Toni T. Mattila

Thermal Cycling Reliability of Sn-Ag-Cu Solder Interconnections—Part 2: Failure Mechanisms

Partxa01 of this study focused on identifying the effects of (i) temperature difference (ΔT), (ii) lower dwell temperature and shorter dwell time, (iii) mean temperature, (iv) dwell time, and (v) ramp rate on the lifetime of ball grid array (with 144 solder balls) component boards. Based on the characteristic lifetime, the studied thermal cycling profiles were categorized into three groups: (i) highly accelerated conditions, (ii) moderately accelerated conditions, and (iii) mildly/nonaccelerated conditions. In this work, the observed differences in component board lifetime are explained by studying the failure mechanisms and microstructural changes that take place in the three groups of loading conditions. It was observed that, under the standardized thermal cycling conditions (highly accelerated conditions), the networks of grain boundaries formed by recrystallization provided favorable paths for cracks to propagate intergranularly. It is noteworthy that the coarsening of intermetallic particles was strong in the recrystallized regions (the cellular structure had disappeared completely in the crack region). However, under real-use conditions (mildly/nonaccelerated conditions), recrystallization was not observed in the solder interconnections and cracks had propagated transgranularly in the bulk solder or between the intermetallic compound (IMC) layer and the bulk solder. The real-use conditions showed slight coarsening of the microstructure close to the crack region, but the solder bulk still included finer IMC particles and β-Sn cells characteristic of the as-solidified microstructures. These findings suggest that standardized thermal cycling tests used to assess the solder interconnection reliability of BGA144 component boards create failure mechanisms that differ from those seen in conditions representing real-use operation.

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.

Thermal Cycling Reliability of Sn-Ag-Cu Solder Interconnections.Part 1: Effects of Test Parameters

The work presented in partxa01 of this study focuses on identifying the effects of thermal cycling test parameters on the lifetime of ball grid array (BGA) component boards. Detailed understanding about the effects of the thermal cycling parameters is essential because it provides means to develop more efficient and meaningful methods of reliability assessment for electronic products. The study was carried out with a single package type (BGA with 144 solder balls), printed wiring board (eight-layer build-up FR4 structure), and solder interconnection composition (Sn-3.1Ag-0.5Cu) to ensure that individual test results would be comparable with each other. The effects of (i) temperature difference (ΔT), (ii) lower dwell temperature and lower dwell time, (iii) mean temperature, (iv) dwell time, and (v) ramp rate were evaluated. Based on the characteristic lifetimes, the thermal cycling profiles were categorized into three lifetime groups: (i) highly accelerated conditions, (ii) moderately accelerated conditions, and (iii) mildly/nonaccelerated conditions. Thus, one might be tempted to use the highly accelerated conditions to produce lifetime statistics as quickly as possible. However, to do this one needs to know that the failure mechanisms do not change from one lifetime group to another and that the failure mechanisms correlate with real-use failures. Therefore, in partxa02 the observed differences in component board lifetimes will be explained by studying the failure mechanisms that take place in the three lifetime groups.

Effects of thermal cycling parameters on lifetimes and failure mechanism of solder interconnections

The work presented in this paper focuses on a) clarifying the underlying physical failure mechanism of Sn-rich solder interconnections under thermomechanical loading and b) identifying the means to accelerate the failure mechanism by optimizing the dwell-times and ramp-rates of thermal cycling test. The statistical results showed that as the dwell-times were decreased the number of cycles to failure increased but the shortest testing time was achieved with 10-minute dwell-times. Increase of ramp-rate did not affect the number of cycles to failure but the time to failure was significantly reduced. Investigations of the nucleation of cracks in solder interconnections revealed that nucleation is much more dependent on the number of thermal cycles than on the studied test parameters and that the nucleation took place within about the first quarter of the average lifetimes. Furthermore, cracking of the SnAgCu interconnections under all thermal cycling conditions studied took place through the bulk of the solder interconnections along the continuous network of grain boundaries produced by recrystallization. An approach to further accelerate thermal cycling tests is proposed based on the formed understanding of the failure mechanism.

Toward comprehensive reliability assessment of electronics by a combined loading approach

Many electronic applications, such as portable handheld devices or automotive electronics, experience various loadings during their common operation. Recent investigations have shown, however, that the interactions of the different load components can be highly significant. The commonly employed standardized single load tests neglect these interactions and, therefore, do not represent well enough the use environment loading conditions of many electronic devices. Thus, it has become clear that modifications to the reliability evaluation procedures are necessary. But before loading conditions can be combined in a meaningful manner, the failure mechanisms under single load environments and their possible interactions must be clarified. This paper makes a brief review to the reliability of electronic assemblies under different loading conditions from the perspective of failure modes and mechanisms. The failure modes and mechanisms under pure thermal cycling, power cycling, mechanical shock impact, or vibration conditions are discussed first. Thereafter, the interactions of the loading conditions, when they are combined consecutively or concurrently, are discussed.

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

Abstract Thermomechanical reliability of electronics has commonly been studied by employing accelerated temperature cycling (ATC) tests. However, due to the localized heat dissipation in modern electronic devices, operational power cycling (OPC) is considered a more realistic testing alternative. In order to characterize the thermomechanical reliability of modern high-density electronics, the failure modes, mechanisms and lifetimes of a contemporary commercial handheld device were studied under the ATC and OPC conditions. The experimental measurements and finite element analysis (FEA) showed distinct differences in the thermomechanical response of the device component boards under the OPC and ATC conditions. The results from FEA showed that the interconnection deformations during the OPC test were mostly in the elastic region of the solder, whereas those during the ATC tests reached well into the plastic region. The inclusion of the product enclosure further emphasized this difference, as the enclosure restricted the thermal expansion of the component board during OPC testing. The experimental test results were consistent with the FEA results, as the device failed due to solder interconnection cracking under the ATC conditions within 18xa0days of testing, but those under the OPC conditions remained operational even after 460xa0days. Finally, FEA estimations suggest that even three times higher power dissipation levels compared to those found in contemporary handheld devices would result in many years of lifetime in OPC testing.

