Abstract
Long term creep of a ceramic ball grid array (CBGA) solder ball under compressive loading was investigated. An experiment was conducted with two levels of loading and four of temperature. Analysis of the data assumed the composite ball structure could be simulated by an interconnection having the same shape, but of a single equivalent material. Curve fitting determined the stress exponent and activation energy of the equivalent material in the Norton creep model; the values were consistent with the range of values of the individual components available in the literature. Nonlinearity of the change in ball height with time was hypothesized to be due to geometric stiffening, a hypothesis which was confirmed by a simplified model. The model may be used to estimate creep behavior of other ball geometries having the same material set. The final result of this work - a closed form equation describing height decrease as a function of compressive force, temperature and time - can be used to simplify complex modeling of an entire package, and as an aid in designing accelerated thermal cycles which appropriately synchronize solder creep and fatigue.
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