Juha Karppinen

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.

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.

Effect of isothermal annealing and electromigration pre-treatments on the reliability of solder interconnections under vibration loading

Effects of two pre-treatment methods, isothermal annealing and DC-current stressing, on the reliability of solder interconnections under vibration loading has been studied in this paper. The results obtained show that: (1) isothermal annealing and DC-current stressing both compromise the reliability of interconnections during testing in comparison to the samples without pre-treatment and (2) the crack propagation path through the interconnection changes as compared to the samples without pre-treatments. The experimental results are rationalized with the help of detailed microstructural investigations coupled with the finite element method analysis. The DC-current stressing and isothermal annealing pre-treatments both soften the solder interconnections by inducing microstructural changes, which is reflected in the slightly reduced reliability under vibrational loading.

The combined effect of shock impacts and operational power cycles on the reliability of handheld device component board interconnections

A compact high-density test board has been subjected to combinations of power cycling and shock impact loads. The test board was designed to replicate both the mechanical shock impact response and the thermomechanical response of a commercial handheld device component board under product- and board-level tests. The power cycling loading affected the microstructure of the solder interconnections by (1) enhancing the growth of interfacial intermetallic compound layers and (2) driving the coalescence of intermetallic particles inside the solder bulk. These microstructural changes initially improved drop test reliability, but longer exposure time caused the drop reliability to deteriorate. The lifetimes and failure modes of the test boards were found to be similar in concurrent and consecutive test combinations. Furthermore, the lifetime trend of the consecutive power cycling and drop tests was found to be the same in board- and product-level tests. This confirms that product-level reliability can be assessed with board-level tests.

Formation of mechanical strains in the component board of a high-end handheld product during shock impact

The effect of mechanical shock impacts on electronic component boards 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 by a drop of a complete product differ from those encountered in standardized board level tests. In order to better understand the correlation between the board level drop tests and an actual drop of a complete device, the component board of a high-end handheld product has been studied under both board level and product level test conditions.

Thermal investigation of a battery module for work machines

Thermal Design of Li-ion battery cells/modules is necessary to ensure better cycle lifetime of the batteries. High power batteries (40Ah–100Ah) generate a significant amount of heat which needs to be dissipated somehow. In this work thermal experiments and simulation are utilized for better thermal design of battery module. The results indicate that liquid cooling is almost indispensible for high power battery modules. This methodology also ensures that the waiting period for battery cool down can also be reduced significantly which helps in better and proper utilization of the batteries.

Improved methods for development of high reliability electronics

To meet the critical reliability requirements of aerospace electronics, reliability design needs to be integrated into the early stage of product design process. In this paper the role of finite element analysis, thermodynamic calculation, and microstructural simulation in reliability design are introduced, followed by the discussion of failure oriented accelerated tests with emphasis on failure mechanisms. The typical failure mechanisms of three single loading tests as well as two combined thermal and mechanical loading tests are covered. Lastly, the challenges in reliability assessment and failure analysis of MEMS devices are discussed through two case studies, MEMS microphone and gyroscope.

Finite element modeling for reliability assessment of solder interconnections in a power transistor

In this study, the reliability modeling of solder interconnections in a power transistor with a compact metallic packaging is carried out. The finite element simulations are verified by experiments. Different loading conditions, thermal cycling (TC), drop impact, and vibration loadings, are considered in the present study. The micrographs from the failure analyses of the failed samples are also presented in order to show the location and detailed morphology of the cracks. The obtained results add insights into the correlation between different failure modes caused by various loading conditions and the stress state of the critical solder interconnections.

On the role of electromigration in power cycling tests

Effects of electromigration on microstructures of solder interconnections during electrical current stressing have been widely reported. However, a more profound understanding of the role of electron flow in the evolution and mechanical integrity of intermetallic layers is needed. Therefore, in this paper the effects of cyclic power loads on the evolution of solder interconnection interfacial microstructures are presented. It is shown that the direction of the electron flux significantly affects on the growth kinetics of Cu6Sn5 and Cu3Sn intermetallics. In addition, there is a clear difference between constant and cyclic electrical loading conditions. Furthermore, the microstructural evolution caused by electromigration and thermomechanical loads has a significant effect on failure mechanism under fast mechanical loading i.e. drop testing.