| Objectives | Background | Approach |
Use the thermo-mechanical-microstructural (TMM) test apparatus
to assess the durability of the leading candidate lead free solder
alloy, Sn3.9Cu0.6Ag.
New interconnect materials are always necessary as a result of evolving packaging technologies and increasing performance and environmental demands on electronic systems. A large research community is centered on materials characterization (traditionally, of solders) to meet the needs of the electronics community. However, there are wide variations among published data, in addition to a multitude of test methods, specimen configurations and preparation techniques, and a host of other parameters that often remain unquantified. One critical issue is the testing of bulk versus miniature specimens, which weighs the relative ease and consistency of testing large, bulk specimens against the more appropriate scale and constraint conditions of thin specimens.
Much of the previous work related to this project has involved developing a customized experimental apparatus used for testing shear specimens that are of the same length-scale as standard surface-mount interconnects. The equipment is used to characterize various solder materials (including Sn-Pb eutectic) and has the potential for application to a wide variety of other materials (e.g. die attaches, conductive adhesives, underfills.) A solid-state piezoelectric actuator is employed to control displacements over a range of 90mm with a resolution of less than 10nm; load resolution is 0.05N with a maximum of 300N (67 lb.) in tension and 800N (180 lb.) in compression. In addition, the manufacturing processes of the specimens, from assembly and reflow to grinding, polishing and test preparation, are carefully controlled, and prepared specimens are aged to ensure the testing of only stabilized microstructures. The standard TMM test specimen is used for durability tests as well as for in situ microstructural aging studies. Local displacements in the TMM specimen are measured using a digital image correlation technique, which is used both to verify sensor data and to detect local cracking, interfacial delaminations and strain concentrations (around large voids, for example.) The occurrence of target failure mechanisms (i.e. bulk solder fatigue) is verified through optical and electron microscopy.
A test and characterization scheme based on constant work tests is proposed. Monotonic tests are first performed to determine the ultimate toughness (UT) of the test alloy over a suitable range of temperatures and strain rates. The results of these tests are used to establish the total work required for failure, and the relative work levels to be maintained during the cyclic tests. Cyclic, work-controlled durability tests are then executed at three load levels, i.e. three levels of work accumulation per cycle. These tests are run at appropriate strain rates and temperatures such that the effects of plasticity and creep deformation can be isolated. Test temperature levels are 25ºC, 90ºC and 140ºC; shear strain rate levels are 5.0E-5s-1, 1.2E-3s-1 and 1.0E-1s-1. The crack propagation through the test joint is tracked by monitoring the load drop of the specimen as the test progresses. A quantitative durability model using partitioned work accumulations as reliability metrics (similar in form to the CALCE energy-partitioning model) is developed using the experimental data and associated finite element modeling of the test structures and conditions. The environmental scanning electron microscope (E-SEM) provides for the evaluation of the microstructural stability and failure modes for the test material. Analytical support for these and related experimental tests is also provided.