Jussi Hokka

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. Post 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. In 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. On 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.

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...

Reliability assessment of MEMS devices — A case study of a 3 axis gyroscope

Recent technological breakthroughs in Micro-Electro-Mechanical Systems (MEMS) technologies have enabled significant cost reductions of MEMS gyroscopes and they are being increasingly employed in new application areas such as portable consumer electronics. Reliability assessment of MEMS assemblies is, however, more challenging than that of conventional IC assemblies: reliability characterization of MEMS must be made while the devices are in a functional state and the large number of small structural features requires new approaches for their physical failure analyses. In this paper we explore these challenges with a case study of MEMS gyroscopes, which are increasingly being employed in handheld consumer as well as automotive applications. Reliability of the gyroscopes will be characterized under elevated temperature and humidity (85°C/90%RH), and under high-G shock impact loading (up to 35 kG). The board assemblies were characterized for (i) maximum deceleration tolerance and (ii) fatigue lifetime under lower shock impact loads. Under the temperature-humidity characterization failures associated with absorption of moisture in the polymeric materials of the MEMS package showed early failures in 37 % of the samples while remainder of the samples survived 150 days of exposure. The shock impact characterization showed that the mean lifetime of the gyroscope assemblies depends significantly on the orientation of the impact load. Furthermore, package failures were produced at much higher decelerations (above 8 kG) than electrical failures of the device (at about 4 kG). Finite element model was established to predict the failure sites and the model correlated well with experimental observations. Several internal failure modes were e identified: fractured comb arms, fractured comb fingers, stuck MEMS elements, and chipped corners and edges of the elements caused by internal collisions. Transient failures were commonly observed under all testing conditions.

An approach to board-level drop reliability evaluation with improved correlation with use conditions

The work presented in this paper has been carried out in order to find means to improve the existing methods of board-level drop reliability assessment to better represent the use environment loading conditions of modern portable electronic devices. To provide goals and guidance for the development work, eight commercially available smart phones from different manufacturers were evaluated for their free-fall drop response. The results show that the drop response can be divided conceptually into two parts: (i) forced bending/twisting of the component board at the moment of the impact, and (ii) subsequent (resonance) vibration of the component board. The strain magnitudes caused by the forced bending and twisting were much higher than those by the post impact vibrations. Furthermore, sharp strain peaks were often identified within a few milliseconds from the impacts. The results of the Finite Element simulations show that the distribution of strains on the component boards is highly nonuniform; The regions of high strain are localized because they are caused either by the forced bending (by surrounding covers and frames in part i) or by internal collisions between the component board and the surrounding mechanical structures (sharp strain peaks in part ii). The devices were characterized for maximum strain, average rate to maximum strain, vibration frequencies and maximum value of deceleration. The following goals were set for the development of a board-level drop test methodology: a test board that simulates the response of modern portable products should be able to produce board strain well above 3 500 μ (“micro-strain” = [10-6 m/m]) and mean rate to maximum strain close to 7 s-1. Reaching these objectives requires significant changes to the existing JESD22-B111 approach. An investigation on the effects of the shape of deceleration input pulse was carried out first, followed by modifications for the board support to further increase the maximum strain and the average strain-rate of the JESD22-B111 compliant printed wiring board. The support was modified by replacing the four point supports by line supports at both ends of the rectangular shaped board. This approach can achieve the maximum strains of 5 000 μ when coupled with the optimized deceleration input. In addition to the increased strain and strain-rate, dampening of the board vibration became more effective, which is also in a better agreement with the typical drop response of portable electronic products.

Methods for reliability assessment of MEMS devices — Case studies of a MEMS microphone and a 3-axis MEMS gyroscope

Despite the fact that MEMS devices have become common in many electronic products, methods for their reliability assessment are still undeveloped. One significant difference to the reliability assessment of conventional electronic component boards is that MEMS devices require a stimulus or means to measure an output in order to monitor their health while the MEMS assemblies are exposed to loadings. Challenges are faced particularly when the instruments to perform the stimulus or measurements will also be exposed to the loading condition. Furthermore, for MEMS devices simple functional/not-functional failure criteria are often not sufficient and health monitoring during loading must cover several characteristics, each of which have their own application specific acceptance limits. Solutions to these challenges are discussed with the help of two case studies: i) a MEMS microphone and ii) a 3-axis MEMS gyroscope. The number of different failure modes in MEMS devices is also large and some of the failures are transient, such as those caused by temporary sticking of moving parts. The small length scale and complexity of the MEMS structures together with the fact that many of the failure modes are transient make the employment of new methods for their failure analyses necessary. Methods of failure analyses, the role of Finite Element Modeling in failure analyses, and some typical failure modes are also discussed.

Effects of shock impact repetition frequency on the reliability of component boards

Novel methods to shorten the time needed for reliability evaluation of new electronic devices are being developed by many. For example, it has recently been shown that the method of vibration testing can be employed to produce the same failure modes and mechanisms as the JESD22-B111 compliant drop testers in significantly shorter time. However, the correlation between the vibration cycles-to-failure and the shock impacts-to-failure is still unclear. Therefore the primary objective of the work being reported in this paper has been to clarify the effects of impact repetition frequency on the lifetime and failure modes and mechanisms of electronic component boards. The reliability tests were conducted with four types of packages (BGA144, BGA288, QFN72, and μSMD36) that were soldered onto the JESD22-B111 compliant printed wiring boards. Three shock impact repetition frequencies (0.01 Hz, 0.1 Hz and 1.6 Hz) were used to study the sensitivity of lifetime to time between the shock impacts. A new tester with adjustable impact repetition frequency was developed for this study. The effect of the elevated temperature (100°C) on the reliability of the component boards was also included in the scope of this study in order to study the role of creep relaxation and the restoration processes of solder interconnections on the lifetime of the component boards. The results showed that the impact repetition frequency has a significant effect on the average lifetime in three of the four studied packages despite the fact that the components have different structures and interconnection geometrics. The change of impact repetition frequency did not, however, change the failure modes and mechanisms.

High-speed mechanical impact reliability of solder interconnections in high-power LEDs

Light Emitting Diodes (LED) are being implemented more and more into demanding applications like automotive and high-brightness general lighting. From the reliability point of view, the automotive environment is extremely harsh and challenging. Automotive electronics have to withstand exposure to high temperature fluctuations, mechanical shock impacts and vibration. The cyclic thermal load of solder interconnections can be up to 150°C for several thousand cycles. For harsh environments, high-Pb and eutectic AuSn solders are currently being used as the interconnection material. Recent developments of SAC-based solders provide alternative lead-free solutions with lower processing temperature. However, little is known about the reliability and failure mechanism of these solder interconnections, especially under mechanical impact loadings. For a successful reliability test of the solder interconnection under the impact, it is important to find a widely accepted mechanical test method of measuring the degradation of the interconnection. In this study a newly designed high-speed impact tester that is based on the use of a pendulum was used to achieve this target. The purpose was to investigate the mechanical behaviour of lead-free solder interconnections under different loading conditions. The studied carrier had multi dies mounted onto a ceramic sub-mount, which was soldered to a Cu substrate. In total 60 samples were tested with five different test velocities (0.7, 1.0, 1.4, 1.8 and 2.2 m/s). The test results showed that the primary failure mode was the ductile failure in the solder bulk and only some local fractures at the solder interfaces were observed. This indicates that there was no dependency of the test velocity, since all the test velocities induced similar failures in the bulk solder of the solder joints.