The proper functioning of a portable product such as a mobile phone depends largely on the mechanical robustness of the physical interconnections that make up its electronic circuitry. Given the nature of their use, mobile phones are often subjected to environmental conditions that can result in the failure of these interconnections. Mechanical impacts caused by accidental drops or knocks are a potential cause of failure of solder interconnections within the phone. As portable electronics become smaller and incorporate an increasing number of functions, interconnections will become denser and finer. One downside of finer interconnections is that they are more fragile and susceptible to impact loads.

Within any mobile phone or portable digital assistant (PDA) is at least one printed circuit board (PCB), on which IC packages are mounted. Little is known about what happens to the PCB when the device is subjected to an accidental drop. To fill this knowledge gap, a study was performed whereby controlled drop tests were done on various portable electronic devices. The tests were controlled in that the products were dropped at specific orientations using a patented drop tester specifically designed for this purpose (Figure 1). The mechanical response of the PCB was gauged using instrumentation such as strain gauges and accelerometers.

Figure 1 : Patented drop tester conducting drop impact test of a handphone.

Test results showed that the impact orientation influences the impact force experienced by the test sample. Secondary impacts, such as, subsequent bounces and knocks after the initial fall, may be more severe than the initial one and prolong loading on the PCB. Product-level tests also show that the presence of internal components can influence deformation of the PCB. Localised deformation in a PCB can also occur because of its proximity to the impact surface of the product casing. The most severe deformation in product-level tests occurs at the bottom edge of the PCB near the point of contact. Limited clearance between the PCB and neighboring internal components reduces the duration and magnitude of PCB flexure within a product. However, there is also a greater possibility of internal components knocking against the PCB.

This study showed that both exterior design and interior arrangement of the components of a handheld product determines the mechanical response of the PCB when dropped. It also provided much insight into the dynamic response of the PCB within the product, and more importantly, it provides information on how to design or reinforce products against failure due to drop impact and shock. The tests also provide data that is critical for prescribing realistic loads in PCB numerical simulation (Figure 2) and board level drop tests.

Figure 2 : Numerical simulation of the bending of a PCB with a mounted electronic packaging on it in a controlled impact test.

This work is part of a tripartite collaboration involving the Department of Mechanical Engineering, NUS, Institute of Microelectronics and the Cavendish Laboratory, University of Cambridge.