Study On Mechanical Properties And Size Effects Of Lead-Free BGA Solder Joint By Nanoindentation

With the downsizing of solder joints in microelectronic packaging, size effect in micro-joining plays an more important role in the constitutive equation of the solder joints. In this thesis, the constitutive equation of Ball Grid Array (BGA) solder joints was constructed with nanoindentation technology, and was compared with the results from tensile tests. The size effect in constitutive equation of solder joints was studied systemically.The mechanical properties, elastic modulus and creep stress exponent, for the as-casted Sn-37Pb eutectic and Sn-3.0Ag-0.5Cu lead-free bulk tensile specimen measured by nanoindentation were completely consistent with the results from tensile and high temperature creep tests of bulk tensile specimen. Therefore, nanoindentation technology provides a new, simple and feasible method on the measurement of mechanical properties of solder alloys.The preliminary results with nanoindentation method show that there existed an obvious size effect in the constitutive equations of solder alloys and BGA solder joints, which broadens the definition of micro-joining. The size effect in the solder joints means that the traditional method for constructing the constitutive equation based on the tensile and creep tests of the tensile specimen could not provide an accurate evaluation of the reliability of solder joints in microelectronic packaging. Based on the direct measurement of the solder joints, nanoindentation method can provide their constitutive equations including the effect of size effect, which gives a new guide on the more accurate evaluation of the reliability of solder joints.The constitutive equations of the solder joints established with the nanoindentation tests can be related with those of the as-casted tensile specimens with the tensile and creep tests by the introduction of the factor of elastic modulus (K_E), the factor of creep stress exponent (K_n) and the factor of creep activated energy (K_ΔH). K > 1 and K < 1 represent the increase and decrease of the values of the mechanical properties of the solder joints compared with those of the as-casted solder alloys, respectively.