investigated in this work. Both cyclic and constant power loadings were separately combined with vibration over a wide amplitude range. Single load vibration and power cycling tests were conducted for comparison. In addition to lifetime analysis, the failure modes occurring under each test case were carefully studied from cross-sectional samples, and the failure mechanisms were rationalized with the help of finite-element calculations and microstructural analysis. A substantial reduction in interconnection lifetimes was observed in the combined load tests as compared with the lifetime under single load tests. Three different failure modes were found within all the different test cases: 1) ductile crack propagation through bulk solder, 2) recrystallization assisted crack propagation, and 3) mixed mode propagation with both mechanisms. The failure mode changes were dependent mainly on the magnitude of plastic strain induced by the mechanical vibration. The results of this study provide insight in designing more comprehensive reliability tests as well as achieving higher levels of test acceleration without compromising the validity of results.

Finite element analyses and lifetime predictions for SnAgCu solder interconnections in thermal shock tests

Purpose – The purpose of this paper is to study the reliability of SnAgCu solder interconnections under different thermal shock (TS) loading conditions.Design/methodology/approach – The finite element method was employed to study the thermomechanical responses of solder interconnections in TS tests. The stress‐strain analysis was carried out to study the differences between different loading conditions. Crack growth correlations and lifetime predictions were performed.Findings – New crack growth data and correlation constants for the lifetime prediction model are given. The predicted lifetimes are consistent with the experimental results. The simulation and experimental results indicate that among all the loading conditions studied the TS test with a 14‐min cycle time leads to the earliest failure of the ball‐grid array (BGA) components.Originality/value – The paper presents new crack growth correlation data and the constants of the lifetime prediction models for SnAgCu solder interconnections, as well as...