Novel Testing Instrument for Lead-Free Solder Characterization

The European Union Restriction of Hazardous Substances Directive of 2006 has revolutionized the use of materials for electronic packaging: tin-lead solder has had to be replaced with lead-free solder. While years of experience and data acquired in the field are available for tin-lead solder, the same cannot be said for lead-free solder. The lack of data on these new alloys hinders the use of modeling for predicting the reliability of lead-free solder. This paper describes a new testing machine that can assess the properties of solder joints in a copper-solder-copper structure. Finite-element analysis models using the newly acquired data will improve lifetime predictions. During a typical test, a model solder joint is expanded and contracted in shear. This actuation is achieved from the thermal expansion of a steel tube, which provides a smooth movement at a rate that is equivalent to that encountered on printed circuit boards when thermally stressed. The applied force is measured using a load cell; the sample displacement is determined from laser displacement sensors. Hysteresis load-displacement loops are obtained and can be used for evaluating the material plastic response and loading history. A camera/microscope system captures time-lapse photography images, which digital image correlation software can analyze to obtain strain profiles and study crack propagation. The instrument is capable of investigating creep, stress relaxation, and isothermal and thermal-mechanical fatigue behavior of solder interconnects. Results from this new instrument are presented here for isothermal and thermal cycling fatigue tests.