Generalized method for characterizing mechanical loads in mobile devices

Abstract

This paper presents the concept and design of a test vehicle for characterizing applied mechanical loads on small form-factor devices. These loads can include forces, moments, and torques. A case-study utilizing a version of this test vehicle is discussed. The test vehicle weighs 100 grams and has a form factor similar to commonly available media players like Zune HD. Specific usage scenarios were selected to represent extreme loading conditions. The resulting multivariate, stochastic data were analyzed using the Bounded Johnson System distribution. This statistical treatment eases the use of the empirical data in subsequent analysis, such as determination of the appropriate in-lab loading conditions to quantify the mechanical robustness of a product. IntroductionRecent proliferation of mobile consumer electronic devices has led to a need for more efficient methods to better predict the reliability of these devices in various use scenarios.[1] These scenarios range from transporting these devices in pockets, back-packs, and purses; to leaving them in parked vehicles or other non-climate-controlled spaces. Some of these use cases were not seen by consumer products in the past.Miniaturization, growing device complexity, and reduction in development time due to time-to-market pressures exacerbate this need.The first step in predicting reliability is to accurately characterize the expected mechanical and other stress profiles for the product during its use life. The ability to quickly and accurately quantify the design margins for a product based on this knowledge minimizes the risk of costly product returns and the bill-of-material costs due to unnecessary over-design. This also allows the opportunity for concurrent evaluation of multiple device architectures while evaluating trade-off scenarios for reliability, manufacturability, industrial design, etc. This consequently can reduce the number of unnecessary design compromises to be made during development.To evaluate reliability at both component and board levels, mechanical stresses are an essential input. [2-4] These stresses may be obtained from crude estimations, historical data from similar products, or evaluating the actual component or board placed inside a product prototype. The issue with the first two approaches is the accuracy of the estimates; and the third approach constraints the estimates around a specific design and requires a higher level of design maturity. This paper presents a generalized methodology for characterizing mechanical loads that a handheld device may experience while in use. It can be used very early in the development phase to support high quality decisions on competing design options.