Influence of Secondary Impact on Printed Wiring Assemblies—Part II: Competing Failure Modes in Surface Mount Components

Portable electronic devices are commonly exposed to shock and impact loading due to accidental drops. After external impact, internal collisions (termed “secondary impacts” in this study) between vibrating adjacent subassemblies of a product may occur if design guidelines fail to prevent such events. Secondary impacts can result in short acceleration pulses with much higher amplitudes and higher frequencies than those in conventional board-level drop tests. Thus, such pulses are likely to excite the high-frequency resonances of printed wiring boards (PWBs) (including through-thickness “breathing” modes) and also of miniature structures in assembled surface mount technology (SMT) components. Such resonant effects have a strong potential to damage the component, and therefore should be avoided. When the resonant frequency of a miniature structure (e.g., elements of an SMT microelectromechanical system (MEMS) component) in an SMT assembly is close to a natural frequency of the PWB, an amplified response is expected in the miniature structure. Components which are regarded as reliable under conventional qualification test methods may still pose a failure risk when secondary impact is considered. This paper is the second part of a two-part series exploring the effect of secondary impacts in a printed wiring assembly (PWA). The first paper is this series focused on the breathing mode of vibration generated in a PWB under secondary impact, and this paper focuses on analyzing the effect of such breathing modes on typical failure modes with different resonant frequencies in SMT applications. The results demonstrate distinctly different sensitivity of each failure mode to the impacts.

Influence of Secondary Impact on Printed Wiring Assemblies—Part I: High-Frequency “Breathing Mode” Deformations in the Printed Wiring Board

Design rules for portable electronic device are continuously striving for thinner printed wiring assemblies (PWAs) and smaller clearances because of ever-increasing demand for functionality and miniaturization. As a result, during accidental drop and impact events, there is an increased probability of internal secondary impact between a PWA and adjacent internal structures. In particular, compared to the initial impact, acceleration pulses caused by contact during secondary impacts are typically characterized by significant increase of amplitudes and frequency bandwidth. The resonant response in the thickness direction of printed wiring boards (PWBs) (termed the dynamic “breathing mode” of response, in this study) acts as a mechanical bandpass filter and places miniature internal structures in some components (such as microelectromechanical systems (MEMS)) at risk of failure, if any of them have resonant frequencies within the transmitted frequency bandwidth. This study is the first part of a two-part series, presenting qualitative parametric insights into the effect of secondary impacts in a PWA. This first part focuses on analyzing the frequency spectrum of: (i) the impulse caused by secondary impact, (ii) the energy transmitted by the dynamic “breathing” response of multilayer PWBs, and (iii) the consequential dynamic response of typical structures with high resonant frequencies that are mounted on the PWB. Examples include internal deformable structures in typical surface mount technology (SMT) components and in MEMS components. The second part of this series will further explore the effects of the breathing mode of vibration on failures of various SMT components of different frequencies.

Influence of Secondary Impact on Failure Modes in PWAs With High Resonant Frequency

Thinner printed wiring assemblies (PWA) and smaller clearances are driven by the continuing increase of functionality and miniaturization in portable electronic devices. The probability of secondary impact during accidental drop and impact between a circuit card and adjacent components increases with the decrease in the size and weight of the product. In particular, compared to the initial impact, impulses caused by contact during secondary impacts are typically characterized by significant increase of amplitudes and extremely short pulse widths. As a result, stress wave transmission and reflection in printed wiring boards (PWBs) can be at a frequency range close to the resonant frequencies of PWA components with miniature internal structures, such as MEMS. This study focuses on analyzing the high frequency content of the accelerations due to stress wave propagation, reflections and dispersions in the thickness direction of multilayered PWBs, caused by secondary impact, and on the consequential effects on typical failure modes with high resonant frequencies.

SnAgCu (SAC) Durability Under Vibration Loading at Different Isothermal Temperature Conditions

The high cycle fatigue (HCF) durability of SAC solders has not been investigated to the same extent as SnPb solder [1] [2] [3], especially as a function of temperature. This is a first essential step towards understanding the interaction between thermal cycling damage and vibration damage [4] [5]. In this study, vibration durability is investigated under step-stress broad-band random vibration excitation at different temperatures. The test vehicle consists of various common surface mount components soldered onto a test PWB. The solder system, plating system and thermal pre-conditioning are systematically varied. Twenty printed wiring assemblies are tested at a time in a specially designed fixture on an electrodynamic shaker. The test setup is first characterized before conducting the durability experiment, by collecting strain histories at different sites on the PWB, mounted at different locations on the fixture, under different loading conditions. These strain results are useful to be able to compare the performance of the assemblies at different strain conditions and as inputs for subsequent finite element analysis (FEA) to estimate acceleration factors for various field environments [6]. In order to check the temperature dependence of the vibration durability, the broad-band vibration durability tests have been repeated at room temperature, high temperature and low temperature. Test results are reported in this paper, and important trends are identified for SAC and SnPb solder systems. Destructive failure analysis (cross-sectioning, polishing and microscopy) is used to confirm that the failure is by solder fatigue.

A Technique to Characterize Rate-Dependent Failure Envelopes in Surface Mount Assemblies

This paper characterizes the fatigue failure envelopes for solder damage in Printed Wiring Assemblies (PWAs) subjected to dynamic loading. An empirical, rate-dependent, power-law durability model, motivated by mechanistic considerations, is used to characterize the failure envelopes in terms of PWA flexural strain and strain rate. Explicit nonlinear finite element analysis (FEA) is used to make the damage constants independent of the specimen geometry and characterize the durability in terms of the ratio of solder plastic strain to its failure strain. A case study, using a simple PWA specimen containing a single area array component, is presented to demonstrate the proposed approach.