Thermo-mechanical and fracture mechanical characterization of lead-free solder joints in microelectronics

The development of the component assemblies fabrication has been characterized during the last few years by the acceleration of the miniaturization of the products and simultaneously by the research of a better reliability. Hereby, there are two important challenges due to the increase of the integration density: the first one is the replacement of the previous used solder alloy, which is the Tin-Lead (SnPb) by alternative lead-free solder alloys. The second one is the growth of the solder joint specifications. It does not mean exclusively that the volume of solder joints has to be reduced, but also that the new lead-free solder alloys must be able to work at higher operating temperatures. To choose a good solder alloy, which will influence the capabilities of the micro components or the micro systems, a knowledge of the alloys capabilities and characteristics is required. For Example, to perform finite element calculation of the thermo-mechanical reliability of assemblies to calculate the lifetime of system assembly, the behaviors of the solder alloys are extremely important. For such calculation the thermo-mechanical characteristics of the solder joints have to be provided experimentally. Afterwards, the material models are introduced and implemented in the FE software. In this article, the experimental methods, which are able to determine the material behavior and are able to identify the parameters needed for the FE simulation, will be shown. Two new kinds of test samples will be presented. These test samples are different to the classical ones like the uni-axial creep test specimen called "dog bone" or "bulk". These two test specimens ( Ring and Plug, grooved-lap) can be used to measure the creep behavior of small solder volume. Furthermore, the CT (compact tension) test specimen, which is used to test the fracture behavior of solders, and the crack propagation under different stress-strain loads, will be presented. The best way to use this test specimens will be shown. Thereby, the experimental boundary conditions and the results of the creep tests and the crack propagation tests will be discussed