SOLDER BUMP SIZE AND SHAPE MODELING AND EXPERIMENTAL VALIDATION

Capillary physics was used to predict the size of solder bumps process ed through a heated reflow cycle, Based on the density, surface tensio n, and volume of the solder material the effects of body forces are ne gligible compared to surface tension effects, For this case the predic ted equilibrium shape of the molten solder is a truncated sphere, wher e the base of the sphere is defined by the bump input/output (I/O) pad on the integrated circuit (IC), Experiments using different size bump s on Si wafers were conducted to test the validity of the truncated sp here model, The experimental results matched the model to within 11% f or the predicted height and 8% for the predicted radius, Using dimensi onless variables far the solder volume, bump height, and bump radius a llows these results for reflowed height and radius to each be plotted on a single curve and fitted with a single equation, These results can be used to design IC's, solder bumps, and solder bump assemblies in o rder to insure that the attached IC can be underfilled reliably, and t o insure reliable products.