Understanding and testing for drop impact failure

This paper presents the results of experiments aimed at studying the effects of drop impact on portable electronics and reproducing these effects in controllable tests. Firstly, a series of drop tests were performed on consumer products (mobile phones and PDAs) to understand how the printed circuit board (PCB) within a product behaves in actual drop conditions. These product-level drop tests show that in drop impact, there are three possible types of mechanical response which can stress the 2(nd) level interconnections of CSP and BGA packages, namely: 1) flexing of the PCB on its supports, dominated by the 1st (fundamental) natural frequency; 2) flexing of the PCB resulting from direct impact or knocking against the PCB, typically dominated by higher natural frequencies; and 3) inertia loading on the solder joints due to high accelerations. Next, a series of board-level experiments were designed to separately study each of the three types of mechanical response. Board flexing due to direct impacts is the most severe response due to the strong strain amplitudes generated. Given the same input shock. the conventional board-level test-where the PCB flexes on its supports-produces much lower strain amplitudes. Inertia loading on the solder joints is practically negligible. Since PCB flexing is the main failure driver, a simple vibration test, which reproduces the strains observed in drop impact, is suggested as an alternative to time-consuming drop impact tests.