The effect of secondary impacts on PWB-level drop tests at high impact accelerations

The continuing increase of functionality and miniaturization in handheld electronics has resulted in a decrease in the size and weight of the product. Therefore, internal structures such as printed wiring boards (PWBs) are becoming more slender, thus increasing the likelihood of unintentionally causing contact between the PWB and other internal structures like battery compartments, displays, and other circuit cards, or the interior of the case. Other researchers have concluded that secondary impact against the case of a portable device can be one of the causes for internal structures to experience highly amplified contact stresses and accelerations and cause damage to the subsystems. In this study, the term secondary impact, refers to subsequent impacts between multiple masses in a system after the system has been subjected to an event like a drop or impact. In this paper, the drop test specimen consists of a PWB populated with multiple functional MEMS components. Secondary impacts, between the test PWB and the fixture underneath, are used to generate very high accelerations (20,000 G) for drop testing. This acceleration level is typically well above those encountered in life-cycle conditions or in typical qualification testing. The velocity and acceleration of different locations on the test PWB are investigated with the help of parametric drop testing and dynamic finite element analysis, to determine its response to the magnitude of the impact with the fixture during drop testing. The MEMS components are functionally tested to understand the role of the impact acceleration on failure. The clearance between the PWB and the fixture are parametrically varied in the study to understand the role of the clearance. The experiments and simulations show interesting results because of the participation of multiple dynamic modes.