RESTORATION OF MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LEAD-FREE BISMUTH CONTAINING SOLDER JOINTS AFTER ACCELERATED RELIABILITY TESTING USING A THERMAL TREATMENT

摘要

Bismuth (Bi)-containing solder alloys have emerged as prime candidates to replace traditional lead (Pb)-free alloys such as SAC 305 (Sn-3.0Ag-0.5Cu). These alloys show stability in mechanical properties after aging, whereas the strength of SAC degrades over time. This finding prompted the development of a patented process in which the Bi-bearing alloy is subjected to a short above-solvus thermal treatment, to extend the life of the solder joint and improve device reliability. During this thermal treatment, all Bi in the alloy dissolves and diffuses through the P-Sn matrix to produce a homogenous microstructure with uniformly sized and distributed Bi precipitates, as well as an equiaxed P-Sn grain structure. In our most recent study, after accelerated thermal cycling (ATC) between -40°C and 70°C, preconditioned Violet (Sn-2.25Ag-0.5Cu-6.0Bi) solder joints demonstrated a 15% improvement in characteristic life over their as-assembled counterparts; this was not the case for SAC 305. In addition to preconditioning, it is proposed that the thermal treatment may 'restore' the microstructure and properties of the alloy after some time in service. Three assembly conditions, each representing some point in the product's life cycle, were analyzed either as-is, or after a restoration treatment. These conditions were as-assembled (early life), room temperature aged (long-term storage), and reliability tested (emulating long-term usage). Room temperature aging was conducted for approximately one year, and reliability testing consisted of alternating ATC (-100 cycles) and vibration (-100,000 cycles); testing was terminated after 2000 ATC cycles and 2 million vibration cycles. Ball Grid Array (BGA) and Plastic Leaded Chip Carrier (PLCC) components were analyzed; alloys under test were SAC 305 and Violet. Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) were utilized to study changes to alloy microstructure. The mechanical behavior of the joints (hardness) was analyzed using nanoindentation.