Compliance metrics for surface mount component lead design

A diagonal lead stiffness metric which measures the flexural compliance of the corner-most leads in the package diagonal direction is developed. The diagonal lead compliance is determined by the component aspect ratio, flexural spring constants, and lead position on the package. The nominal diagonal force transmitted to the corner-most SM (surface-mount) solder connections during operational thermal cycling is controlled by the local diagonal lead stiffness, device-substrate thermal-expansion mismatch, and component dimensions. The author quantifies the diagonal transmitted force and examines the interaction between diagonal lead stiffness, diagonal force, and SM component dimensions. Parametric evaluation of the diagonal spring constant and diagonal force has been performed over a wide range of component aspect ratio and lead-compliance characteristics. It is concluded that the diagonal lead stiffness and diagonal transmitted force are useful comparative metrics for SM leaded components, since these parameters explicitly account for the inherent flexural lead compliance and package dimensions. The diagonal spring constant is a key parameter in currently available tools for predicting long-term SM solder attachment reliability.<
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