High strain rate testing of solder interconnections

Purpose - This paper aims to present a new micro-impact tester developed for characterizing the impact properties of solder joints and micro-structures at high-strain rates, for the microelectronic industry, and the results evaluated for different solder ball materials, pad finishes and thermal histories by using this new tester. Knowledge of impact force is essential for quantifying the strength of the interconnection and allows quantitative design against failure. It also allows one-to-one comparison with the failure force measured in a standard quasi-static shear test.Design methodology approach - An innovative micro-impact head has been designed to precisely strike the specimen at high speed and the force and displacements are measured simultaneously and accurately during the impact, from which the failure energy may be calculated.Findings - The paper demonstrates that, peak loads obtained from the impact tests are between 30 and 100 percent higher than those obtained from static shear tests for all combinations of solder alloy and pad finish. The SnPb solder alloy had the maximum energy to failure for all pad finishes. Of all the lead-free solders, the SnAg solder alloy had the highest energy to failure. Static shearing induces only bulk solder failure for all combinations of solder alloy and pad finish. Impact testing tends to induce bulk solder failure for SnPb solder and a mixture of bulk and intermetallic failure in all the lead-free solder alloys for all pad finishes. In general, the peak loads obtained for solder mask defined pads are significantly higher than those for non-SMD (NSMD) pads. The results obtained so far have highlighted the vulnerability of NSMD pads to drop impact.Practical implications - The work provides a new solution to the microelectronics industry for characterizing the impact properties of materials and micro-structures and provides an easy-to-use tool for research or process quality control.Originality value - The new micro-impact tester developed is able to perform solder ball shear testing at high speeds, of up to 1,000 mm s, and to obtain fracture characteristics similar to those found in drop impact testing using the JEDEC board level testing method JESD22-B111 - but without the complexity of preparing specialized boards. This is not achievable using standard low-speed shear testers.