Failure mechanism of solder interconnections under thermal cycling conditions

Increasing miniaturization, power densities and internal heat dissipation of novel electronic packages have made their solder interconnections more vulnerable to failures. To improve the reliability of electronic devices the underlying physical failure mechanisms of solder interconnections must be clarified in detail in order to find means to control, or even prevent, the development of failures. Therefore, the evolution of microstructures and the development of failures in Snrich lead-free solder interconnections were investigated by employing methods of orientation imaging microscopy: cross-polarized light imaging and electron backscatter diffraction. The as-solidified microstructures of the SnAgCu solder interconnections (composed of a few large Sn colonies) were observed to undergo a notable change of microstructures at the strain/stress concentration regions before cracking. The investigations of microstructures indicate that the change of microstructures take place at two different stages in the course of thermal cycling: 1.) a gradual formation of low angle tilt grain boundaries caused by a rotations of small volumes of the as-solidified microstructures around the [100] and [110] axes. It is suggested that these boundaries are formed by recovery, i.e. the boundaries are a consequence of the rearrangement of dislocations by polygonization. 2.) In subsequent stages the microstructures in the strain concentration regions transformed into a more or less equiaxed grain structure by recrystallization. It is evident that cracking of solder interconnections under thermomechanical loadings is enhanced by the recrystallization, because the network of high-angle grain boundaries extending through the interconnections provide favorable paths for cracks to propagate intergranularly.

Evaluation of the drop response of handheld electronic products

The aim of the product level drop response evaluation presented in this paper is to provide goals and guidelines for the development of a board-level drop test methodology that would better reproduce the field use loading conditions of modern portable electronic devices. Eight commercially available smart phones from different manufacturers were evaluated for their free-fall drop response. For this purpose, miniature accelerometer and strain gauges were attached to various locations on the component board inside the product covers. The maximum strain, average rate to maximum strain, frequency of the effective mode shapes, and maximum deceleration were determined from the measured strain and deceleration histories. The determined values showed significant variation from drop to another and device to another, but it was noteworthy that the extreme magnitude of the strain, average rate to maximum strain, and deceleration can be very high: values as high as 10,000 μ (“micro-strain” = [10−6 m/m]), 26 s−1, and 10 kG were measured, respectively. n nPost analyses of the strain histories revealed that the shock impact response of the devices can be conceptually divided into two consecutive periods: (i) forced high amplitude bending/twisting of the component board at the moment of impact, and (ii) subsequent lower amplitude (resonance) vibration of the component board while the device bounces back from the site of impact. Maximum train values reached during Period (i) were typically much higher than the typical strain peaks during Period (ii). However, during Period (ii) sharp strain peaks were often identified whose maximum value occasionally went well above the maximum value during Period (i). Furthermore, any resonance vibrations initiated by the impact forces were dampened efficiently in all device models. n nIn order to form a better understanding of what is causing the very high strains, the drop response of one of the devices was simulated by employing the Finite Element Method (FEM). The FEM results showed that the regions of high strains are highly localized. During Period (i) they are caused by the forced bending of the board by the surrounding mechanical structures, and during Period (ii) by internal collisions between the vibrating component board and the surrounding mechanical structures. n nOn the basis of the characterization of the commercial portable devices, the following goals were set for the development of a board-level drop test methodology: a test board that simulates the response of portable electronic products to a free-fall drop impact should be able to produce: (1) board strain well above 3500 μ and (2) average strain rate as close as possible to 7 s−1. The experimental characterization of the mobile devices was carried out by Aalto University, while the device-level drop impact simulations were performed by Nokia.

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 Reliability of Microalloyed Sn-Ag-Cu Solder Interconnections Under Cyclic Thermal and Mechanical Shock Loading

In this study, the performance of three microalloyed Sn-Ag-Cu solder interconnection compositions (Sn-3.1Ag-0.52Cu, Sn-3.0Ag-0.52Cu-0.24Bi, and Sn-1.1Ag-0.52Cu-0.1Ni) was compared under mechanical shock loading (JESD22-B111 standard) and cyclic thermal loading (40xa0±xa0125°C, 42xa0min cycle) conditions. In the drop tests, the component boards with the low-silver nickel-containing composition (Sn-Ag-Cu-Ni) showed the highest average number of drops-to-failure, while those with the bismuth-containing alloy (Sn-Ag-Cu-Bi) showed the lowest. Results of the thermal cycling tests showed that boards with Sn-Ag-Cu-Bi interconnections performed the best, while those with Sn-Ag-Cu-Ni performed the worst. Sn-Ag-Cu was placed in the middle in both tests. In this paper, we demonstrate that solder strength is an essential reliability factor and that higher strength can be beneficial for thermal cycling reliability but detrimental to drop reliability. We discuss these findings from the perspective of the microstructures and mechanical properties of the three solder interconnection compositions and, based on a comprehensive literature review, investigate how the differences in the solder compositions influence the mechanical properties of the interconnections and discuss how the differences are reflected in the failure mechanisms under both loading conditions.