Electronics Assembly Knowledge, Vision & Wisdom
Restoration of Lead-Free Bismuth Containing Solder Joints
Restoration of Lead-Free Bismuth Containing Solder Joints
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).
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Corrosion, Contamination, Data Acquisition, ESD and EOS, Inspection, Measurement, Profiling, Reliability, R&D, RFID, Solder Defects, Test, Tombstoning, X-ray and more.
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Authored By:
André M. Delhaise, Ivan Tan1, Polina Snugovsky, Jeff Kennedy
Celestica

Mikaella Brillantes, Doug D. Perovic
Department of Materials Science & Engineering, University of Toronto

David Hillman, David Adams
Rockwell-Collins

Stephan Meschter
BAE Systems

Milea Kammer
Honeywell Aerospace

Ivan Straznicky
Curtiss-Wright

Summary
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 β-Sn matrix to produce a homogenous microstructure with uniformly sized and distributed Bi precipitates, as well as an equiaxed β-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.

Conclusions
In this study, we tested the efficacy of our previously patented thermal treatment as a tool for restoring the microstructure and properties of bismuth-containing solder joints after some time in service. This treatment would serve to improve the long term product reliability as part of a normal repair and overhaul (R&O) product plan. The following conclusions can be drawn from this study:

• After sequential combined environment testing, it was observed that Violet outperformed both SAC 305 and SnPb. It is noted that this result was based on a limited cell of one assembly per alloy, and further testing in Phase 2 of our combined environment testing plan will consist of a larger sample size.

• Almost all joints exhibited a consistent failure mode, with crack propagation through the bulk solder. One exception was observed in the U101 BGA part on the Violet assembly, which failed in the component side interfacial IMC.

• The time zero joints in the restoration study, having been built in January 2018, displayed comparable microstructures and hardness as the room temperature aged samples, which had been aging for over two years, for all three alloy-part configurations under test. This indicates that the initial seven months of storage was enough to produce most of the microstructural evolution in these joints.

• For all alloys, parts, and assembly conditions, hardness degraded after reliability testing; this is accompanied by coarsening of IMCs and Sn recrystallization (due to damage accumulation).

• For the Violet PLCC joints, the restoration treatment produced statistically significant increases in hardness, indicating the microstructure was ‘rejuvenated.’ Bi precipitated out of solution in a fine, uniform distribution; this is a more mechanically robust microstructure.

• The SAC PLCC and Violet BGA joints underwent no discernable changes to microstructure or hardness after restoration treatment. This is possible evidence that the thermal treatment may be more beneficial for improving the properties of joints with ample Bi content, sufficient to form a second phase precipitate.

It is also noted that the results from this study may not be an adequate representation of the performance of the alloy in a practical product. Nanohardness measurements are taken from very small regions of the sample and are only influenced by features on the micro- to nano-scale, including second phase particles, alloy composition, and grain boundaries. In addition, the mechanical response may be influenced by unseen subsurface second phase particles. Nanoindentation results are independent of features such as joint geometry, package type, and interfacial IMCs, which often factor into performance in reliability testing and product usage. For these reasons, we stress that while the results from this study (namely the Violet PLCC joints) are promising, further work (discussed below) is required to evaluate the efficacy of the restoration treatment on the performance of the alloy as a solder joint in a more practical configuration.

Initially Published in the SMTA Proceedings

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