![]() ![]() If the solder paste and the ball have the same alloy mixture there will be little to indicate any change. The gold will have gone in to solution during the original ball attachment process. As the solder ball is already attached to the package, this will result in a change in the thickness of the intermetallic layer between the alloy of the ball and the nickel. Ball drop could also occur and has been seen on BGAs running over a wave soldering system with top side preheat, however the ball may not be round but rather elongated when examined.ĭuring normal reflow, low temperature balls will become liquid and be in a liquid state for a period of time during reflow. The appearance of the ball or solder will also look different if separated in air or nitrogen environments. This would happen if in a liquid state the solder surfaces are stretched and separated. If the balls drop during reflow or rework, the surface of the pad will probably still be perfectly wetted with solder. In the case of drop testing or shear testing the failure site will have a hard fractured surface on the pad and the ball. Section of a ball separating from the packageĮach type of ball drop can generally be illustrated by the way the ball separates from the surface of the package termination pads. Either lack of flux, poor pad solderability or ball oxidisation could be the root cause of this problem.įigure 2. ![]() The gold-plated pads are still clearly visible, indicating that the balls were never soldered to the BGA pad. Automatic optical inspection is used along with sample shear testing of the ball-to-substrate interface after reflow.įigure 1 shows the loss of two balls from a BGA. This is less common today due to the control of area array manufacturing processes and the quality control procedures in component assembly lines. Other literature defines it as 'Balls separating from the package interface during drop testing, vibration, flexure or temperature cycling.' Obviously this is the point of doing environmental testing: to find the weakness of the interconnection or prove the reliability of the joints.īalls frequently separated from the package in the early years of BGA use during handling and placement. It can be difficult to detect even under package optical or X-ray inspection as the separation gap can be extremely small.įirst, it is important to define what 'dropped ball' really means in practice: 'Dropped ball – the loss of one or more balls from the base of an area array component and found during inspection of the component or after first or second side reflow.' This seems appropriate as a definition but many people may have a different description. This type of defect can be found at different stages in the manufacturing process depending on its root cause. Over the last few years I have been asked to examine examples of ball grid array (BGA) defects which are referred to as dropped ball. Bob Willis, process engineering consultant ![]()
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