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Drop Test Performance of BGA Assemblies
Analysis Lab |
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Authored By:Weiping Liu and Ning-Cheng Lee Indium Corporation, Clinton, NY USA Simin Bagheri, Polina Snugovesky, Jason Bragg Russell Brush, and Blake Harper Celestica International Inc., Toronto, Ontario, Canada TranscriptDriven by environmental considerations, the electronics industry has been migrating toward lead-free soldering since the late 1990s. Presently, the prevailing solder alloys are mainly Tin-Silver-Copper alloys with high silver content. Although high silver SAC alloys are widely adopted, the fragility of solder joints of area array packages, such as BGAs or CSPs, causes major concern for portable devices. Low Silver SAC alloys are proposed as a solution, but have had limited success. Among those promising new materials, Titanium has been reported as a very effective dopant to SAC alloy for improvement of drop test performance in a simplified simulation study. In this work, BGA solder spheres using SAC105 with 0.02% addition of Titanium were evaluated for BGA assembly drop test performance. The results are presented and discussed. SummaryDriven by environmental considerations, the electronics industry has been migrating toward lead-free soldering since the late 1990s. Presently, the prevailing solder alloys are mainly SnAgCu (SAC) alloys with high silver content, such as Sn3.8Ag0.7Cu (SAC387) and Sn3.0Ag0.5Cu (SAC305). Although high Ag SAC alloys are widely adopted, the fragility of solder joints of area array packages, such as BGAs or CSPs, causes major concern for portable devices. Low Ag SAC alloys such as SAC105 are proposed as a solution, but with only limited success. Other alloys such as SAC alloys modified with a variety of additives are also attempted. Again, the outcome is mixed. Among those promising new materials, Ti has been reported as a very effective dopant to SAC alloy for improvement of drop test performance in a simplified simulation study In this work, BGA solder spheres using SAC105 with 0.02% addition of Ti (SAC105Ti) were evaluated for BGA assembly drop test performance. The results will be presented and discussed below. ConclusionsBoard-level drop test performance was evaluated and compared for the following four different solder combinations in BGA/CSP assembly: 1) SnPb paste with SnPb balls, 2) SnPb paste with SAC105Ti balls, 3) SAC305 paste with SAC105Ti balls, and 4) SAC305 paste with SAC105 balls. The Ti doping improved the drop test performance significantly, despite the voiding side effect caused by its oxidation tendency. It is anticipated that the voiding can be prevented with the development of a more oxidation resistant flux. The consistently poor drop test performance of 105Ti/SnPb is caused by the wide pasty range resulting from mixing SAC105 Ti with Sn63 solder paste. The effect of Ti in this system is overshadowed by the high voiding outcome due to this wide pasty range material. In view of this, the use of a SAC105 BGA with an SnPb solder paste is not recommended, with or without the Ti addition. High reflow temperatures shifted the fracture site to the interface at the package side, presumably through building up the IMC thickness beyond the threshold value. A lower reflow temperature is recommended. The electrical response is consistent with the complete fracture data, but the complete fracture trend is inconsistent with that of the partial fracture trend, and neither data can provide a full understanding about the failure mode. By integrating the complete fracture and the partial fracture into a "Virtual Fracture", the failure mechanism becomes obvious and data sets become consistent with each other. Initially Published in the IPC Proceedings |
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