Extraction of material parameters for creep experiments on real solder-joints by FE analysis

Abstract

A modern approach to determine material data of solder alloys such as SnAg and SnAgCu is to measure the mechanical behaviour direct on a CSP/BGA solder connection. Advantages of that technique of measurement on industrial manufactured solder bumps are the considerations of miniaturized volumes and the material diffusion from connection pad into the alloy during reflow soldering process. Compared to the tensile test the shear experiment differs in the way of initiation the force load into the solder alloy. The shear force load inducts a multiaxial state of stress. This is the reason for the confrontation with a higher effort into the conversion procedure to determine specific coefficients for the material law. In several publications creep data were published based on shear force load measurements and applied cylinder model to convert primary data into equivalent values. In practice, the specimen bumps may have been different in their shape, depending on pad geometry, solder volume and weight of electronic component. How does the shape of solder joints influence the creep behaviour? A form parameter has been introduced to be able to describe a wide range of solder bump shapes. Every bump shape from barrel to hyperbolic can now be regarded. The form parameter also takes place in the conversion of experimental data into equivalent data. The determined creep material laws, based on the improved analytic model, describe the deformation behaviour of solder joints more accurately, than the commonly assigned creep laws using the pure cylinder model. The shape effect is shown on a FEM analysis of the experimental setup of creep measurements on shape varied Sn96.5Ag3.5 solder bumps. In general, during FEM based material modelling the coefficients of the material laws need to be stepwise changed until the right behaviour occurs. These iterations can stretch over a long time. The improved analytical model shows the potential to shorten the coefficient determination of material laws