Abstract
Solder coated polymer balls have been successfully employed for attaching packages to circuit boards with minimum standoff height, while accommodating large mismatches in thermal expansion coefficients. Dramatic improvements in temperature cycling performance are often realized by using them in place of solid solder balls, with five-fold increases in mean cycles to failure reported by a number of investigators. The sales literature, provided by suppliers of solder coated solder balls, attribute this superior temperature cycling performance to the soft, compliant polymer core of the product.
Our study of the mechanics of solder coated polymer balls has revealed that their stiffness is in fact comparable to that of solid solder balls. Their rigidity results from a composite construction in which a nearly incompressible polymer material is surrounded by a copper shell that is not easily deformed from its spherical shape. We have employed finite element analysis and mechanical measurements to obtain load versus deflection curves for both normal compression and shear displacements of solder coated polymer ball connections. The enhanced temperature cycling performance of solder coated polymer ball connections is also derived from their composite construction. A cross-section through one reveals that near the solder pads, the ratio of copper to polymer is quite high, and consequently so is its resistance to shear. At the mid-plane of the connection, the ratio of copper to polymer is low, which minimizes its shear resistance. Thus, when a solder coated polymer ball connection is subjected to a shear load, as in temperature cycling, most of its deformation occurs around its mid-section. By contrast, when a solid solder ball is subjected to a shear load, most of its deformation occurs near its attachment pads, where its cross-sectional area and hence its stiffness are minimal. In either type of attachment, failure occurs when sufficient plastic strain damage accumulates in the solder to initiate a fracture. By distributing its shear strain over its midsection, a solder coated polymer ball minimizes plastic strain in its solder, where as a solder ball concentrates it near its bond pads.
We have used finite element analysis to compute the cumulative plastic strain in various solder coated polymer ball assemblies subjected to cyclic shear loading induced by thermal excursions. By combining these results with an Engelmaier solder fatigue model, we predicted mean number of temperature cycles to failure of the solder connections. Our results compare favorably with published experimental data from temperature cycle tests. We have employed this analysis technique to examine how fatigue life is impacted by various connection parameters such as package size, stand-off height and solder composition, as well as those specific to solder coated polymer balls, which include size and mechanical properties of the core and ratios of solder and copper thicknesses to core diameter. Our overall objective is to enable design of complex stacked assemblies of multichip modules that meet customer reliability requirements for various use environments.
