Prediction of drop impact reliability of BGA solder joints on FPC

Abstract

Flexible printed circuit (FPC) is extensively used in portable electronic products because of its excellent flexibility and twistability. Compared with rigid boards, FPC usually experiences much larger deformation in its usages. However, most available packaging-components at the moment are rigid, FPC-contained products may cause failures under accidental mechanical conditions, such as drop, bending and twisting. Many literatures have been published on the reliability of rigid boards, while much less results were reported for the research of drop performance of FPCs. In this paper, the mechanical reliability of a BGA mounted on FPC under drop impact has been studied by simulations. To make the model more close to the practical products, one plastic cuboid with the size of 89 mm ? 69.5 mm ? 8 mm taken as the housing is involved in the simulation model. The FPC mounted with a BGA is fixed in the cuboid through four screw bars and dropped from 1.5 meters height to the concrete ground. Drop direction, BGA layout location and solder joint shape and size, as the three main factors are used in the reliability study. In practical drop reliability tests, six orthogonal drop tests are taken. As well as considered that the simulation sample is rectangle, the simulations of different drop directions are divided into two groups according to if the long housing edge or short housing edge impact with ground firstly. To simulate the different drop impact angle based on the orthogonal drop, several drop cases were built by setting the angle from 0? to 90? between the ground and model. In all of above drop cases, the single difference is the drop direction. To study the impaction of BGA location, simulation cases with the BGA moved from edge to center had been simulated also. The height and the diameter of solder joint is another important factor to the connection reliability. To predict its impaction, solder joint height varied from 0.15 mm to 0.23 mm and diameter varied from 0.25 mm to 0.35 mm were considered in the models. Through the corresponding simulations, analysis and comparison, it is found that the strain rate could be up to a peak of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> /s, but most of the time, the strain rate locates in the range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /s. The max stress decreases with the increasing of the solder joint height. Drop reliability can be improved by increasing the height of solder joints. The max stress increases when the diameter of the solder joint increased from 0.25 mm to 0.3 mm, while decreases when the diameter increased from 0.3 mm to 0.35 mm. Drop reliability can be only improved when the diameter is larger than 0.3 mm.